Sample Size, $n$

Power, $1-\beta$

Type I error rate, $\alpha$



X-axis

min

max


Calculate Sample Size Needed to Test 1 Mean: 1-Sample, 1-Sided


This calculator is useful for tests concerning whether a mean, $\mu$, is equal to a reference value, $\mu_0$. The Null and Alternative hypotheses is either

$H_0:\mu=\mu_0$
$H_1:\mu\lt\mu_0$

or
$H_0:\mu=\mu_0$
$H_1:\mu\gt\mu_0$

Formulas

This calculator uses the following formulas to compute sample size and power, respectively: $$n=\left(\sigma\frac{z_{1-\alpha}+z_{1-\beta}}{\mu-\mu_0}\right)^2$$
$$1-\beta=\Phi\left(\frac{|\mu-\mu_0|}{\sigma/\sqrt{n}}-z_{1-\alpha}\right)$$
where

R Code

R code to implement these functions:

mu=115
mu0=120
sd=24
alpha=0.05
beta=0.20
(n=(sd*(qnorm(1-alpha)+qnorm(1-beta))/(mu-mu0))^2)
ceiling(n)# 143
z=(mu-mu0)/sd*sqrt(n)
(Power=pnorm(abs(z)-qnorm(1-alpha)))

References

Rosner B. 2010. Fundamentals of Biostatistics. 7th Ed. Brooks/Cole. page 224 and 230.


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