How to find sample size for Observational Study based on mean and Standard Deviation of variable: If variable is measured on Ratio Scale.
II] Based on mean and Standard Deviation of Variable: If variable is measured on Ratio Scale.
The sample size is calculated using following Formula
Source: Lwanga SK, Lameshaw S. Sample size determination in health studies. WHO, Geneva, 1991.
| Symbol | Description |
|---|---|
| n | Sample size |
| Z² | Value for the selected alpha level (1.96 at 95% confidence level) |
| d | Desired level of precision (should not be more than Standard Deviation) |
| S | Standard Deviation of a variable |
For Example: If you want to estimate hemoglobin level of under five children's. So pilot study or published previous study shows that mean hemoglobin level of under five children was 10.0 gm% with Standard deviation of 4.00gm%.
A. Mean hemoglobin level of under five children was 10.0gm% with Standard deviation of 4.00gm%
S= 4.00gm%.
B. Confidence: 95%, so z =1.96
C. Absolute Precision: d= 0.5 (Should not be more than Standard Deviation)
So required sample size for study is 246 samples.
Note: Sample size can be decrease by increasing precision of a study. Precision can be consider up to Standard deviation