Statisticians
use the Greek letter alpha (
)
to indicate the probability of rejecting the statistical hypothesis tested when
in fact, that hypothesis is true. Before conducting any statistical test, it is
important to establish a value for alpha. For most psychologists, and for many
other scientists, it is customary to set alpha at 0.05.
This is the equivalent
of asserting that you will reject the hypothesis tested if the obtained statistic
is among those that would occur only 5 out of 100 times that random samples are
drawn from a population in which the hypothesis is true. If your obtained statistic
leads you to reject the hypothesis tested, it's not because you believe that the
obtained statistic could not have occurred by chance.
It's that you are asserting
that the odds of obtaining that statistic by chance only are sufficiently low
(one out of twenty) that it reasonable to conclude that your results are not due
to chance. Could you be in error? Of course you could, but at least you know the
probability of such an error. It is exactly equal to the value you have previously
established for alpha.
