title: Mortality cohort effects from mid-19th to mid-20th century Britain: did they exist?
abstract: OBJECTIVE
  Identification is a central problem with age-period-cohort analysis. Because age + cohort = period, there is no unique solution to the linear effect using generalized linear modeling, but cohort effects have caused greater controversy than age and period effects. To illustrate the magnitude of cohort effects given the presence of collinearity, we reanalyze data from the seminal study by Kermack et al, with an update.
  
  
  METHODS
  Relative mortality data in England and Wales between year 1845 and 1995 were analyzed using partial least squares regression. There were seven age groups ranging from 5 to 74 years old and 16 periods with 22 cohorts.
  
  
  RESULTS
  Our reanalysis seemed to support the existence of cohort effects in the mortality trends. Period and cohort effects were generally consistent with changes in the social, economic, and environmental factors taking place in the last two centuries. Our analysis also showed a declining trend in period effects up to 1950s.
  
  
  CONCLUSIONS
  Partial least squares and related methods provide intuitive pointers toward the separation of linear age, period, and cohort effects. Because statistical algorithms cannot distinguish between relative and actual mortality rates, cohort effects may be underestimated because of contamination by negative age effects.
authors.affiliation: Department of Public Health, Institute of Epidemiology and Preventive Medicine, College of Public Health, National Taiwan University, Taipei, Taiwan. Electronic address: yukangtu@ntu.edu.tw.
authors.forename: Yu-Kang
authors.initials: YK
authors.lastname: Tu
authors.affiliation: Department of Epidemiology, Columbia University, New York, NY.
authors.forename: Katherine
authors.initials: K
authors.lastname: Keyes
authors.affiliation: MRC Integrative Epidemiology Unit (IEU), University of Bristol, Bristol, UK.
authors.forename: George
authors.initials: G
authors.lastname: Davey Smith
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D015988:Cohort Effect; D015331:Cohort Studies; D004032:Diet / Q000639:trends; D004739:England / Q000453:epidemiology; D049672:History, 19th Century; D049673:History, 20th Century; D006801:Humans; D016018:Least-Squares Analysis; D016014:Linear Models; D008875:Middle Aged; D009026:Mortality / Q000639:trends*; D012044:Regression Analysis; D012959:Socioeconomic Factors / Q000266:history; D014852:Wales / Q000453:epidemiology; D055815:Young Adult
keywords: Age–period–cohort analysis; Cohort effects; Mortality; Partial least squares
references.citation: Soc Hist Med. 1993 Apr;6(1):101-23
references.pmid: 11613020
references.citation: PLoS One. 2011 Apr 27;6(4):e19401
references.pmid: 21556329
references.citation: Int J Epidemiol. 2001 Aug;30(4):684-7
references.pmid: 11511583
references.citation: Annu Rev Public Health. 1991;12:425-57
references.pmid: 2049144
references.citation: Stat Med. 1987 Jun;6(4):469-81
references.pmid: 3629048
references.citation: J Public Health Med. 1998 Dec;20(4):441-8
references.pmid: 9923952
references.citation: Epidemiology. 2012 Jul;23(4):583-93
references.pmid: 22407139
references.citation: Popul Dev Rev. 2010;36(2):365-90
references.pmid: 20734557
references.citation: Int J Epidemiol. 2004 Aug;33(4):691-700; discussion 705-9
references.pmid: 15282230
references.citation: BMC Med Res Methodol. 2011 May 27;11:82
references.pmid: 21619595
references.citation: Stat Med. 1998 Jun 30;17(12):1325-39
references.pmid: 9682323
references.citation: Soc Sci Med. 2010 Apr;70(7):1100-8
references.pmid: 20122771
references.citation: BMJ. 2001 Apr 14;322(7291):885-9
references.pmid: 11302898
references.citation: Epidemiology. 2010 Jul;21(4):440-8
references.pmid: 20234316
references.citation: Stat Med. 1987 Jun;6(4):449-67
references.pmid: 3629047
references.citation: Int J Epidemiol. 2001 Aug;30(4):696-703
references.pmid: 11511585
references.citation: Stat Med. 1982 Jul-Sep;1(3):245-59
references.pmid: 7187097
references.citation: Biometrics. 1997 Mar;53(1):161-77
references.pmid: 9147590
references.citation: J Clin Epidemiol. 1999 Jun;52(6):569-83
references.pmid: 10408997
references.citation: Int J Epidemiol. 2001 Aug;30(4):678-83
references.pmid: 11511582
references.citation: J Chronic Dis. 1985;38(10):811-30
references.pmid: 4044767