Which term describes a slope that cannot vary across groups?

Which term describes a slope that cannot vary across groups?

• A. The slope is fixed and does not vary across groups
• B. The slope can vary across groups
• C. The intercept is fixed and does not vary across groups
• D. There is no slope in the model

Answer: (A)

When the influence of a predictor is constant across all groups, that slope is fixed. In a multilevel or mixed-effects model, a fixed slope means the coefficient for the predictor applies uniformly, so each group experiences the same change in the outcome for a one-unit change in the predictor. If the slope could vary by group, you’d have a random slope, allowing different groups to have different effects of the predictor. The intercept being fixed refers to the baseline level being the same across groups, which is a separate aspect. And saying there is no slope would mean the predictor has no effect, not that its effect is the same across groups.

When the influence of a predictor is constant across all groups, that slope is fixed. In a multilevel or mixed-effects model, a fixed slope means the coefficient for the predictor applies uniformly, so each group experiences the same change in the outcome for a one-unit change in the predictor. If the slope could vary by group, you’d have a random slope, allowing different groups to have different effects of the predictor. The intercept being fixed refers to the baseline level being the same across groups, which is a separate aspect. And saying there is no slope would mean the predictor has no effect, not that its effect is the same across groups.