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r:anova

ANOVA Examples

Examples adapted from the following work:

Pagano, R. R. (2002). Understanding statistics in the behavioral sciences (6th ed.). Belmont, CA: Wadsworth.

One way ANOVA

From pages 357 - 360.

The null hypothesis:

$H_0: \mu_1 = \mu_2 = \mu_3$
$H_1:$ At least two means differ.

The data in grouped, tabular format:

Group 1 Group 2 Group 3
2 10 10
3 8 13
7 7 14
2 5 13
6 10 15

Create the data:

x  <- c(2,3,7,2,6,10,8,7,5,10,10,13,14,13,15)
g  <- c(rep("group1", 5), rep("group2", 5), rep("group3", 5))
xg <- data.frame(x,g)

Examine the data:

    x      g
1   2 group1
2   3 group1
3   7 group1
4   2 group1
5   6 group1
6  10 group2
7   8 group2
8   7 group2
9   5 group2
10 10 group2
11 10 group3
12 13 group3
13 14 group3
14 13 group3
15 15 group3

Run the test:

fit.1 <- aov(x ~ g, data = xg)
summary(fit.1)
            Df Sum Sq Mean Sq F value   Pr(>F)    
g            2  203.3   101.7   22.59 8.54e-05 ***
Residuals   12   54.0     4.5                     
---
Signif. codes:  0***0.001**0.01*0.05 ‘.’ 0.1 ‘ ’ 1
Effect Size $\omega^2$

Calculate size of effect (p. 367):

$\omega^2 = \frac{SS_B - (k - 1)s_w^2}{SS_T + s_w^2}$

Where:

  1. $SS_B$: Sum of Squares Between (Sum Sq of g in R summary output)
  2. $k$: Number of groups
  3. $s_w^2$: Within-groups variance (Mean Sq of Residuals in R summary output)
  4. $SS_T$: Sum of Squares Total (Not shown in R summary output; add Sum Sq values)

Found this function to compute $\omega^2$ on StackOverflow:

http://stats.stackexchange.com/questions/2962/omega-squared-for-measure-of-effect-in-r

omega_sq <- function(aovm){
        sum_stats <- summary(aovm)[[1]]
        SSm <- sum_stats[["Sum Sq"]][1]
        SSr <- sum_stats[["Sum Sq"]][2]
        DFm <- sum_stats[["Df"]][1]
        MSr <- sum_stats[["Mean Sq"]][2]
        W2 <- (SSm-DFm*MSr)/(SSm+SSr+MSr)
        return(W2)
}

Then run the function on the model:

omega_sq(fit.1)
[1] 0.7422024

Two-Way ANOVA

There are three null hypotheses in a two-way ANOVA:

  1. $H_0$ Main Effect A (Rows): $\mu_{a1} = \mu_{a2}$
  2. $H_0$ Main Effect B (Columns): $\mu_{b1} = \mu_{b2} = \mu_{b3}$
  3. $H_0$ Interaction Effect A:B: $\mu_{a_1b_1} = \mu_{a_1b_3} = \mu_{a_2b_1} = \mu_{a_2b_2} = \mu_{a_2b_3}$

The data in grouped, tabular format:

Exercise
Time of Day Light Moderate Heavy
Morning 6.5 7.4 8.0
7.3 6.8 7.7
6.6 6.7 7.1
7.4 7.3 7.6
7.2 7.6 6.6
6.8 7.4 7.2
Evening 7.1 7.4 8.2
7.9 8.1 8.5
8.2 8.2 9.5
7.7 8.0 8.7
7.5 7.6 9.6
7.6 8.0 9.4

First, load the data:

hours <- c(6.5, 7.3, 6.6, 7.4, 7.2, 6.8, 7.4, 6.8, 6.7,
           7.3, 7.6, 7.4, 8.0, 7.7, 7.1, 7.6, 6.6, 7.2,
           7.1, 7.9, 8.2, 7.7, 7.5, 7.6, 7.4, 8.1, 8.2,
           8.0, 7.6, 8.0, 8.2, 8.5, 9.5, 8.7, 9.6, 9.4)
tod   <- c(rep("morning", 18), rep("evening", 18))
exer  <- rep(c(rep("light", 6), rep("moderate", 6), rep("heavy", 6)), 2)
an.ex <- data.frame(hours, tod, exer) ; rm(hours, tod, exer)

These are ordered factors:

an.ex$tod  <- ordered(an.ex$tod, levels = c("morning", "evening"))
an.ex$exer <- ordered(an.ex$exer, levels = c("light", "moderate", "heavy"))

Examine the data frame (truncated here):

hours     tod     exer
1    6.5 morning    light
2    7.3 morning    light
...
7    7.4 morning moderate
8    6.8 morning moderate
...
35   9.6 evening    heavy
36   9.4 evening    heavy

Run the model:

fit.1 <- aov(hours ~ tod + exer + tod:exer, data = an.ex)
summary(fit.1)
            Df Sum Sq Mean Sq F value   Pr(>F)    
tod          1  9.000   9.000  48.416 9.93e-08 ***
exer         2  4.754   2.377  12.787 9.63e-05 ***
tod:exer     2  1.712   0.856   4.604    0.018 *  
Residuals   30  5.577   0.186                     
---
Signif. codes:  0***0.001**0.01*0.05 ‘.’ 0.1 ‘ ’ 1

Alternate method:

library(car) # alternate way
fit.2 <- lm(hours ~ tod + exer + tod:exer, data = an.ex)
Anova(fit.2, type = "II")
r/anova.txt · Last modified: 2017/02/22 03:54 by seanburns