Radio Shack TRS-80 Model 100 Basic Manual page 101

Table 7-2 below summarizes the average ratio to trend calculations.
Yt
n Ya
148.2+ 8.41*n Ya / Yt
Year Period Actual Sales Trend Value Ratio to Trend
1
1
160 156.6 1.02
2 5 155
190.3 0.81
3 9 215
223.9 0.96
4 13 225 257.5 0.87
5 17 275 291.2 0.94
6 21 300 324.8 0.92
Sum of the ratios 5.52
Average ratio to Trend
0.92
Table 7-2. How the Average Ratio to Trend is Calculated
To predict sales for the first quarter into the future, you would compute the trend line
forecast and then multiply by the average ratio to trend for the first quarter to adjust
for the season. The program can be easily modified to compute the average ratio to
trend for the first period. Enter the following changes:
140 FOR X = 1 TO N STEP 4
150 R
=
Y(X)
I
(A + 6*X) + R
160 NEXT X
170 PRINT "QUARTER 1 RATIO IS";
R/6
If you LIST the complete Sales Trend program, you should now have:
10 CLS
20 N = 24
25 DIM Y(24)
30 FOR X
= 1
TO N
50 READ Y(X)
60 SX
=
SX
+
X:XX = XX
+
X*X
70 SY = SY + Y(X):XY
=
XY + X*Y(X)
80 NEXT X
80 6
=
(N*XY - SX*SY)
I
(N*XX - SX*SX)
100 A = (SY - 6*SX)
I
N
110 PRINT "FORECAST FOR PERIOD X IS"
120 PRINT A;" + ";6;" * X"
14111 FOR X = 1 TO N STEP
a
150 R
=
Y(X) I (A + 6*X) + R
160 NE}{T X
170 PRINT "QUARTER 1 RATIO IS";
R/6
200 DATA 160, 175, 140, 230
210 DATA 155, 215, 155, 225
220 DATA 215, 265, 220, 325
230 DATA 225, 270, 265, 280
240 DATA 275, 350, 255, 345
250 DATA 300, 330, 315, 380
95