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Maths

Scatter diagrams

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Rogue values

A rogue value is a value that doesn’t quite fit with other findings from the same set of data.

These are various ways that we may get a rogue value. The data may have been:

  • deliberately given incorrectly
  • recorded incorrectly
  • plotted incorrectly

Look at this graph showing the height and weight of a group of children.

A scatter graph

Most of the points are close together but two points, at (144, 15) and (84, 62) are much further apart.

(144, 15) is likely to be a rogue value because it suggests that a person 144 cm tall weighs 15 kg. Very unlikely!

(84, 62) means a person is 84 cm tall and weighs 62 kg, which is possible and so this may be an isolated value.

We need to decide if the value is rogue or just isolated.

  • For rogue values we should be able to suggest a reason.
  • An isolated value is possible and may 'fit' if we had more data.

We use the axes to interpret a point, then decide if we have a rogue value.

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