Cramming Sam's top tips from chapter 11

Click on the topic to read Sam's tips from the book


  • Moderation occurs when the relationship between two variables changes as a function of a third variable. For example, the relationship between watching horror films and feeling scared at bedtime might increase as a function of how vivid an imagination a person has.
  • Moderation is tested using a linear model in which the outcome (fear at bedtime) is predicted from a predictor (how many horror films are watched), the moderator (imagination) and the interaction of the predictor variables.
  • Predictors should be centred before the analysis.
  • The interaction of two variables is their scores multiplied together.
  • If the interaction is significant then the moderation effect is also significant.
  • If moderation is found, follow up the analysis with simple slopes analysis, which looks at the relationship between the predictor and outcome at low, mean and high levels of the moderator.


  • Mediation is when the strength of the relationship between a predictor variable and outcome variable is reduced by including another variable as a predictor. Essentially, mediation equates to the relationship between two variables being ‘explained’ by a third. For example, the ­relationship between watching horror films and feeling scared at bedtime might be explained by scary ­images appearing in your head.
  • Mediation is tested by assessing the size of the indirect effect and its confidence interval. If the confidence interval contains zero then we tend to assume that a genuine mediation effect doesn’t exist. If the confidence interval doesn’t contain zero, then we tend to conclude that mediation has occurred.