Suggested answer to exercise: Comparing peak flow meters, 3

Question 3: Compute the average PEF using the two meters and draw a scatter plot of difference against average. Do the differences and averages of the numbers of attacks appear to be related?

Suggested answer

First we compute the average. Click Transform, Compute. Type a variable name into the Target Variable box. I used 'average'. Now type or click '(' into the Numeric Expression box, put the variable 'PEF (Wright meter)' into the Numeric Expression box, type or click '+', put the variable 'PEF (mini meter)' into the Numeric Expression box, and type or click ')/2'. You should now have '(wright + mini)/2' in the Numeric Expression box. Click OK.

Click Graphs, Legacy Dialogs, Scatter/Dot, Define. Put the variable 'difference' into the Y Axis box and 'average' into the X Axis box. Click OK.

You should get the following graph, after changing the text size, scale, number format, etc.:

The variability and mean of the differences do not appear to change with the average, though there does appear to be a point with a much larger difference than the rest.

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Last updated: 20 February, 2012.

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