Subjects may change in the course of the experiment or between repeated measures of the dependent variable due to the passage of time per se. Some of these changes are permanent (e.g., biological growth), while others are temporary (e.g., fatigue). Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim. If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to naturally occurring developmental processes.
During a class early in the school year, the children viewed the 20-minute cartoon (Control condition). Two days later, the Generalization Probe was conducted. The experimenter fell ill soon afterwards, and so it wasn't until a class late in the school year that the children viewed the 20-minute interactive video (Experimental condition). Two days after that, a second Generalization Probe was conducted. The mean score for the children on the first Generalization Probe was 1.2 and their mean score on the second Generalization Probe was 3.4. We conclude that the 20-minute interactive video improved the children's self-protection skills in a potential abduction situation.
The name of each child in the classes was written on a separate slip of paper. All the slips were put in a bowl and mixed up thoroughly. Students were assigned to the Experimental Group and to the Control Group alternately as their names were pulled out of the bowl one at a time. During a class early in the school year, a Generalization Probe was conducted for all children. The experimenter fell ill soon afterwards, and so it wasn't until a class late in the school year that children in the comparison groups were separated, with the Control Group children viewing the 20-minute cartoon and the Experimental Group children viewing the 20-minute interactive video. Two days after that, a second Generalization Probe was conducted. To see the results, click here (Figure 1). We conclude that the 20-minute interactive video improved the children's self-protection skills in a potential abduction situation.
The first item is an example in which maturation is a threat to internal validity. Almost a full school year separated the two Generalization Probes. Thus, the children at the time of the second probe differ from themselves at the time of the first probe in two ways: they viewed the interactive video two days earlier and they were ten months older. The improved score across the two Generalization Probes may be due to intervening exposure to the interactive video or to normal psychological development during those ten months in a child's life. For example, first grade children may naturally learn to become more assertive over the school year.
In the second item, if the improvement across the two Generalization Probes was simply a function of the passage of the school year for the Experimental Group, then we would expect to see a similar trend for the Control Group. Because we do not observe this, we can be more confident that the improved score for the Experimental Group was not the result of maturation.