This is the 2007 version. Click here for the 2017 chapter 01 table of contents.

Experimenter Effects

Earlier we discussed measurement effects , which were defined as "any effect on the data due to the act of measurement itself." One measurement effect described was the observer effect , a measurement effect produced by the actions or mere presence of an observer. Closely related to this is the concept of an experimenter effect , an effect on subjects due to the actions or presence of an experimenter. Demand characteristics can also be regarded as an experimental effect, when they are caused by the actions or attitudes of the researcher.

What is an "experimenter effect"?

Experimenters may accidentally treat subjects in experimental and control groups differently if they know which group is getting the real treatment and which is getting a fake or placebo treatment. For example, if the experimenter knows one group is receiving real medicine and the other is receiving a sugar pill, the experimenter might see improvements in the experimental group where none really exist.

How did Rosenthal demonstrate expectancy effects in graduate students?

Such expectancy effects are very powerful. One scientist—Robert Rosenthal of Harvard—spent over 30 years investigating them. Rosenthal's findings are quite remarkable. For example, he found that graduate students in psychology told that one group of rats was "brighter" than another typically found that this group was faster to learn its way through mazes.

Evidently the graduate students were a little quicker to hit the buttons on their timers when they expected rats to be "bright." Either that or perhaps the rats were somehow influenced by the expectations of the experimenters. They might have been handled differently prior to the maze run. Perhaps some of the graduate students were cheating and supplying data they thought their professor expected. One way or another, expectancy had a strong effect on results. Rosenthal documented similar effects of expectancy in a wide variety of settings.

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Copyright © 2007-2011 Russ Dewey