What is meant by an additive and interactive effect as a threat to internal validity?
Additive and interactive effects refers to the fact that the threats to internal validity can sometimes combine to produce a bias in the study which threatens our ability to conclude that the independent variable is the cause of the differences in the dependent variable. One example of this kind of threat is called the selection-history effect. The selection-history effect occurs when an event occurs in a multi-group design (such as the one shown in Figure 8.2) that differentially affects the different comparison groups. For example, if someone came into one groups room and shouted the president has been shot by did not go into the other groups room, we would expect a differential effect. Another example is the selection-maturation effect. The selection-maturation effect occurs when an event occurs in a multi-group design where the participants in one of the groups experience a different rate of maturation than the participants in a different group. 8.15. How does differential attritio
