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| N for paired t-test = (1.96+0.842)^2 ^ / (difference in paired means / sd of the DIFFERENCE in the means)^2 ^ for 80% power, two-tailed type I error of 5% e.g. if the raw difference is 0.303 and sd equals 0.266 then we require 7.85 / (0.303/0.266)^2 ^ = 6 subjects required. | N for paired t-test = (1.96+0.842)^2 ^ / (difference in paired means / sd of the DIFFERENCE in the means)^2 ^ for 80% power and two-tailed type I error of 5% so __Example__ Using the above formula if the raw paired subject difference is 0.303 and the sd of this difference equals 0.266 then we require 7.85 / (0.303/0.266)^2 ^ = 6 subjects. |
Sample size for a one sample t-test
This spreadsheet computes the total sample size for a one sample t-test with given power.
This can also be done using this on-line calculator. Both these programs use the more liberal normal quantiles as opposed to t quantiles for sample size evaluation.
The web calculator here computes the number of subjects for a paired t-test using the formula on page 39 of
Kraemer HC and Thiemann S (1987) How many subjects? Statistical power analysis in research. Sage.:London. (There should be a copy of this book in the CBU library).
The formula is: N for paired t-test = (1.96+0.842)2 / (difference in paired means / sd of the DIFFERENCE in the means)2 for 80% power and two-tailed type I error of 5% so
Example Using the above formula if the raw paired subject difference is 0.303 and the sd of this difference equals 0.266 then we require 7.85 / (0.303/0.266)2 = 6 subjects.