Longitudinal study

A longitudinal survey is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the life span, and in sociology to study life events throughout lifetimes or generations. The reason for this is that, unlike cross-sectional studies, in which different individuals with same characteristics are compared, longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Because of this benefit, longitudinal studies make observing changes more accurate, and they are applied in various other fields. In medicine, the design is used to uncover predictors of certain diseases. In advertising, the design is used to identify the changes that advertising has produced in the attitudes and behaviors of those within the target audience who have seen the advertising campaign.

Because most longitudinal studies are observational, in the sense that they observe the state of the world without manipulating it, it has been argued that they may have less power to detect causal relationships than experiments. But because of the repeated observation at the individual level, they have more power than cross-sectional observational studies, by virtue of being able to exclude time-invariant unobserved individual differences, and by virtue of observing the temporal order of events. Some of the disadvantages of longitudinal study include the fact that they take a lot of time and are very expensive. Therefore, they are not very convenient.

Longitudinal studies allow social scientists to distinguish short from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population are always poor, or that the whole population experiences poverty for 10% of the time. It is impossible to conclude which of these possibilities is the case using one-off cross-sectional studies.

Types of longitudinal studies include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, and studying them at intervals through time. Panel studies sample a cross-section, and survey it at (usually regular) intervals.

A retrospective study is a longitudinal study that looks back in time. For instance, a researcher may look up the medical records of previous years to look for a trend.

Examples

Study name	Type	Country or region	Year started	Participants	Remarks
Alzheimer's Disease Neuroimaging Initiative	Panel	International	2004	n/a	-
Australian Longitudinal Study on Women's Health (ALSWH)	Cohort	Australia	1996	50,000	Includes four cohorts of women: born between 1921-1926, 1946-1951, 1973-1978 and 1989-1995
Colombian Longitudinal Survey by Universidad de los Andes (ELCA)	Panel	Colombia	2010	15,363	Follows rural and urban households for increasing the comprehension of social and economic changes in Colombia
Avon Longitudinal Study of Parents and Children (ALSPAC)	Cohort	United Kingdom	1991	14,000	-
Born in Bradford	Cohort	United Kingdom	2007	12,500	-
1970 British Cohort Study (BCS70)	Cohort	United Kingdom	1970	17,000	Monitors the development of babies born in the UK in one particular week in April 1970
British Household Panel Survey	Panel	United Kingdom	1991	n/a	Modeled on the US PFID study
Busselton Health Study	Panel	Australia	1966	10,000	-
Caerphilly Heart Disease Study	Cohort	United Kingdom	1979	2,512	Male subjects (Wales)
Canadian Longitudinal Study on Aging (CLSA-ÉLCV)	Cohort	Canada	2012	1,000	Planned as a 20-year study.
Child Development Project	Cohort	United States	1987	585	Follows children recruited the year before they entered kindergarten in three cities: Nashville and Knoxville, TN and Bloomington, Indiana
Children of Immigrants Longitudinal Study (CILS)	Cohort	United States	1992	5,262	Florida
Congenital Heart Surgeons' Society (CHSS)	Cohort	Canada	-	5,000	Various studies, managed by the Data Center Studies on Congenital Heart Diseases
Dunedin Multidisciplinary Health and Development Study	Cohort	New Zealand	1972	1,037	Participants born in Dunedin during 1972-73
Study of migrants and squatters in Rio’s Favelas	Cohort	Brazil	1968	n/a	The work of Janice Perlman, reported in her book, Favela (2014)
Fragile Families and Child Wellbeing Study	Cohort	United States	1998	n/a	Study being conducted in 20 cities
Framingham Heart Study	Cohort	United States	1948	5,209	Massachusetts
Genetic Studies of Genius	Cohort	United States	1921	1,528	The world's oldest and longest-running longitudinal study
Socio-Economic Panel (SOEP)	Panel	Germany	1984	12,000	-
Growing Up in Scotland (GUS)	Cohort	United Kingdom	2003	14,000	Scotland
Health and Retirement Study	Cohort	United States	1988	22,000	-
Household, Income and Labour Dynamics in Australia Survey	Panel	Australia	2001	25,000	-
Human Speechome Project	Cohort	United States	2005	1	Single participant was the son of the researcher, studying language development. Project concluded in 2008.
Study of Australian Children	Cohort	Australia	2004	10,000	-
Luxembourg Income Study (LIS)	Cohort	International	1983	n/a	30 countries
Midlife in the United States	Cohort	United States	1983	6,500	-
Manitoba Follow-Up Study (MFUS)	Cohort	Canada	1948	3,983 men	Canada's largest and longest running investigation of cardiovascular disease and successful aging
Millennium Cohort Study (MCS)	Cohort	United Kingdom	2000	19,000	Study of child development, social stratification and family life
Millennium Cohort Study	Cohort	United States	2000	200,000	Evaluation of long-term health effects of military service, including deployments
Minnesota Twin Family Study	Cohort	United States	1983	17,000 (8,500 twin pairs)	-
National Child Development Study (NCDS)	Cohort	United Kingdom	1958	17,000	-
National Longitudinal Surveys (NLS)	Cohort	United States	1979	NLSY79-12,686, NLSY97-approx. 9000	Includes 4 Cohorts: NLSY79 (Born 1957-64), NLSY97 (Born 1980-84), NLSY79 Children and Young Adults, National Longitudinal Surveys of Young Women and Mature Women (NLSW)
National Longitudinal Survey of Children and Youth (NLSCY)	Cohort	Canada	1994	35,795	Inactive since 2009.
National Health and Nutrition Examination Survey (NHANES)	Cohort	United States	1971	8,837 (since 1999)	Continual since 1999
Pacific Islands Families Study	Cohort	New Zealand	2000	1,398	-
Panel Study of Belgian Households	Panel	Belgium	1992	11,000	-
Panel Study of Income Dynamics	Panel	United States	1968	70,000	Possibly the oldest household longitudinal surveys in the US
Rotterdam Study	Cohort	Netherlands	1990	15,000	Focus is on inhabitants of Ommoord, a suburb of Rotterdam
Seattle 500 Study	Cohort	United States	1974	500	Study of the effects of prenatal health habits on human development
Stirling County Study	Cohort	Canada	1952	639	Long term study epidemiology of psychiatric disorders. Two cohorts were studied (575 from 1952-1970; 639 from 1970-1992).
Study of Health in Pomerania	Cohort	Germany	1997	15,000	Investigates common risk factors, sub-clinical disorders and manifest diseases in a high-risk population
Study of Mathematically Precocious Youth	Cohort	United States	1972	5,000	Follows highly intelligent people identified by age 13.
Irish Longitudinal Study on Ageing (TILDA)	Cohort	Ireland	2009	8,500	Studies health, social and financial circumstances of older Irish population
New Zealand Attitudes and Values Study	-	New Zealand	2009	n/a	-
Seattle Longitudinal Study	Cohort	United States	1956	6,000	-
Understanding Society: The UK Household Longitudinal Study	Panel	United Kingdom	2009	100,000	Incorporates the British Household Panel Survey
Up Series	Cohort	United Kingdom	1964	14	Documentary film project by Michael Apted
Study on Global Ageing and Adult Health (SAGE)	Cohort	International	2002	65,964	Studies the health and well-being of adult populations and the ageing process in six countries: China, Ghana, India, Mexico, Russian Federation and South Africa
Wisconsin Longitudinal Study	Cohort	United States	1957	10,317	Follows graduates from Wisconsin high schools in 1957
ONS Longitudinal Study	Panel	England and Wales	1974 (data from 1971)	1% sample of the population of England and Wales. The LS contains records on over 500,000 people usually resident in England and Wales at each point in time	The sample comprises people born on one of four selected dates of birth and therefore makes up about 1% of the total population. The sample was initiated at the time of the 1971 Census, and the four dates were used to update the sample at the 1981,1991 2001 and 2011 Censuses and in routine event registrations. Fresh LS members enter the study through birth and immigration and existing members leave through death and emigration. Thus, the LS represents a continuous sample of the population of England and Wales, rather than a sample taken at one time point only. It now includes records for over 950,000 study members. In addition to the census records, the individual LS records contain data for events such as deaths, births to sample mothers, emigrations and cancer registrations. Census information is also included for all people living in the same household as the LS member. However, it is important to emphasise that the LS does not follow up household members in the same way from census to census. Support for potential users and more information available at www.ucl.ac.uk/celsius

See also

• Time series
• Repeated measures design

External links

• ESDS Longitudinal data service
• Centre for Longitudinal Studies