This document discusses sample size estimation and the factors that influence determining an appropriate sample size for research studies. It provides examples of calculating sample sizes based on prevalence of a disease, mean values, standard deviations, permissible errors, and confidence levels. The key points are: - Sample size depends on prevalence/magnitude of the attribute being studied, permissible error, and power of the statistical test - Larger sample sizes are needed to detect smaller differences and have sufficient power - Examples are provided to demonstrate calculating sample sizes based on prevalence of anemia, mean blood pressure values, and acceptable margins of error

1. Sample Size Estimation Nayyar Raza Kazmi

2. How big sample do you need Fundamental Research Question Should be addressed after determining the primary objective and study design

3. How big sample do you need Too few cases May fail to detect a clinically important difference Too Many Involve extra cases Therapy or intervention may have risks Costs More

4. Sample Size depends on Prevalence of the disease under study or mean magnitude of the attribute of interest On degree of permissible error (error of rejecting a true alternative hypothesis) Signifance Power of the test (1-B)

5. Suppose the prevalence of anemia in NWFP is 60%. What should be the minimum sample size if the permissible error at 95% confidence level is 10%

6. As the error is 10%, the prevalence in the sample could vary to the extent of plus or minus 10 x 60/100, or 6 At 95% confidence level, 6 will equal 2 x SEP. So SEP=3 SEP is 3, p is 60 and q is (100-60) or 40.

7. N = 60 x 40/ 3 x 3 N= 2400/9 N= 267 SEP= pq n n= pq SEP 2

8. Mean Systolic Blood Pressure (SBP) of 5000 factory workers is 80mm with SD of 12. What should be the sample size if the permissible error is 5% at 95% confidence level?

9. As 5% of 80 is 4, the mean SBP of sample could be 80 ± 4 Since Confidence level is 95%, 4 will equal 2 x SEM. SEM is therefore, 2 SEM= SD n n = SD 2 SEM 2

10. N = 12 x 12/ 2 x 2 N= 144/ 4 N= 36