In a research study, there is usually
an experimental group and a comparison or
control group.
Those who are in the experimental group
will get whatever is being researched in
the study. Those in the control group
will usually get the standard treatment
for the condition.
If there is no standard treatment,
those in the control group might get a
placebo that looks the same but is not
intended to have the same effect as the
experimental treatment, or they might get
nothing at all. A control group is
needed because people will naturally
change a bit from one day to the next
even without being in this study.
Some days people feel good and some 
days they feel a bit worse.
Sometimes just knowing they're in a
study can change the way a person feels.
Even some blood tests can be different
from one time to the next because of
things like diet, stress, hormones, or 
mood and sometimes
things can change without any known
reason.
Since everyone in this study will
experience different natural changes
researchers can measure the changes in
each person,
add them together and find the average
change for each group.
If the average change in all the groups 
is the same, then the changes in any one
person, whether for better or worse, are
probably due to natural variation and
not whatever is being researched.
However, if the average change in the
experimental group is much better or
worse than in the control group, it's
likely that the new treatment is what's
making a difference.
A control group is used to help
researchers note the difference between
these natural changes and the true
effects of whatever is being researched.
