17.5 – Testing regression coefficients

Introduction
Linear regression inference
R code
Test two slopes
Example
Questions
Quiz
Chapter 17 contents

Introduction

Whether the goal is to create a predictive model or an explanatory model, then there are two related questions the analyst asks about the linear regression model fitted to the data:

Does a line actually fit the data
Is the linear regression statistically significant?

We will turn to the first question pertaining to fit in time, but for now, focus on the second question.

Like the one-way ANOVA, we have the null and alternative hypothesis for the regression model itself. We write our hypotheses for the regression

H₀ : linear regression fits the data vs. H_A : linear regression does not fit

We see from the output in R and Rcmdr that an ANOVA table has been provided. Thus, the test of the regression is analogous to an ANOVA — we partition the overall variability in the response variable into two parts: the first part is the part of the variation that can be explained by there being a linear regression (the linear regression sum of squares) plus a second part that accounts for the rest of the variation in the data that is not explained by the regression (the residual or error sum of squares). Thus, we have

$\begin{align*} SS_{Total} = SS_{regression} + SS_{residual} \end{align*}$

As we did in ANOVA, we calculate Mean Squares $MS_{x}= SS_{x}/DF_{x}$ , where x refers to either “regression” or “residual” sums of squares and degrees of freedom. We then calculate F-values, the test statistics for the regression, to test the null hypothesis.

The degrees of freedom (DF) in simple linear regression are always

$\begin{align*} DF_{total} = n - 1 DF_{regression} = 1 DF_{residual} = DF_{total} - DF_{regression} = n - 2 \end{align*}$

where

$\begin{align*} F = \frac{MS_{regression}}{MS_{residual}} \end{align*}$

and F_DFregression, DF_residual are compared to the critical value at Type I error rate α, with DFregression, DFresidual.

Linear regression inference

Estimation of the slope and intercept is a first step and should be accompanied by the calculation of confidence intervals.

What we need to know if we are to conclude that there’s a functional relationship between the X and Y variable is whether the same relationship exists in the population. We’ve sampled from the population, calculated an equation to describe the relationship between them. However, just as in all cases of inferential statistics, we need to consider the possibility that, through chance alone, we may have committed a Type I error.

The graph below (Fig 1) shows a possible outcome under a scenario in which the statistical analyst would likely conclude that there is a statistically significant linear model fit to the data, but the true relationship in the population was a slope of zero. How can this happen? Under this scenario, by chance alone the researchers sampled points (circled in red) from the population that fall along a line. We will conclude that there is linear relationship — that’s what are inferential statistics work would indicate — but there was none in the population from which the sampling was done; there would be no way for us to recognize the error except to repeat the experiment — the principal of research reproducibility — with a different sample.

Scatterplot of hypothetical x,y data for which the researcher may obtain a statistically significant linear fit to sample of data from population in which null hypothesis is true relationship between x and y.

Figure 1. Scatterplot of hypothetical x,y data for which the researcher may obtain a statistically significant linear fit to sample of data from population in which null hypothesis is true relationship between x and y.

So in conclusion, you must keep in mind the meaning of statistical significance in the context of statistical inference: it is inference done on a background of random chance, the chance that sampling from the population leads to a biased sample of subjects.

Note 1. If you think about what I did with this data for Figure 1 graph, purposely selecting data that showed a linear relationship between Y and X (red circles), then you should recognize this as an example of data dredging or p-hacking, cf. discussion in Head et al (2015); Stefan and Schönbrodt (2023). However, the graph is supposed to be read as if we could do a census and therefore have full knowledge of the true relationship between y and x. The red circles indicate the chance that sampling from a population may sometimes yield incorrect conclusions.

Tests of coefficients

One criterion for a good model is that the coefficients in the model, the intercept and the slope(s) are all statistically significant.

For the statistical of the slope, b₁, we generally treat the test as a two-tailed test of the null hypothesis that the regression slope is equal to zero.

H₀ : b₁ = 0 vs. H_A : b₁ ≠ 0

Similarly, for the statistical of the intercept, b₀, we generally treat the test as a two-tailed test of the null hypothesis that the Y-intercept is equal to zero.

H₀ : b₀ = 0 vs. H_A : b₀ ≠ 0

For both slope and intercept we use t-statistics.

$\begin{align*} t = \frac{b}{SE_{b}} \end{align*}$

We’ll illustrate the tests of the slope and intercept by letting R and Rcmdr do the work. You’ll find this simple data set at the bottom of this page (scroll or click here). The first variable is the number of matings, the second is the size of the paired female, and the third is the size of the paired male. All body mass were in grams.

R code

After loading the worksheet into R and Rcmdr, begin by selecting

Rcmdr: Statistics → Fit model → Linear Regression

Note that more than one predictor can be entered, but only one response (dependent) variable may be selected (Fig 2).

Screenshot Rcmdr linear regression menu. Select one response variable and one or more explanatory variables

Figure 2. Screenshot linear regression menu. More than explanatory (predictor or independent) variables may be selected, but only one response (dependent) variable may be selected.

This procedure will handle simple and multiple regression problems. But before we go further, answer these two questions for the data set.

Question 1. What is the Response variable?

Question 2. What is the Explanatory variable?

Answers. See below in the R output to see if you were correct!

If there is only one predictor, then this is a Simple Linear Regression; if more than one predictor is entered, then this is a Multiple Linear Regression. We’ll get some more detail, but for now, identify and evaluate the test (p-value) of the slope coefficient (labeled after the name of the predictor variable), the test of the intercept coefficient, and some new stuff.

R output

RegModel.1 <- lm(Matings ~ Female, data=birds)
summary(RegModel.1)
Call: lm(formula = Matings ~ Female, data = birds) 
Residuals: 
     Min       1Q     Median         3Q        Max 
-2.32805 -1.59407   -0.04359    1.77292    2.67195 

Coefficients: 
              Estimate    Std. Error     t value    Pr(>|t|) 
(Intercept)    -8.6175        3.5323      -2.440     0.02528 * 
Female          0.3670        0.1042       3.524     0.00243 ** 
--- 
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 
Residual standard error: 1.774 on 18 degrees of freedom 
Multiple R-squared: 0.4082, Adjusted R-squared: 0.3753 F-statistic: 12.42 on 1 and 18 DF, p-value: 0.002427

In this example, slope = 0.367 and the intercept = -8.618. The first new term we encounter is called Multiple R-squared (R²) — it’s also called the coefficient of determination. It’s the ratio of the sum of squares due to the regression to the total sums of squares. R² ranges from zero to 1, with a value of 1 indicating a perfect fit of the regression to the data.

Note 2. If you are looking for a link between correlation and simple linear regression, then here it is: R² is the square of the product-moment correlation, r. Thus, r = √R² see Chapter 17.2).

We introduced $R^2$ in Chapter 17.1 . Interpretation of R² goes like this: If R² is close to zero, then the regression model does not explain much of the variation in the dependent variable; conversely, if R² is close to one, then the regression model explains a lot of the variation in the dependent variable.

Did you get the Answers?

Answer 1: Number of matings

Answer 2: Size of females

Interpreting the output

Recall that

$\begin{align*} SS_{Total} = SS_{regression} + SS_{residual} \end{align*}$

then

$\begin{align*} R_{2}= {SS_{regression}}{SS_{Total}} \end{align*}$

From the output we see that R² was 0.408, which means that about 40% of the variation in numbers of matings may be explained by size of the females alone.

To complete the analysis get the ANOVA table for the regression.

Rcmdr: Models → Hypothesis tests → ANOVA table…

> Anova(RegModel.1, type="II") 
Anova Table (Type II tests) 
            Sum Sq   Df   F value      Pr(>F) 
Female      39.084    1    12.415    0.002427 ** 
Residuals   56.666   18 
--- 
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

With only one predictor (explanatory) variable, note that the regression test in the ANOVA is the same (has the same probability vs. the null hypothesis) as the test of the slope.

Test two slopes

A more general test of the null hypothesis involving the slope might be

H₀ : b₁ = b vs. H_A : b₁ ≠ b

where b can be any value, including zero. Again, t-test would then be used to conduct the test of the slope, where the t-test would have the form

$\begin{align*} t = \frac{b_{1} - b_{2}}{SE_{b_{1} - b_{2}}} \end{align*}$

where SE_b1-b2 is the standard error of the difference between the two regression coefficients. We saw something similar to this value back when we did a paired t-test. To obtain s_b1–_b2, we need the pooled residual mean square and the squared sums of the X values for each of our sample.

First, the pooled (hence the subscript p) residual mean square is calculated as

$\begin{align*} \left (s_{y \cdot x}^2 \right )_{p} = \frac{resSS_{1}}{resDF_{1}} + \frac{resSS_{2}}{resDF_{2}} \end{align*}$

where resSS and resDF refer to residual sums of squares and residual degrees of freedom for the first (1) and second (2) regression equations.

Second, the standard error of the difference between regression coefficients (squared!!) is calculated as

$\begin{align*} s_{b1-b2} = \left (s_{y \cdot x}^2 \right )_{p} = \frac{\left (s_{y \cdot x}^2 \right )_{p}}{\sum x_{1}^2} + \frac{\left (s_{y \cdot x}^2 \right )_{p}}{\sum x_{2}^2} \end{align*}$

where the subscript “1” and “2” refer to the X values from the first sample (eg, the body size values for the males) and the second sample (eg, the body size values for the females).

Note 3. To obtain the squared sum in R and Rcmdr, use the Calc function (eg, two sum the squared X values for the females, use SUM(‘Female’*’Female’)).

We can then use our t-test, with the degrees of freedom now

$\begin{align*} DF = n_{1} + n_{2} - 4 \end{align*}$

Alternatively, the t-test of two slopes can be written as

$\begin{align*} t = \frac{b_{1} - b_{2}}{\left (SE_{b1}^2 + SE_{b2}^2 \right )^2} \end{align*}$

with again $DF = n_{1} + n_{2} - 4$ .

In this way, we can see a way to test any two slopes for equality. This would be useful if we wanted to compare two samples (eg, males and females) and wanted to see if the regressions were the same (eg, metabolic rate covaried with body mass in the same way — that is, the slope of the relationship was the same). This situation arises frequently in biology. For example, we might want to know if male and female birds have different mean field metabolic rates, in which case we might be tempted to use a one-way ANOVA or t-test (since one factor with two levels). However, if males and females also differ for body size, then any differences we might see in metabolic rate could be due to differences in metabolic rate or to differences in the covariable body size. The test is generally referred to as the analysis of covariance (ANCOVA), which is the subject for Chapter 17.6. In brief, ANCOVA allows you to test for mean differences in traits like metabolic rate between two or more groups, but after first accounting for covariation due to another variable (eg, body size). However, ANCOVA makes the assumption that relationship between the covariable and the response variable is the same in the two groups. This is the same as saying that the regression slopes are the same. Let’s proceed to see how we can compare regression slopes, then move to a more general treatment in Chapter 17.6.

Example

For a sample of 13 tadpoles (Rana pipiens), hatched in the laboratory, oxygen consumption at rest was recorded with a YSI meter, model 51B, and a YSI oxygen – temperature probe (Yellow Spring Instruments Co., Yellow Springs, Ohio). Individual tadpoles were placed into a mason jar (volume 885 ml), with a magnetic stir bar housed in a slotted Petri dish glued to the bottom of the jar to gently stir the water. Water samples from the aquarium were used and petroleum jelly served as a sealant once the vessel had been fitted with the probe. Oxygen consumption was recorded in ppm and $\dot{V}O_{2}$ values (ml O₂ STP · hr^-1). The tadpoles were scored for developmental stage by Gosner’s (1960) criteria. The tadpoles ranged from Gosner stage 35 to 44.

Note 4. Gosner stages 1 through 25 include the embryonic stages; By stage 31, toes develop on the limb buds; Following stage 40, substantial body changes are observed in the growing tadpole, transitioning from tailed tadpole to tail-less adult form. Complete metamorphosis is assigned Gosner stage 45.

Figure 3. Pearson Scott Foresman, Public domain, via Wikimedia Commons

The question we addressed: metamorphosis of Anuran tadpoles to adults (frogs) likely involves increased metabolic demands, demands that may exceed simple allometric predictions based on increase in body size during development. Thus, we predicted that mass-specific metabolic rate for metamorphic frogs would exceed mass-specific metabolism of pre-metamorphic frogs.

M. Dohm unpublished results from my undergraduate days (1986!), you’ll find this simple data set at the bottom of this page (scroll or click here). For simplicity, we grouped all tadpoles between stages 35 and 38 as Stage I and all tadpoles between 39 and 44 as stage II. No tadpoles who completed metamorphosis were included in the dataset. It shouldn’t surprise you — with such a small sample size, the project lacked power to test the hypothesis, but many such studies, it’s worth looking at in the case anyway because the prevailing sentiment is that the metabolic cost of metamorphosis is significant — in other words, we expect a large effect size — and remains an ongoing research effort (see Burraco et al 2019).

You should confirm your self, but the slope of the regression equation of oxygen consumption (ml O₂/hour) on body mass (g) was 444.95 (with SE = 65.89). Plot of data shown in Figure 4.

Oxygen consumption of tadpoles by body weight

Figure 4. Scatterplot of oxygen consumption by tadpoles (blue: Gosner developmental stage I [35 – 38]; red: Gosner developmental stage II [39 – 44]), vs body mass (g).

The project looked at whether metabolism as measured by oxygen consumption was consistent across two developmental stages. Metamorphosis in frogs and other amphibians represents profound reorganization of the organism as the tadpole moves from water to air. Thus, we would predict some cost as evidenced by change in metabolism associated with later stages of development. Figure 5 shows a box plot of tadpole oxygen consumption by Gosner (1960) developmental stage.

boxplot oxygen consumption of tadploes by developmental stage

Figure 5. Boxplot of oxygen consumption by Gosner developmental stages (blue: stage I; red: stage 2).

Looking at Figure 4 we see a trend consistent with our prediction; developmental stage may be associated with increased metabolism. However, older tadpoles also tend to be larger, and the plot in Figure 4 does not account for that. Thus, body mass is a confounding variable in this example. There are several options for analysis here (eg, ANCOVA), but one way to view this is to compare the slopes for the two developmental stages. While this test does not compare the means, it does ask a related question: is there evidence of change in rate of oxygen consumption relative to body size between the two developmental stages? The assumption that the slopes are equal is a necessary step for conducting the ANCOVA, which we describe in Chapter 17.6.

So, divide the data set into two groups by developmental stage (12 tadpoles could be assigned to one of two developmental stages; one tadpole was at a lower Gosner stage than the others and so is dropped from the subset).

Gosner stage I (stages 35 – 39)

Body mass	$\dot{V}O_{2}$
1.76	109.41
1.88	329.06
1.95	82.35
2.13	198
2.26	607.7

Gosner stage II (stage 40 – 44)

Body mass	$\dot{V}O_{2}$
2.28	362.71
2.35	556.6
2.62	612.93
2.77	514.02
2.97	961.01
3.14	892.41
3.79	976.97

The slopes and standard errors were

	Gosner Stage I	Gosner stage II
slope	750.0	399.9
standard error of slope	444.6	111.2

Are the two slopes equal?

Looking at the table, we would say, no. slopes look different (750 vs 399.9). However, the errors are large and, given this is a small data set, we need to test statistically; are the slopes indistinguishable (Ho: b_I = b_II), where b_I is the slope for the Gosner Stage I subset and b_II is the slope for the Gosner Stage II subset?

To use our tests discussed above, we need sum of squared X values for Gosner Stage I and sum of squared for Gosner stage II results. We can get these from the ANOVA tables. Recall that after applying

ANOVA regression Gosner Stage I, R work

Anova(gosI.1, type="II")
Anova Table (Type II tests)

Response: VO2
            Sum Sq   Df    F value   Pr(>F)
Body.mass    89385    1     2.8461   0.1902
Residuals    94220    3

ANOVA regression Gosner Stage II

Anova(gosII.1, type="II")
Anova Table (Type II tests)

Response: VO2
             Sum Sq    Df   F value  Pr(>F) 
Body.mass    258398     1    12.935  0.0156 *
Residuals     99882     5

Rcmdr: Models – Compare model coefficients..

compareCoefs(gosI.1, gosII.1)
Calls:
1: lm(formula = VO2 ~ Body.mass, data = gosI)
2: lm(formula = VO2 ~ Body.mass, data = gosII)

            Model 1    Model 2
(Intercept)   -1232       -441
SE              891        321

Body.mass       750        400
SE              445        111

SSgosnerI = 89385

SSgosnerII = 258398

We also need the residual Mean Squares (SSresid/DF resid) from the ANOVA tables

MSgosnerI = 94220/3 = 31406.67

MSgosnerII = 99882/5 = 19976.4

[under construction] Therefore, the pooled residual MS (s²_x.y)_p = nnn and the pooled SE of the difference s_b1-b2 = nnn using the formulas above.

Now, we plug in the values I can get a t-test = (750 – 444.6)/nnn = nnn

The DF for this t-test are n1 + n2 – 4 = 4 + 6 – 4 = 6.

Using Table of Student’s t distribution (Appendix), I find the two-tailed critical value for t at alpha = 5% with DF = 6 is equal to 3.758. Since nn >> 3.758, we conclude that the two slopes are statistically different and P < 0.001 that the null hypothesis (Ho: b_w = b_all) was true. (edit: fix nn).

Questions

1. Write up three learning outcomes for this page. Hint: Point your favorite generative AI to this page and ask for help.

2. Metabolic rates like oxygen consumption over time are well-known examples of allometric relationships. That is, many biological variables (eg, $\dot{V}O_{2}$ is related as a · M^b, where M is body mass, b is scaling exponent (the slope!), and a is a constant (the intercept!)), and best evaluated on log-log scale. Redo the linear regression of oxygen consumption vs. body mass for the tadpoles, but this time, apply log10-transform to VO2 and to Body.mass. For example

Data in this page, bird matings

Body.Mass	Matings
29	0
29	2
29	4
32	4
32	2
35	6
36	3
38	3
38	5
38	8
40	6

Data in this page, Oxygen consumption, $\dot{V}O_{2}$ , of Anuran tadpoles

Gosner	Body mass	VO2
NA	1.46	170.91
I	1.76	109.41
I	1.88	329.06
I	1.95	82.35
I	2.13	198
II	2.28	362.71
I	2.26	607.7
II	2.35	556.6
II	2.62	612.93
II	2.77	514.02
II	2.97	961.01
II	3.14	892.41

Gosner refers to Gosner (1960), who developed a criteria for judging metamorphosis staging.

Quiz Chapter 17.5

Testing regression coefficients

Chapter 17 contents

12.2 – One way ANOVA

Introduction
Three important terms you’ll need for one-way ANOVA
Note on experimental design
ANOVA sources of variation
Mean squares
Worked example with R
Questions
Quiz
Data set
Chapter 12 contents

Introduction

In ANalysis Of VAriance, or ANOVA for short, we likewise have tools to test the null hypothesis of no difference between between categorical independent variables — often called factors when there’s just a few levels to keep track of — and a single, dependent response variable. But now, the response variable is quantitative, not qualitative like the $\chi^2$ tests.

Analysis of variance, ANOVA, is such an important statistical test in biology that we will take the time to “build it” from scratch. We begin by reminding you of where you’ve been with the material.

We already saw an example of this null hypothesis. When there’s only one factor (but with two or more levels), we call the analysis of means and “one-way ANOVA.” In the independent sample t-test, we tested whether two groups had the same mean. We made the connection between the confidence interval of the difference between the sample means and whether or not it includes zero (ie, no difference between the means). In ANOVA, we extend this idea to a test of whether two or more groups have the same mean. In fact, if you perform an ANOVA on only two groups, you will get the exact same answer as the independent two-sample t-test, although they use different distributions of critical values (t for the t-test, F for the ANOVA — a nice little fact for you square the t-test statistic, you’ll get the F-test statistic: $t^{2} = F$ ).

Let’s say we have an experiment where we’ve looked at the effect of different three different calorie levels on weight change in middle-aged men.

I’ve created a simulated dataset which we will use in our ANOVA discussions. The data set is available at the end of this page (scroll down or click here).

We might graph the mean weight changes ( $\pm$ SEM); Below are two possible outcomes of our experiment (Fig 1).

Figure 1. Hypothetical results, box plots.

Figure 1. Hypothetical results of an experiment, box plots. Left, no difference among groups; Right, large differences among groups.

As statisticians and scientists, we would first calculate an overall or grand mean for the entire sample of observations; we know this as the sample mean whose symbol is $\bar{X}$ . But this overall mean is made up of the average of the sample means. If the null hypothesis is true, then the all of the sample means all estimate the overall mean. Put another way, being a member of a treatment group doesn’t matter, ie, there is no systematic effect, all differences among subjects are due to random chance.

$\begin{align*} H_{0} : \bar{X} = \bar{X}_{A} = \bar{X}_{B} = \bar{X}_{C} \end{align*}$

The hypotheses among are three groups or treatment levels then are:

$\begin{align*} H_{0} : \bar{X}_{A} = \bar{X}_{B} = \bar{X}_{C}&& \end{align*}$

Null that there are no differences among the group means. And the alternative hypotheses include any (or all) of the following possibilities

$\begin{align*} H_{0} : \bar{X}_{A} \ne \bar{X}_{B} = \bar{X}_{C} \end{align*}$

or maybe

$\begin{align*} H_{0} : \bar{X}_{A} = \bar{X}_{B} \ne \bar{X}_{C} \end{align*}$

or… have we covered all possible alternate outcomes among three groups?

In either case, we could use one-way ANOVA to test for “statistically significant differences.”

Three important terms you’ll need for one-way ANOVA

FACTOR: We have one factor of interest. For example, a factor of interest might be

Diet fed to hypertensive subjects (men and women)
Distribution of coral reef sea cucumber species in archipelagos
Antibiotic drug therapy for adolescents with Acne vulgaris (click see Webster 2002 for review).

Note 1: Character vs factor. In R programming, while it is true that a treatment variable can be a character vector like Gender*, such vectors are handled by R as string. In contrast, by claiming Gender as a factor, R handles the variable as a nominal data type. Hence, ANOVA and other GLM functions expect any treatment variable to be declared as a factor:

Gender <- c("Gender non-conforming", "Men", "Non-binary", "Transgender", "Women", "Declined"); Gender
[1] "Gender non-conforming" "Men" "Non-binary" "Transgender" 
[5] "Women" "Declined"

# it’s a string vector, not a factor

is.factor(Gender)
[1] FALSE

Change string vector to a factor.

Gender <- as.factor(Gender); is.factor(Gender) [1] TRUE

* For discussions of ethics of language inclusivity and whether or not such information should be gathered, see Henrickson et al 2020.

LEVELS: We can have multiple levels (2 or more) within the single factor. Some examples of levels for the Factors listed:

Three diets (DASH), diet rich in fruits & vegetables, control diet)
Five archipelagos (Hawaiian Islands, Line Islands, Marshal Islands, Bonin Islands, and Ryukyu Islands)
Five antibiotics (ciprofloxacin, cotrimoxazole, erythromycin, doxycycline, minocycline).

RESPONSE: There is one outcome or measurement variable. This variable must be quantitative (ie, on the ratio or interval scale). Continuing our examples then

Reduction in systolic pressure
Numbers of individual sea cucumbers in a plot
number of microcomedo.

Note 2: A comedo is a clogged pore in the skin; a microcomedo refers to the small plug. Yes, I had to look that up, too.

The response variable can be just about anything we can measure, but because ANOVA is a parametric test, the response variable must be normally distributed!

Note on experimental design

As we discuss ANOVA, keep in mind we are talking about analyzing results from an experiment. Understanding statistics, and in particular ANOVA, informs how to plan an experiment. The basic experimental design is called a completely randomized experimental design, or CDR, where treatments are assigned to experimental units at random.

In this experimental design, subjects (experimental units) must be randomly assigned to each of these levels of the factor. That is, each individual should have had the same probability of being found in any one of the levels of the factor. The design is complete because randomization is conducted for all levels, all factors.

Thinking about how you would describe an experiment with three levels of some treatment, we would have

Table 1. Summary statistics of three levels for some ratio scale response variable.

Level 1

$\bar_{X}_{1}$

sample mean₁

$s_{1}$

sample standard deviation₁

Level 2

$\bar_{X}_{2}$

sample mean₂

$s_{2}$

sample standard deviation₂

Level 3

$\bar_{X}_{3}$

sample mean₃

$s_{3}$

sample standard deviation₃

Note 3: Table 1 is the basis for creating the box plots in Figure 1.

ANOVA sources of variation

ANOVA works by partitioning total variability about the means (the grand mean, the group means). We will discuss the multiple samples and how the ANOVA works in terms of the sources of variation. There are two “sources” of variation that can occur:

Within Group Variation
Among Groups Variation

So let’s look first at the variability within groups, also called the Within Group Variation.

Consider and experiment to see if DASH diet reduces systolic blood pressure in USA middle-aged men and women with hypertension (Moore et al 2001). After eight weeks we have

We get the corrected sum of squares, SS, for within group

$\begin{align*} within-groups \ SS = \sum_{i=1}^{k}\left [ \sum_{j=1}^{n_{i}}\left ( X_{ij}-\bar{X}_{i} \right )^2 \right ] \end{align*}$

and the degrees of freedom, DF, for within groups

$\begin{align*} within-groups \ DF = \sum_{i=1}^{k}\left ( n_{i}-1 \right ) = N - k \end{align*}$

where i is the identity of the groups, $X_{ij}$ is the individual observations within group i, $\bar{X}_{i}$ is the group i mean, n_i is the sample size within each group, N is the total sample size of all subjects for all groups, and k is the number of groups.

Importantly, this value is also referred to as “error sums of squares” in ANOVA. It’s importance is as follows — In our example, the within group variability would be zero if and only if all subjects within the same diet lost exactly the same amount of weight. This is hardly ever the case of course in a real experiment. Because there are almost always some other unknown factors or measurement error that affect the response variable, there will be some unknown variation among individuals who received the same treatment (within the same group). Thus, the error variance will generally be larger than zero.

The first point to consider: your ANOVA will never result in statistical differences among the groups if the error variance is greater than the second type of variability, the variability between groups.

The second type of variability in ANOVA is that due to the groups or treatments. Individuals given a calorie-restricted diet will lose some weight; individuals allowed to eat a calorie-rich diet likely will gain weight, therefore there will be variability (a difference) due to the treatment. So we can calculate the variability among groups. We get the corrected sum of squares for among groups

$\begin{align*} among-groups \ SS = \sum_{i=1}^{k}n_{i}\left ( X_{i}-\bar{X}\right )^2 \end{align*}$

and the degrees of freedom for among groups

$\begin{align*} among-groups \ DF = k - 1 \end{align*}$

where i is the identity of the groups, $\bar{X}$ is the grand mean as defined in Measures of Central Tendency (Chapter 3.1), $\bar{X}_{i}$ is the group i mean, n_i is the sample size within each group, N is the total sample size of all subjects for all groups, and k is the number of groups.

The sums of squares here is simply subtracting the mean of each population from the overall mean.

If the Factor is not important in explaining the variation among individuals then all the population means will be similar and the sums of squares among populations would be small.
If the Factor is important in explaining some of the variation among the individuals then all the population means will NOT be the same and the sums of squares among populations would be large.

Finally, we can identify the total variation in the entire experiment. We have the total sum of squares.

$\begin{align*} Total \ SS = among-groups \ SS + within-groups \ SS \end{align*}$

Thus, the insight of ANOVA is that variation in the dataset may be attributed to a portion explained by differences among the groups and differences among individual observations within each group. The inference comes from recognizing that if the among group effect is greater than the within group effect, then there will be a difference due to the treatment levels.

Mean squares

To decide whether the variation associated with the among group differences are greater than the within group variation, we calculate ratios of the sums of squares. These are called Mean Squares or MS for short. The ratio of the Mean Squares is called F, the test statistic for ANOVA.

For the one-way ANOVA we will have two Mean Squares and one F, tested with degrees of freedom for both the numerator $MS_{groups}$ , and denominator $MS_{error}$ ).

The Mean Square for (among) groups is

$\begin{align*} MS_{group} = \frac{among-groups \ SS}{among-groups \ DF} \end{align*}$

The Mean Square for error is

$\begin{align*} MS_{error} = \frac{within-groups \ SS}{within-groups \ DF} \end{align*}$

And finally, the value for F, the test statistic for ANOVA, is

$\begin{align*} F = \frac{MS_{group}}{MS_{error}} \end{align*}$

Worked example with R

A factor with three levels, A, B, and C

group <- c("A", "A", "A", "B", "B", "B", "C", "C", "C")

and their responses, simulated

response <- c(10.8, 11.8, 12.8, 6.5, 7, 8, 3.8, 2.8, 3)

We create a data frame

all <- data.frame(group, response)

Of course, you could place the data into a worksheet and then import the worksheet into R. Regardless, we now have our dataset.

Now, call the ANOVA function, aov, and assign the results to an object (eg, Model.1)

Model.1 <- aov(response ~ group, data=all)

Now, visualize the ANOVA table

summary(Model.1)

and the output from R, the ANOVA table, is shown below

Note 4: Rcmdr: Statistics → Means → One-way ANOVA… see figures 2 and 3 below

Table 2. Output from aov() command, the ANOVA table, for the “Difference” outcome variable.

           Df   Sum Sq    Mean Sq   F value       Pr(>F) 
group       2   111.16      55.58     89.49    0.0000341 ***
Residuals   6     3.73       0.62
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Let’s take the ANOVA table one row at a time

The first row has subject headers defining the columns.
The second row of the table “groups” contains statistics due to group, and provides the comparisons among groups.
The third row, “Residuals” is the error, or the differences within groups.

Moving across the columns, then, for each row, we have in turn,

the degrees of freedom (there were 3 groups, therefore 2 DF for group),
the Sums of Squares, the Mean Squares,
the value of F, and finally,
the P-value.

R provides a helpful guide on the last line of the ANOVA summary table, the “Signif[icance] codes,” which highlights the magnitude of the P-value.

Note 5: On the use of “error” and “residuals.” Error refers to the difference between an observed value and the true population value. Residual is the difference between an observed value and a value predicted by a model. In ANOVA, the “error variance” or “residual variance” is the variation within groups unexplained by the tested factor. It is what remains after the effects of the model are accounted for.

What to report? ANOVA problems can be much more complicated than the simple one-way ANOVA introduced here. For complex ANOVA problems, report the ANOVA table itself! But for the one-way ANOVA it would be sufficient to report the test statistic, the degrees of freedom, and the p-value, as we have in previous chapters (eg, t-test, chi-square, etc.). Thus, we would report:

F = 89.49, df = 2 and 6, p = 0.0000341

where F = 89.49 is the test statistic, df = 2 (degrees of freedom for the among group mean square) and 6 (degrees of freedom for the within group mean square), and p = 0.0000341 is the p-value.

In Rcmdr, the appropriate command for the one-way ANOVA is simply

Rcmdr: Statistics → Means

Screenshot Rcmdr select one-way ANOVA

Figure 2. Screenshot Rcmdr select one-way ANOVA.

which brings up a simple dialog. R Commander anticipates factor (Groups) and Response variable. Optional, choose Pairwise comparisons of means for post-hoc test (Tukey’s) and, if you do not want to assume equal variances (see Chapter 13), select Welch F-test.

Screenshot Rcmdr select one-way ANOVA options

Figure 3. Screenshot Rcmdr select one-way ANOVA options.

Questions

1. Review the example ANOVA Table and confirm the following

How many levels of the treatment were there?
How many sampling units were there?
Confirm the calculation of $MS_{group}$ and $MS_{error}$ using the formulas contained in the text.
Confirm the calculation of F using the formula contained in the text.
The degrees of freedom for the F statistic in this example were 2 and 6 (F_2,6). Assuming a two-tailed test with Type I error rate of 5%, What is the critical value of the F distribution (see Appendix 20.4)

2. Repeat the one-way ANOVA using the simulated data, but this time, calculate the ANOVA problem for the “No.difference” response variable.

3. Leaf lengths from three strains of Arabidopsis thaliana plants grown in common garden are shown in Fig 2. Data are provided for you in the following R script.

arabid <- c("wt","wt","wt","AS1","AS1","AS1","AS2","AS2","AS2")
leaf <- c(4.909,5.736,5.108,6.956,5.809,6.888,4.768,4.209,4.065)
leaves <- data.frame(arabid,leaf)

Write out a null and alternative hypotheses
Conduct a test of the null hypothesis by one-way ANOVA
Report the value of the test statistic, the degrees of freedom, and the P-value
Do you accept or reject the null hypothesis? Explain your choice.

Box plot of length of leaves from three strains of Arabidopsis thaliana

Figure 4. Box plot of lengths of leaves of a 10-day old plant from on of three strains of Arabidopsis thaliana.

4. Return to your answer to question 7 from Chapter 12.1 and review your answer and modify as appropriate to correct your language to that presented here about factors and levels.

5. Conduct the one-way ANOVA test on the Comet assay data presented question 7 from Chapter 12.1. Data are copied for your convenience to end of this page. Obtain the ANOVA table and report the value of the test statistics, degrees of freedom, and p-value.
a. Based on the ANOVA results, do you accept or reject the null hypothesis? Explain your choice.

Quiz Chapter 12.2

One-way ANOVA

Data used in this page

Difference, no difference

Table 12.2 Difference or No Difference

Group	No.difference	Difference
A	12.04822	11.336161
A	12.67584	13.476142
A	12.99568	12.96121
A	12.01745	11.746712
A	12.34854	11.275492
B	12.17643	7.167262
B	12.77201	5.136788
B	12.07137	6.820242
B	12.94258	5.318743
B	12.0767	7.153992
C	12.58212	3.344218
C	12.69263	3.792337
C	12.60226	2.444438
C	12.02534	2.576014
C	12.6042	4.575672

Simulated data

Comet assay, antioxidant properties of tea

Data presented in Chapter 12.1

17.5 – Testing regression coefficients

Introduction

Linear regression inference

Tests of coefficients

R code

R output

Did you get the Answers?

Interpreting the output

Test two slopes

Example

Are the two slopes equal?

Questions

Chapter 17 contents

12.2 – One way ANOVA

Introduction

Three important terms you’ll need for one-way ANOVA

Note on experimental design

ANOVA sources of variation

Mean squares

Worked example with R

Questions

Data used in this page

Table 12.2 Difference or No Difference

Comet assay, antioxidant properties of tea

Chapter 12 contents