JoHo

Researchers might be interested in more than one independent variable from the beginning of the experiment or they might want to add additional variables after they had looked at the results. When researchers ask about the effect of an extra independent variable, they are usually interested in an interaction effect. This interaction effect looks at if the effect of the original independent variable depends on the level of other independent variable. An example of this is hands-free driving and reaction time. Researchers wanted to know if young people show a longer reaction time while driving and talking to someone on the phone (hands-free) than older people. Previous research has already shown that talking on the phone while driving, causes someone to react more slowly to obstacles on the road. In that research, there was just one independent variable (the use of a mobile phone). Then, researchers wanted to know if the effect depended on age. That became the second independent variable. An interaction effect can be explained as a difference of the differences.