title: [Power estimation and follow-up period evaluation in Korea radiation effect and epidemiology cohort study].
abstract: OBJECTIVE
  The objective of this study was to calculate sample size and power in an ongoing cohort, Korea radiation effect and epidemiology cohort (KREEC).
  
  
  METHODS
  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.
authors.affiliation: Department of Preventive Medicine, Seoul National University, Korea.
authors.forename: In Seong
authors.initials: IS
authors.lastname: Cho
authors.forename: Minkyo
authors.initials: M
authors.lastname: Song
authors.forename: Yunhee
authors.initials: Y
authors.lastname: Choi
authors.forename: Zhong Min
authors.initials: ZM
authors.lastname: Li
authors.forename: Yoon Ok
authors.initials: YO
authors.lastname: Ahn
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D009381:Neoplasms, Radiation-Induced / Q000453:epidemiology*; D056910:Republic of Korea / Q000453:epidemiology; D012306:Risk; D055815:Young Adult