Subjects bring with them into the investigation unique characteristics, some learned and some inherent. Examples include sex, height, weight, color, attitude, personality, motor ability, and mental ability. If assigning subjects to comparison groups results in unequal distribution of these subject-related variables, then there is a possible threat to internal validity. Suppose that subjects in two comparison groups are unlike with respect to the independent variable and one of these subject-related variables. If scores on the dependent measure differ between the groups, the discrepancy may be due to the independent variable or to the subject-related variable.
It so happened that there were an equal number of boys and girls in the classes, so for convenience the boys were assigned to the Control Group and the girls to the Experimental Group. One day at school, the boys were told to go to one room and the girls to another room, where they were exposed to their respective conditions. Two days later, the Generalization Probe was conducted. 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. Two days later, the Generalization Probe was conducted. 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 selection 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 gender. The higher Generalization Probe score by the Experimental Group may be due to exposure to the interactive video or to the sex of each child.
In the second item, by randomly allocating subjects to conditions, the only way sex (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. We can be more confident that the better Generalization Probe score by the Experimental Group was not the result of selection.