Real difference
To work out whether there is a The difference between two mean values which is only a real difference if their ranges do not overlap. in data, add A marking on a point of a graph that indicates the lowest and highest repeat measurement for that mean. 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 A measurement that appears very different to other repeat measurements. It should be included in the data unless a reason is found to explain it. have been removed).
In this example, it looks as if the value of B is greater than A:
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.
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.
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.