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Masters in Educational Practice: Child and adolescent development 0-19


Typical and atypical development


Atypical development: What is meant by delay and disorder?

The graph below is an example of a normal bell curve/distribution curve. The majority of children in the classroom sit under the typical range, i.e. within the red section.  

Figure 1:

Sometimes children exhibit behaviours that fall outside of the typical range in terms of the bell curve. This can be seen as behaviours emerging in a way or at a pace that is noticeably different from their peers. Children are usually considered atypical if they score one standard deviation or more away from the majority of their peers.  

Often by using assessments, e.g. administered by an educational psychologist visiting a school or a private assessment, a child’s ability can also be expressed in terms of percentiles by using data on other children to see where they would sit if ranked within a group of peers of the same age.  

It is important to be clear that one distinguishes between skills that are slow to emerge (delay) and those that are categorically different in quality, form and function (disorder) from their peers.


  • Identify a child in your care who can be described as ‘typically developing’.
  • Identify a child in your care who can be described as ‘atypically developing’.
  • What does ‘normal’ mean in terms of normal distribution?
  • Ensure that you are confident of understanding the term ‘percentiles’ and ‘standard deviations’ in terms of scores on assessments and where the child sits in terms of the normal distribution curve.

What is meant by delay and disorder?


A chronological delay in the appearance of normal developmental milestones achieved during infancy and early childhood. This may be caused by biological, psychological or environmental factors.

Global delay

This usually means delay in development in more than one domain. This may be associated with intellectual impairment or a learning disability.


A child who has a disordered developmental profile has gaps in their attainment of developmental milestones. Progress occurs in a non-sequential pattern. Sometimes this is referred to as a ‘deviant’ pattern of development.

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