Question

The sales of a commodity in tones varied from January 1979 to December 1979 as

follows:

280 300 280 280 270 240

230 230 220 200 210 200

Fit a trend line by the method of semi-average.
​

Answer 1

Since the number of months is even (12), we can equally divide the given data in two equal parts and obtain the averages of the first six months and last six months. REFER IMAGE 1.

Thus we obtain semi-average I = 276.667 and semi average II = 213.333

To fit a trend line we plot each value at the mid-point (month) of each half, (i.e) we plot 276.667 in the middle of March and April; we plot 213.333 in the middle of September and October. We join the two points by a straight line. This is the required line. REFER IMAGE 2.