A longitudinal study is an observational study which involves repeated observations over long periods of time, sometimes even decades. Longitudinal studies are often used in Sociology to observe changes in life times or through generations. Longitudinal studies can also be used to study change in the lives of organisations and institutions as well as individual people.

A longitudinal study is a correlation research study that involves observations of the same items over long periods of time. In sociology it’s often used to study developmental trends across the life span. The reason for this is that 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. A large number of variables is often examined because the researchers are unsure what data may prove to be important or required later in the research; although the researcher still has to decide what variables to study, examination of so many limits the extent to which they impose their own theories on the research.

The ‘Up’ series consists of seven documentary films that have followed the lives of fourteen British children since 1964, when they were seven years old. The children were selected to represent the range of socio-economic backgrounds in Britain at that time, with the explicit assumption that each child’s “class” predetermines their future. Every seven years, the director, Michael Apted, films new material from as many of the fourteen he can get to participate. The latest film, 49 up, was released in September 2005; filming for the next instalment in the series, 56 Up, is expected in late 2011 or early 2012.

In 1973, D.J. West and D.P. Farrington reported on a study they had made of 411 London school boys following their development from age 8 to 18 in order to determine what factors were associated with delinquency.

Unlike cross sectional studies, which look at different people, longitudinal studies look at the same person for a long period of time, this could mean that they are less likely to be the result of cultural differences throughout generations, because of this, it means that people can track the changes easily if there are any made.

Because many longitudinal studies are observational, it means that it cannot be strongly manipulated. However it has been argued that there is less power to detect casual relationships.

There are a number of different types of longitudinal studies, including:

1. Individual studies, when individuals are tracked and studied.

Household panel surveys, when individuals are followed and observed within their household and information is collected.

Cohort studies, when people from certain age groups are studied to explore their different trajectories as they age.

Record linkage studies, administrative or census data are linked across time.

Advantages of longitudinal studies are:

1. High in validity – people usually do not remember past events and if they were asked about their past, they would not remember.

2. Picking up long-term changes

Disadvantages are:

1. It takes a long period of time to gather results

2. A need to have a large sample size and accurate sampling to reach representativeness

3. Participant may drop out, this is called subject attrition.

Courtesy of Lee Bryant, Director of Sixth Form, Anglo-European School, Ingatestone, Essex