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Suggested answer to exercise: Normal plot for angina and pronethalol, 6

Question 6: Compute the average of the square roots of
the number of attacks on placebo adnd pronethalol
and draw a scatter plot of difference against average.
Do the difference and average after square root transformation appear to be related?

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Suggested answer

First we compute the average.
Click __Transform__, __Compute__.
Type a variable name into the Target Variable box.
I used 'averagesr'.
Now type or click '(' into the Numeric Expression box.
Choose the __Arithmetic__ Function Group, choose __Sqrt__ and use
the up arrow to put it into the Numeric Expression box.
Replace the '?' symbol in the brackets by variable 'placebo',
type or click '+',
put 'Sqrt' into the Numeric Expression box,
and put the variable 'proneth' where the question mark is.
then type or click ')/2'.
You should now have '(SQRT(placebo) + SQRT(proneth)/2' in the Numeric Expression box.
Click __OK__.

Before drawing the graph, I gave this new variable a label in Variable View.
I used 'Average, square root number of attacks'.

Click __Graphs__, __Legacy Dialogs__, __Scatter/Dot__, __Define__.
Put the variable 'Difference, square root number of attacks' into the Y Axis box
and 'Average, square root number of attacks' into the X Axis box.
Click __OK__.

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

If we put the two scatter plots side by side we get:

The plot for the square root transformed data shows a less strong relationship
between difference and average than does that for the untransformed raw data,
though some relationship is still apparent.

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Last updated: 20 February, 2012.

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