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TROPICAL MEDICINE COMMUNITY HEALTH CARE & RESEARCH 2015

By… Assoc. Prof. Dr. Sompong Srisaenpang, M.D., Ph.D.

http://home.kku.ac.th/sompong

SAMPLE SIZE DETERMINATION

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Why ?

n is large enough to provide a reliable answer to the research question

too small n a waste of time too many n a waste of money & other

resources May be unethical e.g. delayed beneficial

therapy

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When ?

Quantitative research Descriptive study Analytical study Cohort study Case-control study Cross-sectional study

Intervention study Experimental study (RCT) Quasi-experimental study (Non-RCT)

In “Research Methodology”

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p-value

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p-value

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Inferential statistics needs representative sample

Two components of representative sample Probability sampling Adequate sample size

Inferential statistics (95% CI, p-value) Predict results in the study population by

using the discovered results in the relevant samples

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Probability sampling

Single stage Simple random sampling Systematic sampling Stratified random sampling Cluster sampling

Multi-stages Each stage must base on 4 methods above

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Non-probability sampling

Convenience (accidental) sampling Purposive (judgemental) sampling Quota sampling Snow ball (network, chain) sampling

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Adequate sample size

n is usually determined by the primary objective of the study

method of calculating n should be given in the proposal, together with the assumptions made in the calculation

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Steps of sample size determination Determine primary objective of the study Translate it into relevant statistics Proportion (describe, compare) Mean (describe, compare) Correlation

Calculate sample size regarding the defined statistics

Justify for related measurement Before-after measurement in the same

individual Matching

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WINPEPI a free sample-size calculator

Search google for “winpepi”

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WINPEPI a free sample-size calculator

Download the latest version & install

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WINPEPI a free sample-size calculator

Start using by clicking on the desktop shortcut

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4 Modules for sample size calculation

Modules Functions

Describe

Compare2

Pairsetc Etcetera

Estimate proportion, mean

Compare 2 proportions, 2 means (assessment of association between variables)

Related measurement Correlation

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