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Power is determined by?
•The power of the study depends on
•Sample size
•Alpha level set in the study (it is
optimal to set at 5% [0.05] in medical
research)
•Effect size](https://image.slidesharecdn.com/samplesizeestimationinmedicalresearch-191105141435/75/Sample-size-calculation-in-medical-research-7-2048.jpg)
Uploaded byKannan Iyanar
Sample size calculation in medical research
The document provides an overview of sample size estimation in medical research, highlighting the importance of using representative samples due to economic, ethical, and time constraints. It discusses how the optimal sample size affects the power of a study, which should ideally be above 80%, and outlines strategies for calculating sample size using data from previous studies or pilot studies. Additionally, it includes information on formulas for estimating sample size for both binary and continuous outcomes while cautioning against overshooting population limits.
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Sample size calculation in medical research
- 1. statisticsforu.com Online statistical service Samplesize estimation in Medical Research Dr. I. Kannan Ph.D Associate Professor of Microbiology Tagore Medical College and Hospital Chennai – 600127 dr.ikannan@tagoremch.com
- 2. statisticsforu.com Online statistical service Whatis sample size? •It is the total representative samples from the given population used for the research study.
- 3. statisticsforu.com Online statistical service Whyonly representative samples? •Difficult to subject the entire population for the research because of ➢Economical reason ➢Ethical reason ➢Time constraints
- 4. statisticsforu.com Online statistical service Howthe sample size should be? •Should be optimal, neither high nor less. •High ➢Study will become costly and time consuming ➢Unethical to include participants than the required numbers. •Less ➢Validity of the study is lost. ➢Low power of the study and the research outcome is not trustable.
- 5. statisticsforu.com Online statistical service Whatis the power of study? •The power is the probability to reject the null hypothesis (Ho), given that the null hypothesis is false. •Minimum expected probability is 80%. •It is applicable to those studies wherein there is null and alternative hypothesis. •The prevalence studies do not have any hypotheses and thus power of the study is not applicable.
- 6. statisticsforu.com Online statistical service Whatis power? Reality Decision based on research H0 is true H0 is false Reject H0 Type I error () Correct Decision (1 – β) Power Accept H0 Correct Decision (1 – ) Type II error (β) 1 minus Type II error (β) is the power of the study
- 7. statisticsforu.com Online statistical service Poweris determined by? •The power of the study depends on •Sample size •Alpha level set in the study (it is optimal to set at 5% [0.05] in medical research) •Effect size
- 8. statisticsforu.com Online statistical service Samplesize and power •Increase in sample size increases the power of the study. •80% and above of power is considered to be enough in medical research. •During sample size calculation, it is mandatory to calculate the expected power of the study to justify your sample size.
- 9. statisticsforu.com Online statistical service Strategyto determine the sample size •Different formulae based on the study design •Data obtained from previous similar studies are used in the formula to determine the sample size. •If there is no similar studies, the researcher has to perform a pilot study to obtain the values that can be used for sample size calculation.
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- 11. statisticsforu.com Online statistical service Samplesize estimation WITHOUT finite population size* For the level of confidence of 95%, which is conventional, Z value is 1.96. *Daniel WW (1999). Biostatistics: A Foundation for Analysis in the Health Sciences. 7thedition. New York: John Wiley & Sons.
- 12. statisticsforu.com Online statistical service Samplesize estimation WITHOUT finite population size (Caution) •This sample size formula is valid if the calculated sample size is smaller than or equal to 5% of the population size •Sample size calculation without considering total population size should be avoided because: ➢It may overshoot the 5% of total population size. ➢Sometimes it may even overshoot the total population size.
- 13. statisticsforu.com Online statistical service Samplesize estimation WITH finite population size* *Daniel WW (1999). Biostatistics: A Foundation for Analysis in the Health Sciences. 7thedition. New York: John Wiley & Sons.
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- 15. statisticsforu.com Online statistical service Formulafor a binary outcome Formula for a binary outcome and equal sample sizes in both groups, assuming: alpha = 0.05 and power =0.80 (beta = 0.20).
- 16. statisticsforu.com Online statistical service Formulafor a binary outcome • When the significance level alpha is chosen at 0.05 one should enter the value 1.96 for a in the formula. • When beta is chosen at 0.20, the value 0.842 should be filled in for b in the formula.
- 17. statisticsforu.com Online statistical service Formulafor a binary outcome • X in the formula is the minimal difference between the groups that the investigator considers biologically plausible and clinically relevant.
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- 19. statisticsforu.com Online statistical service Formulafor a continuous outcome Formula for a continuous outcome and equal sample sizes in both groups, assuming: alpha = 0.05 and power = 0.80 (beta = 0.20)
- 20. statisticsforu.com Online statistical service Finalcomments • There are many books that gives the sample size estimation formulae. • Many software are available to determine the sample size which has the advantage of calculating the expected power as well.