FO RE CAST ING
Figure 6: IBM SPSS’
fitted models for
three specified
growth curves.
IBM SPSS does
have a curve fitting
feature, so I utilized
it and specified three
possible models to
be examined – the
linear, growth and
logistic curves. Figures 6 and 7 give
the resulting output
and plots for these
choices.
NCSS has, in
addition to the stanFigure 7: IBM SPSS’ plot of the data and growth curves.
dard
forecasting
procedures (Box-Jenkins and exponen- with four parameters; there is a Logistic(3)
tial smoothing models), an extensive list model available as well], and Figure 8
of more than 20 nonlinear curve mod- shows the NCSS’ output.
els under its menu label “Growth and
Minitab is a hybrid of a semi-autoOther Models.” The user chooses a matic and manual forecasting product. If
model, and NCSS finds the appropriate you specify that a Box-Jenkins model be
parameters for the particular data set. used, the software finds the appropriate
I chose, for the “Non-Stop” data, the parameters for the model. However, if you
“Logistic(4)” model [i.e., a logistic curve choose Winters’ method, Minitab requires
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