Normal Probabilities PlotSection 7.6

2

Normal Plots

A normal probability plot is a scatter plot of the (normal score*, observation) pairs.

A substantially linear pattern in a normal probability plot suggests that population normality is plausible.

A systematic departure from a straight-line pattern (such as curvature in the plot) casts doubt on the legitimacy of assuming a normal population distribution.

*There are different techniques for determining the normal scores. Typically, we use a statistical package such as a graphing calculator or Excel to create a normal probability plot.

3

Normal Probability Plot Example

Ten randomly selected shut-ins were each asked to list how many hours of television they watched per week. The results are

82 66 90 84 7588 80 94 110 91

Minitab obtained the normal probability plot on the following slide.

4

Normal Probability Plot Example

P-Value: 0.753A-Squared: 0.226

Anderson-Darling Normality Test

N: 10StDev: 11.8415Average: 86

110100908070

.999

.99

.95

.80

.50

.20

.05

.01

.001

Pro

bab

ility

Hours of TV

Normal Probability Plot

Notice that the points all fall nearly on a line so it is reasonable to assume that the population of hours of tv watched by shut-ins is normally distributed.

5

Normal Probability Plot Example

A sample of times of 15 telephone solicitation calls (in seconds) was obtained and is given below. 5 10 7 12 35

65 145 14 3 22011 6 85 6 16

Minitab obtained the normal probability plot on the next slide

6

Normal Probability Plot Example

P-Value: 0.000A-Squared: 2.087

Anderson-Darling Normality Test

N: 15StDev: 63.1129Average: 42.6667

2001000

.999

.99

.95

.80

.50

.20

.05

.01

.001

Pro

bab

ility

Length of Ca

Normal Probability Plot

Clearly the points do not fall nearly on a line. Specifically the pattern has a distinct nonlinear appearance. It is not reasonable to assume that the population of hours of tv watched by shut-ins is normally distributed.

One would most assuredly say that the distribution of lengths of calls is not normal.