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These effects are a very real concern in conducting studies of new
treatments.  Researchers try to limit this kind of effect by 
performing studies that are "blind" in various ways.  Some of these
are:

  o  The subjects of the study do not know whether they receive a 
     placebo or the test treatment, i.e., whether they are in the
     control group or the test group.

  o  Those administering the treatment do not know which subjects 
     receive a placebo or the test treatment.

  o  Those evaluating individual results do not know which subjects
     receive a placebo or the test treatment.

Obviously, at the point at which the data is analyzed, one has to 
differentiate the test group from the control group.  But the analysis
is quasi-public: the researcher describes it and presents the data on
which it is based so that others can verify it.  

It is worth noting that in biological studies where the subjects are
animals, such as mice, there were many cases of skewed results because
those who performed the study did not "blind" themselves.  It is not
considered so important to make mice more ignorant than they already
are, though it is important that in all respects except the one tested,
the control and test groups are treated alike.