Appendix 16.2 Time Series Analysis Using MegaStat

Appendix 16.2
Time Series Analysis Using MegaStat
1
Appendix 16.2 ■ Time Series Analysis Using MegaStat
The instructions in this section begin by describing the entry of data into an Excel worksheet. Alternatively, the
data may be downloaded from this book’s website. Please refer to Appendix 1.1 for further information about
entering data, saving data, and printing results in Excel. Please refer to Appendix 1.2 for more information about
using MegaStat.
Calculation of seasonal factors and deseasonalization:
•
Enter the Tasty Cola data into column A with
label Sales. Only the sales values need to be
entered—the year, month, and time period
need not be entered.
•
Select Add-Ins : MegaStat : Time Series/
Forecasting : Deseasonalization.
•
In the Deseasonalization dialog box, enter
the range A1 : A37 into the “Input Range of
Seasonal Data” window. This range can be
entered by dragging with the mouse—the
autoexpand feature cannot be used in this
dialog box.
•
Select the type of seasonal data—“quarterly”
or “monthly”—by clicking. Here we have
selected “monthly” because the Tasty Cola
data consists of monthly sales values.
•
In the “First data period” box, specify the
month (in this case, January) in which the
first time series value was observed by using
the up or down arrow buttons.
•
In the “First data period” box, enter the year
in which the first time series value was observed (here equal to 1) into the Year box.
•
Check the Plot Values checkbox to obtain
plots of the seasonal observations, the
deseasonalized data, and a trend line fit to
the deseasonalized data.
•
•
Click OK in the Deseasonalization dialog box.
The seasonal factors are displayed in the
“Seasonal Indexes” column of the “Centered
Moving Average and Deseasonalization”
table in the output worksheet. They are also
given in the “adjusted” row at the bottom of
the “Calculation of Seasonal Indexes” table in
the output worksheet.
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Chapter 16
Time Series Forecasting
Simple exponential smoothing:
•
Enter the cod catch data into column A with
label CodCatch.
•
Select Add-Ins : MegaStat : Time Series/
Forecasting : Exponential Smoothing :
Simple Exponential Smoothing.
•
In the Simple Exponential Smoothing dialog
box, enter the range A1 : A25 into the “Input
Range for Data” window. Enter this range by
dragging with the mouse—the autoexpand
feature cannot be used in this dialog box.
•
Type the value of the smoothing constant
(here equal to .02) into the Alpha window.
•
Leave the Initial Value window blank if you
wish to use an initial value equal to the
average of the first six time series observations.
If another initial value is desired, type it into
the Initial Value window.
•
Click OK in the Simple Exponential Smoothing
dialog box.
•
The forecast for a future value of the time series
is found at the bottom of the “Forecast”
column in the output worksheet.
Double exponential smoothing:
•
Enter the calculator sales data into column A
with label Sales.
•
Select Add-Ins : MegaStat : Time Series/
Forecasting : Exponential Smoothing :
Two-factor Exponential Smoothing.
•
In the Two-Factor Exponential Smoothing
dialog box, enter the range A1 : A25 into
the “Input Range for Data” window. Enter
this range by dragging with the mouse—the
autoexpand feature cannot be used in this
dialog box.
•
Type the desired values of the smoothing
constants (here both are set equal to .20) into
the Alpha and Beta boxes.
•
Leave the “Initial Value” and “Initial Trend”
boxes blank if you wish to use initial values
that are estimated by the computer using the
first six time series observations. If you wish to
supply initial values, type an initial value of
the intercept into the “Initial Value” box and
type an initial value of the slope into the
“Initial Trend” box. Here we have supplied the
values 198.0 and 8.1.
•
Click OK in the Two-Factor Exponential
Smoothing dialog box.
•
The forecast for the next time series value is
found at the bottom of the Forecast column in
the output worksheet.