1.8: What is the "Experimenter effect"?
Description
This article is from the Scientific Skepticism FAQ, by Paul Johnson Paul@treetop.demon.co.uk with numerous contributions by others.
1.8: What is the "Experimenter effect"?
It is unconscious bias introduced into an experiment by the
experimenter. It can occur in one of two ways:
o Scientists doing experiments often have to look for small effects
or differences between the things being experimented on.
o Experiments require many samples to be treated in exactly the same
way in order to get consistent results.
Note that neither of these sources of bias require deliberate fraud.
A classic example of the first kind of bias was the "N-ray",
discovered early this century. Detecting them required the
investigator to look for very faint flashes of light on a
scintillator. Many scientists reported detecting these rays. They
were fooling themselves. For more details, see "The Mutations of
Science" in "Science since Babylon" by Derek Price (Yale Univ. Press).
A classic example of the second kind of bias were the detailed
investigations into the relationship between race and brain capacity
in the last century. Skull capacity was measured by filling the empty
skull with lead shot or mustard seed, and then measuring the volume of
filling. A significant difference in the results could be obtained by
ensuring that the filling in some skulls was better settled than
others. For more details on this story, read Stephen Jay Gould's "The
Mismeasure of Man".
For more detail see:
T.X. Barber, "Pitfalls of Human Research", 1976.
Robert Rosenthal, "Pygmalion in the Classroom".
[These were recommended by a correspondent. Sorry I have no more
information.]
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