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Sample Size Considerations in Research Studies

Learn the importance of sample sizes, factors affecting sample size determination, types of measurements, variability, power, and more. Understand how to choose the right equation and avoid underestimating sample sizes in your research studies.

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Sample Size Considerations in Research Studies

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  1. Sample Sizes Considerations Joy C MacDermid

  2. General Principles • Why do we want many subjects? • Generalizability • Power • Subgroup Analysis • Why do we want a minimal number? • Cost • Time

  3. What factors affect sample size? • Type of measure • Variability of measure • Power we want to detect differences • Risk we are willing to take of falsely declaring a difference where none exists • Size of difference that is important

  4. Types of Measurements • Scale • Nominal, Ordinal, Cardinal • Range of possible responses • The less detail of information in a measurement - the more numbers required to establish trends

  5. Variability • Variability is “background noise” which obscures our ability to see where true differences exist. • The more variable a measurement/trait in a particular sample - the more numbers required to differentiate that the differences observed is true

  6. Power • Ability to detect a true difference Frequently set at 80% The more powerful you want to be - the more numbers you will need

  7. Alpha Error • Willingness to falsely declare a difference as real • Usually set at 5% i.e. 95% confidence intervals, alpha=0.05 • Considered worse to put into place a new treatment that is ineffective (and has side effects) than to miss a potentially useful one.

  8. Clinically Important Difference • How much will make an important difference? • The least amount that you would consider important • The smaller you make this - the more subjects you need

  9. Choosing an Equation • Depends on type of data/study • Differences in size of two measurements i.e. means • Difference in numbers of subjects- i.e. proportions

  10. Difference in MeansSample size required per group • N=(Z alpha + Z beta )2 standard deviation2 Important difference2 Z alpha usually 1.96; z beta usually 0.80

  11. Difference in ProportionsSample size required per group • N=(Z alpha + Z beta )2 [Pe(1-Pe) + Pc(1-Pc)]/ (Pe-Pc)2 Z alpha usually 1.96; z beta usually 0.80

  12. Special cases • Repeated measures • Unequal groups • Affect of covariates

  13. Reasons why sample sizes are underestimated • Forget to take into account loss to follow-up • Effectiveness of treatment of often over-estimated • Selection criteria make exclude patients who get most benefit • Controls improve - due to attention

  14. Suggestions • Use outcomes that have more detail • Cardinal if possible • Proportions - count serious and less serious occurrences • Record time to events • Use surrogate outcomes i.e. disease present versus death

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