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Sample Size Estimation
 

Sample Size Estimation

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How to determine Sample Size in Biostatistics

How to determine Sample Size in Biostatistics

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    Sample Size Estimation Sample Size Estimation Presentation Transcript

    • Sample Size Estimation Nayyar Raza Kazmi
    • How big sample do you need
      • Fundamental Research Question
      • Should be addressed after determining the primary objective and study design
    • 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
      • 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)
      • 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%
      • 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.
      • N = 60 x 40/ 3 x 3
      • N= 2400/9
      • N= 267
      SEP= pq n n= pq SEP 2
      • 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?
      • 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
      • N = 12 x 12/ 2 x 2
      • N= 144/ 4
      • N= 36
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