Well, Let me make this attempt:
n= t² x p(1-p)
n = required sample size
t = confidence level at 95% (standard value of 1.96)
p = estimated prevalence of malnutrition in the project area
m = margin of error at 5% (standard value of 0.05)
In the Al Haouz project in Morocco, it has been estimated that roughly 30% (0.3) of the children in the project area suffer from chronic malnutrition. This figure has been taken from national statistics on malnutrition in rural areas. Use of the standard values listed above provides the following calculation.
n= 1.96² x .3(1-.3)
n = 3.8416 x .21
n = .8068
n = 322.72 ~ 323
Resource : http://www.ifad.org/gender/tools/hfs/anthropometry/ant_3.htm
Definitely, in the above example, the Confidence Interval (CI) is 95%, and yours 80% which is below the statistical expectation, then look for the Z value, but I am not sure the exact value.
Simply this equation expect half of the population (50%) and by substitution 95% CI:
Again, this is under 95% CI, so look for 80% in Z, and make your substitution.
Try to learn other methods.
( I am sorry, the wed doesn't support word equation, or I may not be aware of how to handel it well).
Assume you are testing \( p = 0.8 \) vs \( p > 0.8 \)
Lets give the level of the test first; with that we can define the critical region and thus calculating the power. Of course as posted by wadi, we can express the power in terms of the level of the test.
hi , I looking for help my self. Does that formula work for every case scenario. How do you know the population proportion , do you assume the proportion that you are interested in. My dead line is fast approaching and I am confused.