Why is it important for the experimental results of one scientist to be tested by another scientist?

Scientists test hypotheses through experimentation.  The
inquiry usually begins with an idea that the scientist has, based on her observations. 
For example, she could notice that the silver paint on her old automobile has faded,
discolored and peeled, whereas the red paint on similar vehicles has not.  Her theory
might be that silver paint, when exposed to the elements, undergoes a higher rate of
degradation than does red paint.  To test this theory the scientist could paint sheets
of metal with the two colors, expose them to the elements over time, and observe and
record the results.  This process would constitute a scientific
experiment.

Scientific experiments must be controlled and
reproducible.  This means that variables that could affect the outcome other than those
being measured must be controlled.  In this experiment, for example, one would have to
control the variable of metal surface type.  She would have to ensure that both paint
colors were painted on exactly the same shape and type of metal.  Similarly she would
have to control the degree of exposure to the elements of both samples, making sure both
color samples were exposed to the same intensity and duration of
sunlight.

By “reproducible” is meant that another scientist
can duplicate the experimental results.  The experiment must be described in sufficient
detail in the scientific literature for another scientist to reproduce the experiment
and, we hope, confirm the results.

These and other
features, which define proper and valid modern scientific inquiry, are found under the
heading of “The Scientific Method” (see reference).