The three methods most commonly used by Developmental Psychologists to study children are:

• Cross-sectional studies
  - comparisons are made of the behaviour or abilities of two or more different groups of children, with each group covering a different age range. This makes it less expensive to carry out the study, and results are gained faster, as the investigator does not have to wait for the children to get older! An example of a cross sectional study is the research we are carrying out as part of Open2's Child of Our Time website. Explore some of the results.
• Longitudinal studies
  - a group of children is followed up across time, this could be a relatively short period of several months, or in some cases for decades. As a result, the same children are seen at all of the ages being studied.
  
  The Child of Our Time series is one example of a longitudinal study, with a relatively small group of children.
  
  The Millennium Cohort Study is an example of a large scale longitudinal investigation, following a much larger number of children who were also born at the beginning of this century.
• Case Studies
  - one or a very small number of children are seen regularly over a period of time. The time period is variable but usually is six months or more in length. There are some important diary studies which adopt this approach. The advantage of this method is that it can provide a very rich and detailed picture of development as well as providing insights into topics that should be investigated in a larger sample of children.
  
  The Child of Our Time project is a longitudinal study, and the way it is reported also involves case studies of individual children and their families.

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