Cohorts 1) Central concept in demography 2) Definition a. "the aggregation of individuals (within some population definition) who experience the same event within the same time interval" – Ryder (1965, ASR) 3) Examples 4) Observing cohorts a. Repeated cross-sections – e.g., census b. Panel survey – examples c. Both take a long timeAge-period-cohort 1) Three central dimensions of time in demography a. Age structure of demographic events – regularity b. Temporal change in demographic processes c. Cohort change/difference in demographic processes 2) Perfectly collinear a. e.g, 1984 birth cohort is 20 years old in 2004Lexis diagram – visualization of A-P-CReal cohorts and synthetic cohorts 1) Definition of synthetic cohort 2) The Total Fertility Rate – TFR 3) ∑==4915xwxtxttPBTFR where t=year (period), B=number of births, Pw=number of women, and x indexes age (15-49) 4) TFR=2.5 -> what does this mean to you?Another graphical representation - divorce Year2000 2001 2002 2003 2004 2005 2006 2007 2008 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027D 0123456781718192021222324252627u1r2a3t4I5o6n789101112131415161718192021222324252627Hypothetical Cohort Real CohortComparison of period and cohort measures 1) Discrepancies reflect a. Historical change – e.g., medical advance, technological change, legal changes b. Timing shifts – later fertility or less fertility? c. Tempo vs. quantumTFR – example. U.S. in 1955 Age ASFR (ASFR*5)/1000 15-19 90.3 0.452 20-24 242.0 1.210 25-29 190.5 0.953 30-34 116.2 0.581 35-39 58.7 0.294 40-44 16.1 0.081 45-49 1.0 0.005 TFR=3.574Example of cohort and period measures 2.011.972.102.031.983.652.372.002.142.1300.511.522.533.541950 1955 1960 1965 1970Cohort Fertility Period
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