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Sample Size Estimation


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How to determine Sample Size in Biostatistics

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Sample Size Estimation

  1. 1. Sample Size Estimation Nayyar Raza Kazmi
  2. 2. How big sample do you need <ul><li>Fundamental Research Question </li></ul><ul><li>Should be addressed after determining the primary objective and study design </li></ul>
  3. 3. How big sample do you need <ul><li>Too few cases </li></ul><ul><ul><li>May fail to detect a clinically important difference </li></ul></ul><ul><li>Too Many </li></ul><ul><ul><li>Involve extra cases </li></ul></ul><ul><ul><li>Therapy or intervention may have risks </li></ul></ul><ul><ul><li>Costs More </li></ul></ul>
  4. 4. <ul><li>Sample Size depends on </li></ul><ul><ul><li>Prevalence of the disease under study or mean magnitude of the attribute of interest </li></ul></ul><ul><ul><li>On degree of permissible error (error of rejecting a true alternative hypothesis) </li></ul></ul><ul><ul><li>Signifance </li></ul></ul><ul><ul><li>Power of the test (1-B) </li></ul></ul>
  5. 5. <ul><li>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% </li></ul>
  6. 6. <ul><li>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 </li></ul><ul><li>At 95% confidence level, 6 will equal 2 x SEP. So SEP=3 </li></ul><ul><li>SEP is 3, p is 60 and q is (100-60) or 40. </li></ul>
  7. 7. <ul><li>N = 60 x 40/ 3 x 3 </li></ul><ul><li>N= 2400/9 </li></ul><ul><li>N= 267 </li></ul>SEP= pq n n= pq SEP 2
  8. 8. <ul><li>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? </li></ul>
  9. 9. <ul><li>As 5% of 80 is 4, the mean SBP of sample could be 80 ± 4 </li></ul><ul><li>Since Confidence level is 95%, 4 will equal 2 x SEM. SEM is therefore, 2 </li></ul>SEM= SD n n = SD 2 SEM 2
  10. 10. <ul><li>N = 12 x 12/ 2 x 2 </li></ul><ul><li>N= 144/ 4 </li></ul><ul><li>N= 36 </li></ul>