1
Assessing Normality of a Sample
•
normal quantile plots can be used to assess whether the data could have
come from a normal distribution
•
these plots are also called QQ or normal scores plots
•
the sorted values are plotted against the values we would expect to get if
the sample came from a normal distribution
•
a straight line in this plot indicates that the data are normally distributed
•
outliers show up as values distant from the overall pattern
•
curvature indicates departure from normality e.g. skewness
•
the NSCORES command in MINITAB can be used to produce the values
to be plotted against the data
For example
MTB > nscores c1 c2
•
Manually you can create the normal probability scores using the following
commands
First rank the data from C1 to C3.
Then create the quantile of each observation in column C4
and ﬁnally ﬁnd the point of the normal distribution which gives you the
corresponding quantile.
MTB > rank c1 c3
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 Spring '12
 daniel
 Normal Distribution, probability plot, NSCORES

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