WHAT IS LONGITUDINAL DESIGN?
Most psychology studies are "snapshot studies" - they give you information about people's behaviour or thoughts at one particular moment in time.
However "time" is a very important variable in human experience. We change over time and how we are at one moment in time might not be representative of how we are at a later date.
Longitudinal studies treat "time" as one of their independent variables (IV). They test people at different points in time.
Another way of looking at longitudinal studies is that they look at the effects of AGEING on people.
However, not all longitudinal studies are interesting in ageing. For example, if you studied people being treated for depression, checking their symptoms at the start, half way through and at the end of the treatment, this would also be a longitudinal study. However, it's not really looking at ageing: it's looking at the impact of following a course of treatment for a certain amount of time.
Longitudinal studies often last for weeks or months but some of them can last for years or decades.
Case studies also follow a person over a period of time. This means that case studies are longitudinal designs.
However, not all longitudinal studies are case studies. In fact. most longitudinal studies follow a GROUP of people (called a "cohort"), not just one person
There are two types of longitudinal design:
When psychology tests (and exams) use 'longitudinal study' they normally mean prospective longitudinal study.
The website mentions 14,500 families taking part. What happened to the other 1,247? This is SAMPLE ATTRITION
The biggest problem with prospective longitudinal studies is that they take so much time.
The alternative to a longitudinal study that you can carry out in an afternoon is a cross-sectional study.
This sort of study uses several groups of participants who are at different points in time. For example, it might use a group of 5 year olds and a group of 10 year olds - or a group of people just starting psychotherapy and a group of people already half-way through psychotherapy.
The assumption is that the younger/earlier group WILL TURN INTO the older/later group in time.
You should think of longitudinal studies as being like REPEATED MEASURES DESIGN: you test the same group of people on different occasions. Cross-sectional studies are like INDEPENDENT GROUPS DESIGN: you test different groups of people on the same occasion.
Videos and text books often link longitudinal/cross-sectional design to studying DEVELOPMENT. This is true, but these designs can also study things like how well people with mental disorders respond to therapy or how well offenders respond to rehabilitation programmes.
APPLYING LONGITUDINAL DESIGNS IN PSYCHOLOGY
EVALUATING LONGITUDINAL DESIGNS IN PSYCHOLOGY