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Beta is also defined as 1 - Power.
The Power of a test is a function of the difference between the groups.
If the difference is Zero then the Power = Alpha. if the difference is
very large the Power = 1.0 That is why we must specify Beta (1-Power) to
be 5% when the difference in prevalence is 0.07 say. That is both Beta
and delta must be given. If the study is likely to find no difference then
the Power play a major role in defining the maximum size that the true
difference in the population may have.
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