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J Prev Med Public Health > Volume 43(6); 2010 > Article
Journal of Preventive Medicine and Public Health 2010;43(6): 543-548. doi: https://doi.org/10.3961/jpmph.2010.43.6.543
Power Estimation and Follow-Up Period Evaluation in Korea Radiation Effect and Epidemiology Cohort Study.
In Seong Cho, Minkyo Song, Yunhee Choi, Zhong Min Li, Yoon Ok Ahn
1Department of Preventive Medicine, College of Medicine, Seoul National University, Korea. yoahn@snu.ac.kr
2Medical Research Collaborating Center, Seoul National University Hospital/Seoul National University, Korea.
3Institute of Radiation Effect & Epidemiology, Seoul National University Medical Research Centre, Korea.
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 non-exposure 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.
Key words: Cohort studies; Epidemiologic studies; Power; Sample size; Research design
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