COHORT

Cohorts are people sharing a similar experience or event at a particular time. For example all children born in a particular place and time, or all students graduating from high school in a particular place and time are cohorts. Cohorts are frequently used in longitudinal research. Marvin Wolfgang, for example, established a research project to follow all male children born in Philadelphia in 1940 in order to determine their encounters with the police. Cohort is a group of subjects who share a defining characteristic, and experienced a common event in a selected time period, such as birth or graduation.

A panel study is not always a cohort study as individuals in a panel study do not always share a common characteristic. Cohort data can oftentimes be more advantageous to demographers than period data. Cohort data is not affected by tempo effects, unlike period data. Cohort data can be disadvantageous in the sense that it can take a long amount of time to collect the data necessary.

Identification of Parameters in Cohort Models - WOLFGANG JAGODZINSKI, Sociological Methods & Research, Vol. 12, No. 4, 375-398 (1984) - Linear-additive age-period cohort models that permit the maximum number of aging effects, period effects, and cohort effects, full cohort models, are underidentified. In this article, some general rules will be given for detecting identified and underidentified parameters in any full cohort model under zero-sum game restrictions. It will be demonstrated that only one parameter in a dimension is identified whenever the number of parameters in this dimension is uneven. Otherwise, no parameter is identified. Full cohort models for larger tables contain more overidentifying restrictions, but the number of identified parameters never exceeds four.

Cleavage and Ideological Voting in Age Cohorts - van der Brug, Wouter. Paper presented at the annual meeting of the MPSA Annual National Conference.
Abstract: This study compares the effects of religion, social class and left-right on party choice between EU countries, over 15 years and between generations. Generational replacement leads to a decreasing effect of long-term determinants of the vote.

Advances in Age–Period–Cohort Analysis - Herbert L. Smith, University of Pennsylvania, Philadelphia, Sociological Methods & Research, Vol. 36, No. 3, 287-296 (2008).
Social indicators and demographic rates are often arrayed over time by age. The patterns of rates by age at one point in time may not reflect the effects associated with aging, which are more properly studied in cohorts. Cohort succession, aging, and period-specific historical events provide accounts of social and demographic change. Because cohort membership can be defined by age at a particular period, the statistical partitioning of age from period and cohort effects focuses attention on identifying restrictions. Four new articles on age–period–cohort modeling call attention to the multilevel nature of the problem and draw on advances in methods including nonparametric smoothing, fixed and random effects, and identification in structural or causal models.

Secondary sex ratios and male lifespan: Damaged cohort or culled cohorts
Ralph Catalano and Tim Bruckner, School of Public Health, University of California.
Abstract: Controversy remains as to whether abortions result from reduced maternal tolerance of males at the low end of a relatively constant distribution of survivability, or the “culled cohort” explanation, or from shifts in the whole distribution of survivability such that more males fall below a relatively constant criterion of maternal tolerance for low survivability, or the “damaged cohort” explanation. These alternatives make opposing predictions regarding the relationship between the secondary sex ratio and lifespan of male birth cohorts. We test the hypothesis that the secondary sex ratio among Swedish cohorts born in the years 1751 through 1912 predicts male cohort life expectancy at birth (i.e., realized lifespan). Our results support the culled cohort argument.

Three-year incidence of AIDS in five cohorts of HTLV-III-infected risk group members
JJ Goedert, RJ Biggar, SH Weiss, ME Eyster, M Melbye, S Wilson, HM Ginzburg, RJ Grossman, RA DiGioia, WC Sanchez, et al.
In cohort studies of homosexual men, parenteral drug users, and hemophiliacs. By actuarial survival calculations, the 3-year incidence of AIDS among all HTLV-III seropositive subjects was 34.2 percent in the cohort of homosexual men in Manhattan, New York, and 14.9 percent (range 8.0 to 17.2 percent) in the four other cohorts. Out of 117 subjects followed for a mean of 31 months after documented seroconversion, five (all hemophiliacs) developed AIDS 28 to 62 months after the estimated date of seroconversion, supporting the hypothesis that there is a long latency between acquisition of viral infection and the development of clinical AIDS. This long latency could account for the significantly higher AIDS incidence in the New York cohort compared with other cohorts if the virus entered the New York homosexual population before it entered the populations from which the other cohorts were drawn.

GenomEUtwin Scientific Meeting. European Network of Twin Registries and MORGAM Cohorts. - Istituto Superiore di Sanita. Rome, 13-14 December 2004. Abstract book.
Edited by Maria Antonietta Stazi and Valeria Patriarca 2004, v, 76 p. ISTISAN Congressi 04/C7 Twin cohorts provide a unique competitive advantage for investigations of the role of genetics and environment or life style in the aetiology of common diseases. This workshop provides an up-to-date on research within the GenomEUtwin Project (Genome-wide analysis of European twin and population cohorts to identify genes predisposing to common diseases) supported by the European Commission under the 5th Framework Programme. The project has developed and applied to twin and MORGAM cohorts new molecular technologies and statistical strategies to define and characterise the genetic, environmental and life-style risk factors of obesity, migraine, coronary heart disease and stroke, which represent major health care problems worldwide.

Hospital Readmission Rates for Cohorts of Medicare Beneficiaries in Boston and New Haven - Elliott S. Fisher, John E. Wennberg, Therese A. Stukel, and Sandra M. Sharp.
Methods We used Medicare claims data to study cohorts of Medicare beneficiaries 65 years of age or older and residing in Boston or New Haven who were initially hospitalized for one of acute myocardial infarction, stroke, gastrointestinal bleeding, hip fracture, or potentially curative surgery for breast, colon, or lung cancer. Residents of Boston or New Haven who were discharged between October 1, 1987, and September 30, 1989, were enrolled in the cohort corresponding to the earliest such admission and followed for up to 35 months.
Results The relative rate of readmission in Boston as compared with New Haven was 1.64 (95 percent confidence interval, 1.53 to 1.76) for all cohorts combined, with a similarly elevated rate for each of the five clinical cohorts and each age, sex, and race subgroup examined.