Instrumentation: The term instrumentation (sometimes called instrumental
bias or instrumental decay) refers to changes in a measuring instrument
that occur over time. For example, a scale used to weigh participants may
gradually wear out during the course of the study. In this case, the
measurements change during the study, not because of the different
treatments but because of the changes in the scale. Behavioral observation
measures (discussed in Chapter 3 and Chapter 13) are much more subject
to instrumentation than other types of measures. For example, from one
testing to the next, the researcher doing the observing may become more
proficient in making the observations, change the standards on which the
observations are based, or become more skilled or fatigued, and as a
result, judge the same behavior differently at different times. Notice that
the changes in the participants’ scores are not caused by the treatment but
instead by a change in the measurement instrument (the researcher). Like
history and maturation, instrumentation is a particular concern in
research situations in which the series of treatments extends over a
relatively long time.