# Sample Size Calculation – Formula for One Sample t-test

rsample-size

This question might be really easy to answer, but I am not able to figure it out. For a project, I calculate sample sizes for a one sample t-test using the pwr package in r using the pwr.t.test function. As input for the function (as an example) I use the following: pwr.t.test(d = 1/1.91, sig.level = 0.05, power = 0.8, type = "one.sample", alternative = "two.sided"). A sample size of 30.75 (so 31) rolls out. However, if I try to calculate this sample size by hand, I do not get the same answer. Everywhere I look I can find formulas for calculating the sample size in the case of a parallel group, and these formulae do not give the same solution, but no formulas for single group situations. Does anyone have an idea what goes wrong and what specific formula I could use to get to this same answer? Thanks in advance!

From the fragmentary and undocumented R code you show, I suppose you want to do a two-sided, one-sample t test at level $$\alpha = 0.05$$ based on a sample from a normal population with standard deviation $$\sigma=1.91$$ and hope for power $$0.80$$ to detect a difference in population means of $$1.$$

Several methods are in common use, and they may give slightly different answers.

• Find sample size necessary to get power 80% using a comparable z-test. When the required $$n$$ is 30 or larger, the result will be approximately correct.

• Use an exact formula for the power of such a t test, based on a non-central t distribution. Many intermediate level applied statistics texts and mathematical statistics texts show the formula, and software such as R will do the necessary computation for the noncentral t distribution.

• Many statistical computer programs have 'power and sample' size procedures; most use the noncentral t distribution.

• Simulation of many t tests for normal data of a trial sample size $$n$$ from a population with appropriate $$\mu$$ and $$\sigma$$ to find the proportion that reject (approximate power).

You have already seen computer output from R. Below is output from a recent release of Minitab statistical software. It gives $$n = 31$$ as the desired sample size--in agreement with your result from R.

Power and Sample Size

1-Sample t Test

Testing mean = null (versus ≠ null)
Calculating power for mean = null + difference
α = 0.05  Assumed standard deviation = 1.91

Sample  Target
Difference    Size   Power  Actual Power
1      31     0.8      0.805289


Finally, here is a simulation in R, showing that (in appropriate circumstances) $$n = 31$$ gives power about 80%. [I use a 'for' loop because it seems to be more widely understood than more elegant structures in R. With $$m = 10\,000$$ iterations one can expect about two decimal places of accuracy.]

set.seed(314)
n = 31;  mu.0 = 100;  mu.a = 101;  sg = 1.91
m = 10000;  t.stat = numeric(m)
for(i in 1:m) {
x = rnorm(n, mu.a, sg)
t.stat[i] = ( mean(x) - mu.0 )/( sd(x)/sqrt(n) )
}
c = qt(.975, n-1);  c    # critical value
[1] 2.042272
mean(abs(t.stat) >= c)   # aprx power
[1] 0.8037


Note: If discrepancies among the various formulas and computational methods you used are small, that may be due to rounding errors or approximations. If discrepancies are large, you need to verify you have correct formulas and are using correct syntax in programs.