For example, at my son's elementary school, one of the school-wide emphases is concerned with showing respect for self and others . Historical effects are also referred to as time of measurement effects and these effect all members of a population living during a particular historical era. Reactivity involves the possible effects on subsequent behavior from being exposed to a particular procedure . Also found in studies where observations of behavior are used to assessment changes in behavior after treatment. Observer boredom, fatigue, lack of training can effect measurement.
Statistical regression can occur whenever participants score very high or very low on the pretest.
During subsequent measurements, they score closer to the mean regardless of the treatment effects. Because students are selected for treatment due to their low scores on social skills, they are likely to score higher on the posttest. This has to do with the loss or attrition of participants from your study over time. So, you have lost five participants from your study. Your sample is now different from the one with which you started.