Wemedoo Sample Size and Power Calculator
Below we have provided a free sample size and power calculator for all to use. This calculator performs five of the most frequently used calculations and is provided as is without warranty. If you require additional calculations or professional biostatistical services please contact us and we will be more than happy to help you.
Test for Two Means
Result
TEST FOR TWO MEANS
Purpose : Tests for the number of subjects required to detect a significant difference between the means of two samples.
Example : The aim of the study is to examine whether people who go to work by foot or by bicycle have a different body mass index (BMI) compared to those who travel by car or other motorized transportation. Literature shows that active people (travelling by foot or bike) have an average BMI of 24.2 and inactive people have an average BMI of 27.9. The assumption is that standard deviations are 3.8 and 4.1, respectively. The alpha level (probability of obtaining false positive result) is 0.05.
SAMPLE SIZE CALCULATION
How many subjects are necessary to achieve an 80% probability to detect a significant difference between the BMIs of the above groups? The Power is set to 0.80 (80%) and the allocation ratio is 1:1 (equal sample sizes).
Design :
Alpha (α) = 0.05
Power = 0.80
Mean 1 = 24.2
Mean 2 = 27.9
Standard Deviation 1 = 3.8
Standard Deviation 2 = 4.1
Allocation Ratio (N2/N1): 1
Result : A sample size of 18 subjects is required to achieve 80% power to detect significant difference the between the two groups.
POWER CALCULATION
What power achieves 25 subjects per group?
Design :
Alpha (α) = 0.05
Sample Size 1 (N1) = 25
Sample Size 2 (N2) = 25
Mean 1 = 24.2
Mean 2 = 27.9
Standard Deviation 1 = 3.8
Standard Deviation 2 = 4.1
Result : A sample size of 25 subjects per group (a total of 50 subjects) achieves 91% power to detect a significant difference between groups.
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