- 1 Kangaroos skull measurements: crest width
- 1.1
**(1 p)**Interpret plots of the data, distributional centers and shapes - 1.2
**(1 p)**Do the plots above suggest there is an interaction? - 1.3 Fit the two-way interaction model
- 1.4
**(1 p)**Check model assumptions for full model - 1.5
**(1 p)**ANOVA table, test for interaction - 1.6
**(4 p)**Reduce to final model, test assumptions - 1.7
**(2 p)**Summarize the differences

- 1.1

*What effect does sex and species have on the crest width of a kangaroo skull?*

The data to be analyzed here are selected skull measurements on 148 kangaroos of known sex and species. There are 11 columns of data, corresponding to the following features. The measurements are in meters/10000 (mm/10).

column | Variable name | Description |
---|---|---|

1 * | sex | sex (1=M, 2=F) |

2 * | species | species (0=M. giganteus, 1=M.f. melanops, 2=M.f. fuliginosus) |

3 | pow | post orbit width |

4 | rw | rostal width |

5 | sopd | supra-occipital - paroccipital depth |

6 * | cw | crest width |

7 | ifl | incisive foramina length |

8 | ml | mandible length |

9 | mw | mandible width |

10 | md | mandible depth |

11 | arh | ascending ramus height |

Some of the observations in the data set are missing (not available). These are represented by a period `.`

, which in the `read_csv()`

function is specified by the `na = "."`

option.

```
library(tidyverse)
# load ada functions
source("ada_functions.R")
# First, download the data to your computer,
# save in the same folder as this Rmd file.
dat_kang <-
read_csv(
"ADA2_WS_09_kang.csv"
, na = c("", ".")
) %>%
# subset only our columns of interest
select(
sex, species, cw
) %>%
# make dose a factor variable and label the levels
mutate(
sex = factor(sex , labels = c("M","F"))
, species = factor(species, labels = c("Mg", "Mfm", "Mff"))
)
# remove observations with missing values
n_start <- nrow(dat_kang)
dat_kang <- na.omit(dat_kang)
n_keep <- nrow(dat_kang)
n_drop <- n_start - n_keep
cat("Removed", n_start, "-", n_keep, "=", n_drop, "observations with missing values.")
```

`Removed 148 - 148 = 0 observations with missing values.`

```
# A tibble: 6 x 3
sex species cw
<fct> <fct> <dbl>
1 M Mg 153
2 M Mg 141
3 M Mg 144
4 M Mg 116
5 M Mg 120
6 M Mg 188
```

The side-by-side boxplots of the data compare the crest widths across the 6 combinations of sex and species. Comment on the distributional shapes and compare the typical crest widths across groups.

```
# Calculate the cell means for each (sex, species) combination
# Group means
kang_mean <- dat_kang %>% summarise(m = mean(cw))
kang_mean_x <- dat_kang %>% group_by(sex) %>% summarise(m = mean(cw)) %>% ungroup()
kang_mean_s <- dat_kang %>% group_by(species) %>% summarise(m = mean(cw)) %>% ungroup()
kang_mean_xs <- dat_kang %>% group_by(sex, species) %>% summarise(m = mean(cw)) %>% ungroup()
kang_mean
```

```
# A tibble: 1 x 1
m
<dbl>
1 123.
```

```
# A tibble: 2 x 2
sex m
<fct> <dbl>
1 M 111.
2 F 136.
```

```
# A tibble: 3 x 2
species m
<fct> <dbl>
1 Mg 110.
2 Mfm 116.
3 Mff 144.
```

```
# A tibble: 6 x 3
sex species m
<fct> <fct> <dbl>
1 M Mg 103.
2 M Mfm 102.
3 M Mff 128.
4 F Mg 117.
5 F Mfm 128.
6 F Mff 161
```

```
# Interaction plots, ggplot
library(ggplot2)
p1 <- ggplot(dat_kang, aes(x = sex, y = cw, colour = species))
p1 <- p1 + geom_hline(aes(yintercept = 0), colour = "black"
, linetype = "solid", size = 0.2, alpha = 0.3)
p1 <- p1 + geom_boxplot(alpha = 0.5, outlier.size=0.1)
p1 <- p1 + geom_point(data = kang_mean_xs, aes(y = m), size = 4)
p1 <- p1 + geom_line(data = kang_mean_xs, aes(y = m, group = species), size = 1.5)
p1 <- p1 + labs(title = "Kangaroo interaction plot, species by sex")
#print(p1)
p2 <- ggplot(dat_kang, aes(x = species, y = cw, colour = sex))
p2 <- p2 + geom_hline(aes(yintercept = 0), colour = "black"
, linetype = "solid", size = 0.2, alpha = 0.3)
p2 <- p2 + geom_boxplot(alpha = 0.5, outlier.size=0.1)
p2 <- p2 + geom_point(data = kang_mean_xs, aes(y = m), size = 4)
p2 <- p2 + geom_line(data = kang_mean_xs, aes(y = m, group = sex), size = 1.5)
p2 <- p2 + labs(title = "Kangaroo interaction plot, sex by species")
#print(p2)
library(gridExtra)
grid.arrange(grobs = list(p1, p2), nrow=1, top="Kangaroo crestwidth plots")
```

[answer]

Do the lines for each group seem to be very different from parallel?

[answer]

Here it is.

Recall that we assume that the full model is correct before we perform model reduction by backward selection.

```
Non-constant Variance Score Test
Variance formula: ~ fitted.values
Chisquare = 3.094663, Df = 1, p = 0.078549
```

```
Anderson-Darling normality test
data: lm_cw_x_s_xs$residuals
A = 0.38405, p-value = 0.3907
```

[answer]

Provide your conclusion for the test for interaction.

```
Anova Table (Type III tests)
Response: cw
Sum Sq Df F value Pr(>F)
(Intercept) 2244042 1 1643.795 < 2.2e-16 ***
sex 22556 1 16.523 7.927e-05 ***
species 34195 2 12.524 9.788e-06 ***
sex:species 2367 2 0.867 0.4224
Residuals 193853 142
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
```

[answer]

If the model can be simplified (because interaction is not significant), then refit the model with only the main effects. Test whether the main effects are significant, reduce further if sensible. Test model assumptions of your final model.

[answer]

Summarize differences, if any, in sexes and species using relevant multiple comparisons. Give clear interpretations of any significant effects.

*This code is here to get you started. Determine which comparisons you plan to make and modify the appropriate code. Make the code chunk active by moving the {R} to the end of the initial code chunk line.*

```
{R}
library(emmeans)
# Contrasts to perform pairwise comparisons
cont_kang <- emmeans(lm_object, specs = "sex")
cont_kang <- emmeans(lm_object, specs = "species")
cont_kang <- emmeans(lm_object, specs = "sex", by = c("species"))
cont_kang <- emmeans(lm_object, specs = "species", by = c("sex"))
# Means and CIs
cont_kang
# Pairwise comparisons
cont_kang %>% pairs()
```

**EMM plot interpretation**

This **EMM plot (Estimated Marginal Means, aka Least-Squares Means)** is only available when conditioning on one variable. The **blue bars** are confidence intervals for the EMMs; don’t ever use confidence intervals for EMMs to perform comparisons – they can be very misleading. The **red arrows** are for the comparisons among means; the degree to which the “comparison arrows” overlap reflects as much as possible the significance of the comparison of the two estimates. If an arrow from one mean overlaps an arrow from another group, the difference is not significant, based on the adjust setting (which defaults to “tukey”).

```
{R, fig.height = 5, fig.width = 6}
# Plot means and contrasts
p <- plot(cont_kang, comparisons = TRUE)
p <- p + labs(title = "Tukey-adjusted contrasts")
p <- p + theme_bw()
print(p)
```

[answer]