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Suggested answer to exercise: Comparing peak flow meters, 2

Question 2: Compute the differences in measured PEF and draw a Normal plot (Q-Q plot).
Does the distribution look Normal?

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Suggested answer

First we compute the difference.
Click __Transform__, __Compute__.
Type a variable name into the Target Variable box.
I used 'difference'.
Now put the variable 'PEF (Wright meter)' into the Numeric Expression box,
type or click '-',
and put the variable 'PEF (mini meter)' into the Numeric Expression box.
You should now have 'wright - mini' in the Numeric Expression box.
Click __OK__.

We now draw the Normal or Q-Q plot.
Click __Analyze__, __Descriptive statistics__, __Q-Q plots__.
Choose variable 'difference'.
Click __OK__.

I have increased the size of the text on the graph and improved it in various ways:

There does not appear to be a systematic deviation from the Normal distribution,
though there does appear to be a rather extreme difference.

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Last updated: 20 February, 2012.

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