Real difference

To work out whether there is a real difference in data, add range bars to the graph. A conclusion cannot be drawn unless there is a real difference in the data.

A range bar shows the highest and lowest repeat measurement (after any explained odd results have been ignored).

In this example, it looks as if the value of B is greater than A:

Two dots labelled A and B on a blank graph.

With range bars drawn, it is not possible to draw a conclusion that the data shows a real difference.

If the true value lies at the top of A's range bar and the bottom of B's range bar, the value of B could actually be less than A.

Dot and crosses labelled A and B on a blank graph.

In this example, even if the true value lies at the top of A's range bar and the bottom of B's range bar, the value of B is still greater than A. It can be concluded that the data shows a real difference.

Dots and crosses marked A and B on a blank graph

Data that is precise:

  • lies close to the true value
  • has a small range bar

Precise data makes it easier to show a real difference in data. It is difficult to draw a conclusion about a real difference if the range of results is wide.