Radio Shack TRS-80 Model 100 Basic Manual page 102

RUN the new program to confrrm that the trend line equation and the ftrst quarter
average ratio to trend are printed as:
FORECAST FOR PERIOD X IS
148.22463768116
+
8.4086956521739
*
X
QUARTER 1 RATIO IS .9230187061999
Ok
III
To predict sales into the future for quarter 1 of year 7, ftrst compute the trend line
forecast for period 25 (you can reduce the above ftgures to 3 or 4 decimal places):
PR I NT 148.225 + 8.4087
*
25 (ENTER)
the result will be 358.4425
Then multiply by the first quarter average ratio to trend to take seasonal fluctuation
into account:
PRINT 0.92
*
358.443
the answer will be 329.76756
Thus, 329.76756 would represent a prediction of quarter I sales in year 7, taking into
account both long term trend and typical ftrst quarter seasonal variation.
This experiment required the program to use every fourth number in the sales array
Y(X). This was easily accomplished with a slight change to the FOR statement in line
140:
140 FOR X
=
1 TO N STEP 4
This change in the FOR statement tells the computer to increment the index variable X
in steps of 4 starting with the value 1 until X reaches the upper limit N. Thus, X will
assume the values 1, 5, 9, 13, 17 and 21, which corresponds to the first quarter period
number in years 1, 2, 3, 4, 5 and 6 respectively.
Line 159 computes the ratio to trend for period X and then adds this to the sum of the
previous periods ratios.
Line 169 tests the value of the FOR statement index variable X to see if it has reached
the upper limit N. If it has not, the loop consisting of lines 140 to 160 is repeated. If
the index variable X has reached its upper limit, execution continues with line 170.
Line 179 prints the average ratio to trend for quarter 1. Note that the numerical
expression R/6 within the PRINT list computes the average ratio by dividing the sum
of the ratios R by the number of ratios 6.
This experiment took into consideration the seasonality of quarter 1. The next
experiment extends this concept to each of the four quarters.
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