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http://dx.doi.org/10.3961/jpmph.2010.43.6.543

Power Estimation and Follow-Up Period Evaluation in Korea Radiation Effect and Epidemiology Cohort Study  

Cho, In-Seong (Department of Preventive Medicine, College of Medicine, Seoul National University)
Song, Min-Kyo (Department of Preventive Medicine, College of Medicine, Seoul National University)
Choi, Yun-Hee (Medical Research Collaborating Center, Seoul National University Hospital/Seoul National University)
Li, Zhong-Min (Institute of Radiation Effect & Epidemiology, Seoul National University Medical Research Centre)
Ahn, Yoon-Ok (Department of Preventive Medicine, College of Medicine, Seoul National University)
Publication Information
Journal of Preventive Medicine and Public Health / v.43, no.6, 2010 , pp. 543-548 More about this Journal
Abstract
Objectives: The objective of this study was to calculate sample size and power in an ongoing cohort, Korea radiation effect and epidemiology cohort (KREEC). Method: Sample size calculation was performed using PASS 2002 based on Cox regression and Poisson regression models. Person-year was calculated by using data from '1993-1997 Total cancer incidence by sex and age, Seoul' and Korean statistical informative service. Results: With the assumption of relative risk=1.3, exposure:non-exposure=1:2 and power=0.8, sample size calculation was 405 events based on a Cox regression model. When the relative risk was assumed to be 1.5 then number of events was 170. Based on a Poisson regression model, relative risk=1.3, exposure:non-exposure=1:2 and power=0.8 rendered 385 events. Relative risk of 1.5 resulted in a total of 157 events. We calculated person-years (PY) with event numbers and cancer incidence rate in the nonexposure group. Based on a Cox regression model, with relative risk=1.3, exposure:non-exposure=1:2 and power=0.8, 136 245PY was needed to secure the power. In a Poisson regression model, with relative risk=1.3, exposure:non-exposure=1:2 and power=0.8, person-year needed was 129517PY. A total of 1939 cases were identified in KREEC until December 2007. Conclusions: A retrospective power calculation in an ongoing study might be biased by the data. Prospective power calculation should be carried out based on various assumptions prior to the study.
Keywords
Cohort studies; Epidemiologic studies; Power; Sample size; Research design;
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