## Testing pairwise differences

In the **previous
tutorial**, we computed marginal means at the 3 different
`Species`

levels from the `iris`

dataset. However, one might also want to **statistically
test** the differences between each levels, which can be achieved
through **contrast analysis**. Although the procedure is
much more powerful, its aim is analogous to the ** post
hoc** analysis (pretty much consisting of pairwise

*t*-tests), which are heavily utilized in behavioral sciences as a way to follow up on hypotheses about global differences tested by ANOVAs with more specific hypotheses about pairwise differences.

Let’s carry out contrast analysis on the simple model from the previous tutorial:

```
library(ggplot2)
library(see)
library(modelbased)
model <- lm(Sepal.Width ~ Species, data = iris)
means <- estimate_means(model)
ggplot(means, aes(x = Species, y = Mean)) +
geom_line(aes(group = 1)) +
geom_pointrange(aes(color = Species, ymin = CI_low, ymax = CI_high)) +
theme_modern()
```

**Contrast analysis** can be achieved through the
`estimate_contrasts`

function:

`estimate_contrasts(model)`

```
> Marginal Contrasts Analysis
>
> Level1 | Level2 | Difference | 95% CI | SE | t(147) | p
> ------------------------------------------------------------------------------
> setosa | versicolor | 0.66 | [ 0.49, 0.82] | 0.07 | 9.69 | < .001
> setosa | virginica | 0.45 | [ 0.29, 0.62] | 0.07 | 6.68 | < .001
> versicolor | virginica | -0.20 | [-0.37, -0.04] | 0.07 | -3.00 | 0.003
>
> Marginal contrasts estimated at Species
> p-value adjustment method: Holm (1979)
```

We can conclude that all pairwise differences are statistically significant.

## Complex model

Again, as contrast analysis is based on marginal means, it can be applied to more complex models:

```
model <- lm(Sepal.Width ~ Species * Petal.Width, data = iris)
contrasts <- estimate_contrasts(model)
contrasts
```

```
> Marginal Contrasts Analysis
>
> Level1 | Level2 | Difference | 95% CI | SE | t(144) | p
> -----------------------------------------------------------------------------
> setosa | versicolor | 1.59 | [ 0.64, 2.54] | 0.39 | 4.04 | < .001
> setosa | virginica | 1.77 | [ 0.77, 2.78] | 0.41 | 4.29 | < .001
> versicolor | virginica | 0.18 | [-0.17, 0.54] | 0.15 | 1.27 | 0.205
>
> Marginal contrasts estimated at Species
> p-value adjustment method: Holm (1979)
```

For instance, if we add `Petal.Width`

in the model, we can
see that the difference between *versicolor* and
*virginica* becomes not significant (and even changes sign).

Note that we can plot simple contrast analysis through
**lighthouse plots** with the help of the `see`

package:

```
library(see)
plot(contrasts, estimate_means(model)) +
theme_modern()
```

These represent the estimated means and their CI range (in black), while the grey areas show the CI range of the difference (as compared to the point estimate).

## Changes in difference

Interestingly, we can also see how these differences are modulated by
another continuous variable. Based on the model above (including the
interaction with `Petal.Width`

), we will compute the
contrasts at 100 equally-spaced points of `Petal.Width`

, that
we will then visualise.

```
contrasts <- estimate_contrasts(model, at = "Petal.Width", length = 100)
# Create a variable with the two levels concatenated
contrasts$Contrast <- paste(contrasts$Level1, "-", contrasts$Level2)
# Visualise the changes in the differences
ggplot(contrasts, aes(x = Petal.Width, y = Difference)) +
geom_ribbon(aes(fill = Contrast, ymin = CI_low, ymax = CI_high), alpha = 0.2) +
geom_line(aes(colour = Contrast), size = 1) +
geom_hline(yintercept = 0, linetype = "dashed") +
theme_modern() +
ylab("Difference")
```

As we can see, the difference between *versicolor* and
*virginica* increases as `Petal.Width`

increases.