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.
Background Information
Example
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.
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 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
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.
