Internal Validity Tutorial


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h. Selection-Maturation Interaction

Subject-related variables and time-related variables may interact. Suppose that subjects in two comparison groups differ with respect to the independent variable and a subject-related variable such as age. Suppose also that the dependent variable is measured twice for each group, once at Time A and later at Time B, and that the independent variable is introduced in the interim. If the change in scores on the dependent measure from Time A to Time B differs between the two groups, this discrepancy may be due to the independent variable or to distinctive naturally occurring developmental processes for the two age categories that comprise the two comparison groups.

Background Information

Example

It so happened that there were an equal number of boys and girls, so for convenience the boys were assigned to the Control Group and the girls to the Experimental Group. 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 the 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 (1). We conclude that the 20-minute interactive video improved the children's self-protection skills in a potential abduction situation.

Nonexample

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 the 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 (1). We conclude that the 20-minute interactive video improved the children's self-protection skills in a potential abduction situation.

Analysis

The first item is an example in which a selection-maturation interaction is a threat to internal validity. If the improvement across the two Generalization Probes was a function of the passage of the school year only for the Experimental Group, then we would also expect to see similar improvement for the Control Group. Because we do not observe this, we might erroneously conclude that the enhanced score for the Experimental Group was not the result of maturation. However, the children in the two comparison groups are unlike with respect to whether or not they viewed the interactive video two days prior to the second Generalization Probe and with respect to gender. Perhaps relevant assertion skills are naturally learned sometime during first grade for girls and later during second grade for boys. The better second Generalization Probe score by the Experimental Group relative to the Control Group may be due to exposure to the interactive video or to the differential development of boys and girls.

In the second item, by randomly allocating subjects to conditions, the only way gender (and other subject-related variables) can be unevenly distributed between the two comparison groups is through chance. Thus, the children in the two groups appear to be unlike only with respect to whether or not they viewed the interactive video two days prior to the second Generalization Probe. Because of this and the fact that the Generalization Probe scores increased for the Experimental Group and not for the Control Group, we can be more confident that the improvement was not the result of gender, maturation, or an interaction between the two.

h. Selection-Maturation Interaction

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