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Normal Quantile Plots
How can you tell whether or not a distribution is normal?
•histogram or stemplot can tell us whether or not distribution is obviously nonnormal
•if distribution appears roughly symmetric and unimodal, need a more sensitive way to
tell
Normal Quantile Plot
(Figure 1.34)
1. Arrange the observed data values from smallest to largest.
Record what percentile of the data
each value occupies.
For example, the smallest observation in a set of 10 is at the 10% point, the
second smallest is at the 20% point, and so on.
60
65
70
75
76
80
86
90
95
99
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
2. Do normal distribution calculations to find the zscores at these same percentiles.
For
example, z= 1.282 is the 10% point of the standard normal distribution.
zscore
1.282
0.842
0.524
0.253
0
0.253
0.524
0.842
1.282
percentile
10%
20%
30%
40%
50%
60%
70%
80%
90%
3. The variable of interest we call x.
Plot each data point x (on y axis) against the corresponding
z (on x axis).
• If the data distribution is close to
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This note was uploaded on 02/01/2010 for the course PAM 2100 taught by Professor Abdus,s. during the Spring '08 term at Cornell University (Engineering School).
 Spring '08
 ABDUS,S.

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