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© Dr Azmi Mohd Tamil, 2012




                   Calculate Your Own
                   Sample Size – Part 9
                                Diagnostic Study

                          Jones SR, Carley S & Harrison M.
                       An introduction to power and sample size
                        estimation. Emergency Medical Journal
                                2003;20;453-458. 2003
                                                                  1
© Dr Azmi Mohd Tamil, 2012




                             Diagnostic Study
              Based upon literature review, identify
              the sensitivity & specificity of the
              diagnostic test being studied.
              Calculate the sample size based on the
              sensitivity using the following formula;




                                                         2
© Dr Azmi Mohd Tamil, 2012




                             Terms
              TP = True Positive
              FN = False Negative
              SN = Sensitivity
              z = Confidence Interval normal
              distribution value i.e. for 95%, z = 1.96
              P = prevalence of disease in the test
              population
              W = accuracy = 0.05
                                                          3
© Dr Azmi Mohd Tamil, 2012




                     Example (SN=95%)
              SN = 95%
              z = 1.96
              P = 30%
              W = 0.05




                                        4
© Dr Azmi Mohd Tamil, 2012




                     Example (SN=95%)




                                        5
© Dr Azmi Mohd Tamil, 2012




                             Diagnostic Study
              Calculate again the sample size based
              on the specificity using the following
              formula;




                                                       6
© Dr Azmi Mohd Tamil, 2012




                      Example (SP=80%)
              SP = 80%
              z = 1.96
              P = 30%
              W = 0.05




                                         7
© Dr Azmi Mohd Tamil, 2012




                      Example (SP=80%)




                                         8
© Dr Azmi Mohd Tamil, 2012




                             Diagnostic Study
              N for Sensitivity 95% = 243
              N for Specificity 80% = 351
              If interested in both sensitivity and
              specificity, then take the higher number
              (e.g. 351).




                                                         9
© Dr Azmi Mohd Tamil, 2012




                  What If There Is
                No Prior Information?
              Instead of saying "Sample sizes are not
              provided because there is no prior
              information on which to base them“, do
              this instead;
                Find  previously published information
                Conduct small pre-study
                If a very preliminary pilot study, sample
                 size calculations not usually necessary


                                                             10
© Dr Azmi Mohd Tamil, 2012




                             Conclusion
              You can calculate your own sample size.
              Tools are available and most of them are free.
              Decide what is your study design and choose
              the appropriate method to calculate the
              sample size.
              If despite following ALL these notes
              fastidiously, your proposal is still rejected by
              the committee due to sample size, kindly SEE
              THEM, not us.



                                                                 11
© Dr Azmi Mohd Tamil, 2012




                             References

             Jones SR, Carley S & Harrison M.
             An introduction to power and sample
             size estimation. Emergency Medical
             Journal 2003;20;453-458. 2003




                                                   12
© Dr Azmi Mohd Tamil, 2012




                             THANK
                              YOU


                                     13

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9. Calculate samplesize for diagnostic study

  • 1. © Dr Azmi Mohd Tamil, 2012 Calculate Your Own Sample Size – Part 9 Diagnostic Study Jones SR, Carley S & Harrison M. An introduction to power and sample size estimation. Emergency Medical Journal 2003;20;453-458. 2003 1
  • 2. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study Based upon literature review, identify the sensitivity & specificity of the diagnostic test being studied. Calculate the sample size based on the sensitivity using the following formula; 2
  • 3. © Dr Azmi Mohd Tamil, 2012 Terms TP = True Positive FN = False Negative SN = Sensitivity z = Confidence Interval normal distribution value i.e. for 95%, z = 1.96 P = prevalence of disease in the test population W = accuracy = 0.05 3
  • 4. © Dr Azmi Mohd Tamil, 2012 Example (SN=95%) SN = 95% z = 1.96 P = 30% W = 0.05 4
  • 5. © Dr Azmi Mohd Tamil, 2012 Example (SN=95%) 5
  • 6. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study Calculate again the sample size based on the specificity using the following formula; 6
  • 7. © Dr Azmi Mohd Tamil, 2012 Example (SP=80%) SP = 80% z = 1.96 P = 30% W = 0.05 7
  • 8. © Dr Azmi Mohd Tamil, 2012 Example (SP=80%) 8
  • 9. © Dr Azmi Mohd Tamil, 2012 Diagnostic Study N for Sensitivity 95% = 243 N for Specificity 80% = 351 If interested in both sensitivity and specificity, then take the higher number (e.g. 351). 9
  • 10. © Dr Azmi Mohd Tamil, 2012 What If There Is No Prior Information? Instead of saying "Sample sizes are not provided because there is no prior information on which to base them“, do this instead;  Find previously published information  Conduct small pre-study  If a very preliminary pilot study, sample size calculations not usually necessary 10
  • 11. © Dr Azmi Mohd Tamil, 2012 Conclusion You can calculate your own sample size. Tools are available and most of them are free. Decide what is your study design and choose the appropriate method to calculate the sample size. If despite following ALL these notes fastidiously, your proposal is still rejected by the committee due to sample size, kindly SEE THEM, not us. 11
  • 12. © Dr Azmi Mohd Tamil, 2012 References Jones SR, Carley S & Harrison M. An introduction to power and sample size estimation. Emergency Medical Journal 2003;20;453-458. 2003 12
  • 13. © Dr Azmi Mohd Tamil, 2012 THANK YOU 13