Suggested answer to exercise: Two sample t test for the energy expenditure data, 2

Question 2: For each group, draw a Normal plot (Q-Q plot) for energy expenditure. Does the distribution look Normal?

Suggested answer

There are two ways to do this.

We could select lean women only, do the plot, then select obese women only, and do the plot.
We could split the file by group and get the two plots at the same time.

For the first method, click Data, Select Cases, choose If condition is satisfied. Click If. Choose the variable Group of women and put into the right hand box, type "=1". Click Continue. Click OK.

Now we draw the graph. Click Analyze, Descriptive statistics, Q-Q plots. Choose variable '24 hour energy expenditure'. Click OK.

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

We can draw the graph for Group 2 in the same way:

Both graphs show some positive skewness by being curved and becoming progressively less steep.

The alternative method uses Split File. First we must select all cases. Click Data, Select Cases, choose All cases, click OK. Now we split the file. click Data, Split File, choose Organize output by group. Choose the variable Group of women and put into the Groups Based on box. Click OK.

Now create the graph as above. You will get both plots, with a label saying to which group they apply.

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LLast updated: 20 February, 2012.