Sample size and sampling for quantitative research A. Bowling; Research Methods in Health(2002): pp

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1 Sample size and sampling for quantitative research A. Bowling; Research Methods in Health(2002): pp SNU DHPM; Research Methodology Seminar 19 Apr CiEHR; Mi-Sook Kwak 0

2 Contents Calculation of sample size, statistical significance and sampling The sampling unit Calculation of sample size and statistical power Testing hypotheses, statistical significance, the null hypothesis Typeand Typeerrors One- or two-sided hypothesis testing Statistical, social and clinical significance Sampling frames Sampling Confidence intervals and the normal distribution External validity of the sample results Methods of sampling Random sampling Non-random sampling: quota sampling Sampling for qualitative research Sampling for telephone interviews 1

3 Calculation of sample size, statistical significance and sampling 2

4 Contents Calculation of sample size, statistical significance and sampling The sampling unit Calculation of sample size and statistical power Testing hypotheses, statistical significance, the null hypothesis Typeand Typeerrors One- or two-sided hypothesis testing Statistical, social and clinical significance Sampling frames Sampling Confidence intervals and the normal distribution External validity of the sample results 3

5 The sampling unit e.g.,,, : e.g., :? ( )?,? (Multi-level analysis) e.g., :,,,. e.g.,,, (Ecological fallacy) (extrapolation) e.g., : VS. : 4

6 Calculation of sample size and statistical power (Power) (H1), (H0) ( ) (H0) (H1) (Statistical power) =0.05(0.1, 0.01, 0.001), >0.8 > : H0, <= H1 (Subgroup) : :,., 5

7 Testing hypotheses, statistical significance, the null hypothesis (Null hypothesis): H0. (Testing hypotheses) (Statistical Significance) (Probability Theory) ( ) : 0.05, 0.01, e.g., < %., e.g., = 0.001, 6

8 Typeand Typeerrors 1 (TypeErrors) 2 (TypeErrors) ß and ß : ß (Ho) (Reject) (Accept ) (1 ) 1-1- ß ß(2 ) 7

9 ß /2=0.10(ß=0.20) /2=0.025( =0.05) 1- = ß= : = 0.10, 0.05, 0.01, 0.001, ( ). (1 ) = 0.05, ß (2 ) =

10 1, 2 :, 1 :. : (Multiple significance testing) 1 1 : (significance tests) / (endpoint) 1, Research Methodology Seminar; 19 Apr

11 One- or two-sided hypothesis testing or, e.g., :. : ( ). 6% 3% 10

12 Statistical, social and clinical significance 2 : (side effect),, (smallest clinically important difference): 20%. : 5%. 20: 5%. ( /),. 11

13 Sampling frames // : (External validity) e.g., : ( ) e.g., :, e.g., :. : 1984, 1984 : NHS, : 98% NHS, :, e.g., / 12

14 Sampling, e.g., But,. (sampling error) ( ).,

15 Confidence intervals and the normal distribution (The normal distribution) e.g.,, IQ e.g., : If : : 0, : 1 14

16 (Sampling distribution of the means) : (Central limit theorem),, : 30 ( ) = (= ) (<).. (standard error): 15

17 (Confidence intervals), e.g., 95% 95% x SE and x SE p x {p(1 - p)n} and p x {p(1 - p)n} (Distribution of errors) = but, X (Systematic error) / (Random error) ( ), = 0 16

18 External validity of the sample results (External validity) (Internal validity) :? :,,? (target population)????? e.g.,, (experimental group), (control group)?? 17

19 Methods of sampling 18

20 Contents Methods of sampling Random sampling Non-random sampling: quota sampling Sampling for qualitative research Sampling for telephone interviews 19

21 Random sampling ( ). (Unrestricted random sampling) (Simple random sampling) / (Systematic ramdom sampling) / (Stratified random sampling) (Cluster sampling) (Multistage sampling) (Sampling with probability proportional to size(pps)) 20

22 (Unrestricted random sampling). (Simple random sampling) 21

23 / (Systematic ramdom sampling) : ( ) e.g., But, : / (Stratified random sampling) e.g.,,,, or (internal homogeneity) (external heterogeneity) (Proportional stratified sampling): Research Methodology Seminar; 19 Apr. (Disproportional 2007 stratified sampling): 22

24 (Cluster sampling) :,. e.g., > 200 :, : but,. (Multistage sampling), e.g., 00-> -> (Sampling with probability proportional to size(pps)),. :, : 23

25 Non-random sampling: quota sampling e.g.,, :. : :,. 24

26 Sampling for qualitative research (Convenience sampling). e.g.,, (Purposive sampling) / e.g., e.g., : X 25

27 / (Snowballing) (Sampling frame). :. : (Theoretical sampling) :., 26

28 Sampling for telephone interviews :,. (e.g., NHS) (Random digit dialling: RDD), (multi stage) 4 (systematic).. or or :, e.g., : 500, 55 : 2 27

29 Q & A kwakms@snu.ac.kr 28

Sample Size, Power and Sampling Methods

Sample Size, Power and Sampling Methods Mary Ann McBurnie, PhD Senior Investigator, Kaiser Permanente Center for Health Research Steering Committee Chair, Community Health Applied Research Network (CHARN)