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How Science Works

Links between Variables - Causal - Association - Chance.

Scientists look at how changing
one variable affects another variable.

If a change in the independent variable alone
causes a change in a dependent variable
then the link is said to be causal.
Causal means that a change in one variable
causes a change in another variable.
For example changing the velocity of a car
changes its braking distance.
Scientists try to identify causal links as these give
the clearest information and can lead to a fair test.

If a change in the independent variable and a change
in another variable are both linked by a change in a
third variable then the link is said to be due to association.
Association means that the changes are associated with
each other. For example, the rate of growth of a crop can
be affected by soil acidity. The amount of fertiliser used on
the crop will affect both the rate of growth and the soil acidity.
The information from links due to association is less clear
and therefore less useful than the information from causal links.

If a change in the independent variable and a change
in another variable are not related at all
then the link is said to be due to chance occurrence.
Chance occurrence means that the two variables just
happen to change in that way but one variable has
no effect on the other. For example, the number of emails
being sent and the number of cars being driven on the
road have both increased in the last twenty years. The
information from links due to chance occurrence is not valid.

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