Outside events may influence subjects in the course of the experiment or between repeated measures of the dependent variable. 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. Suppose also that Event A occurs between Time A and Time B. If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to Event A.
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. One day at school, the children in the Control Group were told to go to one room and the children in the Experimental Group to another room, where they were exposed to their respective conditions. Immediately afterwards, while walking back to their regular classroom, all the children in the Control Group saw a man laughing and joking with their school principal. Two days later, the Generalization Probe was conducted, during which many of the Control Group children recognized the stranger as the man who made their principal laugh. The mean score for children in the Control Group was 1.2 and the mean score for children in the Experimental Group 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. One day at school, the children in the Control Group were told to go to one room and the children in the Experimental Group to another room, where they were exposed to their respective conditions. The rooms were adjacent to each other, and, when the special class was over, the two groups left their rooms at exactly the same time. Immediately afterwards, while walking back to their regular room, some of the children saw a man laughing and joking with their school principal. Two days later, the Generalization Probe was conducted, during which some of the children recognized the stranger as the man who made their principal laugh. It appears that the number who did so was equally proportioned between the two groups. The mean score for children in the Control Group was 1.2 and the mean score for children in the Experimental Group was 3.4. 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 history is a threat to internal validity. The children in the two comparison groups are unlike with respect to whether or not they viewed the interactive video and with respect to another event encountered in the course of the experiment, that being whether or not they saw the confederate laughing and joking with their school principal. The higher Generalization Probe score by the Experimental Group may be due to exposure to the interactive video or to the fact that only the Control Group subjects witnessed this other event. Seeing the confederate laugh and joke with their school principal may have made him a less intimidating figure for the Control Group subjects, which, in turn, may have caused them to be less likely to verbally refuse and run away from him on the subsequent Generalization Probe.
In the second item, the experimenter attempted to control for differential exposure to outside influences by treating the two comparison groups as equally as possible other than which video she showed them. While some of children saw the confederate laughing and joking with their school principal, in this case it appears that the number who did so was equally proportioned between the two groups. If this is true, then we can be more confident that the better Generalization Probe score by the Experimental Group was not the result of history.