Psych 325 1st Edition Lecture 13Chapter Five: Common Threats to ValidityThursday March 12Part II: People Change-TimeoOften study people over timeoHistory: change occurring in a large group or cultureoMaturation: normal development in a person/people of same ageoWhy is this a problem?oWhat is the solution?-History: classic recent example:oYou hypothesize that job loss increases depression levelsoYou measure depression before and after assessment of job loss in NYC residentsoAlso, 9/11 occurred at the same time as your assessmentoCan you be confident that it was really job loss that caused changes in depression scores?-MaturationoExample from p. 112 text: mental imagery training with adolescent ballet dancersoMeasure dance ability before and after 24-week mental imagery training programoThe group improves in dance ability. What is it due to the training? -Maybe not--dance ability is known to improve with natural physical development-Regression toward the meanoExtreme scores are likely to be followed by less extreme scoresoWhy? Measurement error.-All scores reflect true score and measurement error-Measurement error is normally distributed around true score-Extreme scores reflect chance and are unlikely to repeat-True score may stay the sameoWhy is this a problem?oWhat is the solution?oWhat is it?-Regression to the mean (RTM) is a statistical trend that is a reality of life in statistics. It basically happens on the posttest where the measures (for example test scores) on the average move back toward the mean on average. The net effect of RTM is that the lower scores on the pretest tend to be higher on the posttest, and the higher scores on the pretest tend to be lower on the posttest. It is important to note that regression is always to the population mean of a group. But really, there is no change that takes place from the pretest to the posttest due to the treatment or the dependent variable. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.RTM is like a number of everyday terms for example "law of averages", "things will even out" or "we are due for a good day after a string of bad ones". Essentially what allthese slogans are stating is that "extreme occurrences tend to be balanced by less extreme occurrences“oWhen does it occur?-It occurs when the sample is selected on the basis of extreme pretest scores. Examples of this are workers with the lowest drive, students with the worst math skill, or patients with the worse symptoms. RTM is anticipated in these cases where there is non-random sampling or assignment in experiments. oHow does it happen?-When one takes a measurement in a study, the measurement consists of two components; the true score/measure and the
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