Sample size and sampling for quantitative research A. Bowling; Research Methods in Health(2002): pp.116-192 SNU DHPM; Research Methodology Seminar 19 Apr. 2007 CiEHR; Mi-Sook Kwak 0
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
Calculation of sample size, statistical significance and sampling 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 3
The sampling unit e.g.,,, : e.g., :? ( )?,? (Multi-level analysis) e.g., :,,,. e.g.,,, (Ecological fallacy) (extrapolation) e.g., : VS. : 4
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
Testing hypotheses, statistical significance, the null hypothesis (Null hypothesis): H0. (Testing hypotheses) (Statistical Significance) (Probability Theory) ( ) : 0.05, 0.01, 0.001 e.g., < 0.05 5%., e.g., = 0.001, 6
Typeand Typeerrors 1 (TypeErrors) 2 (TypeErrors) ß and ß : ß (Ho) (Reject) (Accept ) (1 ) 1-1- ß ß(2 ) 7
ß /2=0.10(ß=0.20) /2=0.025( =0.05) 1- =0.95 1-ß=0.80 1 1 : = 0.10, 0.05, 0.01, 0.001, 0.10 2 ( ). (1 ) = 0.05, ß (2 ) = 0.20 8
1, 2 :, 1 :. : (Multiple significance testing) 1 1 : (significance tests) / (endpoint) 1, Research Methodology Seminar; 19 Apr. 2007 9
One- or two-sided hypothesis testing or, e.g., :. : ( ). 6% 3% 10
Statistical, social and clinical significance 2 : (side effect),, (smallest clinically important difference): 20%. : 5%. 20: 5%. ( /),. 11
Sampling frames // : (External validity) e.g., : ( ) e.g., :, e.g., :. : 1984, 1984 : NHS, : 98% NHS, :, e.g., / 12
Sampling, e.g., But,. (sampling error) ( ).,. 1 2 13
Confidence intervals and the normal distribution (The normal distribution) e.g.,, IQ e.g., : If : : 0, : 1 14
(Sampling distribution of the means) : (Central limit theorem),, : 30 ( ) = (= ) (<).. (standard error): 15
(Confidence intervals), e.g., 95% 95% - 1.96 x SE and + 1.96 x SE p - 1.96 x {p(1 - p)n} and p + 1.96 x {p(1 - p)n} (Distribution of errors) = but, X (Systematic error) / (Random error) ( ), = 0 16
External validity of the sample results (External validity) (Internal validity) :? :,,? (target population)????? e.g.,, (experimental group), (control group)?? 17
Methods of sampling 18
Contents Methods of sampling Random sampling Non-random sampling: quota sampling Sampling for qualitative research Sampling for telephone interviews 19
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
(Unrestricted random sampling). (Simple random sampling) 21
/ (Systematic ramdom sampling) : ( ) e.g., 1000 10 100 But, : / (Stratified random sampling) e.g.,,,, or (internal homogeneity) (external heterogeneity) (Proportional stratified sampling): Research Methodology Seminar; 19 Apr. (Disproportional 2007 stratified sampling): 22
(Cluster sampling) :,. e.g., 200000-> 200 :, : but,. (Multistage sampling), e.g., 00-> -> (Sampling with probability proportional to size(pps)),. :, : 23
Non-random sampling: quota sampling e.g.,, :. : :,. 24
Sampling for qualitative research (Convenience sampling). e.g.,, (Purposive sampling) / e.g., e.g., : X 25
/ (Snowballing) (Sampling frame). :. : (Theoretical sampling) :., 26
Sampling for telephone interviews :,. (e.g., NHS) (Random digit dialling: RDD), (multi stage) 4 (systematic).. or or :, e.g., 65 80 : 500, 55 : 2 27
Q & A kwakms@snu.ac.kr 28