The reliability of the instrument used to gauge the dependent variable or manipulate
the independent variable may change in the course of an experiment. Examples include
changes in the calibration of a mechanical measuring device as well as the proficiency of
a human observer or interviewer. 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 the ability of a recording device to
detect instances of the target behavior improves (declines) as the experiment progresses.
If scores on the dependent measure differ at these two times, the discrepancy may be due
to the independent variable or to more (less) sensitive recordings of the target behavior
at Time B relative to at Time A.