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How Science Works

What is a Fair Test?

In a fair test, a change in the independent variable alone
causes a change in the dependent variable and
all other variables remain unchanged (they are kept constant).

It is easier to control the variables and carry out a fair test
 inside a laboratory than outside. Experiments conducted outside
are called "in the field" or a "field investigation".
If experiments must be done in the field, then a scientist
will try to make sure that changes in other variables
are the same for all of the experiments.
For example, crops would have the same type of soil and
be exposed to the same amount of rainfall as far as possible.

If the experiment is conducted on people
(for example, to determine the effectiveness of a drug)
it is preferable that the people are all of the same sex
with a similar age, weight and diet (food).
This has the disadvantage that the conclusions
of the experiment might only be valid for the
particular group of people who took the drug
(young thin vegetarian women for example).

In biology (or medicine) a control group is used to show
that it is the treatment (drug) which is causing the effect.
The control group have exactly the same conditions
but are not given the drug. The people in the experiment
do not know whether they are in the control group or not
as everyone is given the same looking pill but the
pill taken by the control group has no drug in it
and is called a placebo (pronounced pler-seeb-o).

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