# Parallel Slopes Model

Take a look at the parallel slopes model in multiple regression.

When creating regression models with one numerical and one categorical explanatory variable, we aren’t limited to interaction models, as we just saw. Another type of model we can use is known as a** **parallel slopes model. Unlike interaction models where the regression lines can have different intercepts and different slopes,** **parallel slopes models still allow for different intercepts but force all lines to have the same slope. The resulting regression lines are therefore parallel. Let’s visualize the best-fitting parallel slopes model to `evals_ch6`

.

Unfortunately, the `geom_smooth()`

function in the `ggplot2`

package doesn’t have a convenient way to plot parallel slopes models. Evgeni Chasnovski, therefore, created a special purpose function called `geom_parallel_slopes()`

that’s included in the `moderndive`

package. We won’t find `geom_parallel_slopes()`

in the `ggplot2`

package, but rather the `moderndive`

package. Thus, if we want to use it, we’ll need to load both the `ggplot2`

and `moderndive`

packages. Using this function, let’s now plot the parallel slopes model for teaching scores. Notice how the code is identical to the code that produced the visualization of the interaction model, but now the `geom_smooth(method = "lm", se = FALSE)`

layer is replaced with `geom_parallel_slopes(se = FALSE)`

.

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