Question

Suppose a study is being designed to measure the effect, on systolic blood pressure, of lowering sodium in the diet. From a pilot study it is observed that the standard deviation of systolic blood pressure in a community with a high sodium diet is 12 mmHg while that in a group with a low sodium diet is 10.3 mmHg. If a=0.05 and b=0.10, how large a sample from each community should be selected if we want to be able to detect a 2 mmHg difference in blood pressure between the two communities?

Answer 1

We have:

Power, 1−β = 0.90

Type I error rate, α = 0.05

Group 'A' mean, μA - Group 'B' mean, μB = 2

Group 'A' Standard Deviation, σA = 12

Group 'B' Standard Deviation, σB = 10.3

Sampling Ratio, κ=nA/nB = 1

The formula to be used is:

Putting all the values in the formula, we get:

Sample Size, nA = 535

535 people from each community should be selected if we want to be able to detect a 2 mmHg difference in blood pressure between the two communities.