It is accordingly good practice to have twelve points included here in making up the moving average, so as to give as smooth a curve as possible. If there had been a complete wave or cycle every six months instead of once a year, it would have been desirable to use six points in the moving average, rather than twelve points. The practice in many offices is to use the last twelve months in making up a moving average, though it frequently occurs that a smoother curve would be obtained if some other number of months were used.

As will be seen by reference to Fig. 91, points for a moving-average curve are usually plotted in the middle of the horizontal space covered by those points included in the moving average. In executive-control curves such as are seen here, it seems desirable to make an exception to the general rule and plot the last point on the moving-average curve so that it falls on the same vertical co-ordinate line as the last point included in the average. If the moving-average curve were made as

284 GRAPHIC METHODS

in Fig. 91 it would be following so far behind the periodic record curve that the ordinary reader would not realize that the moving-average curve is really up-to-date. For executive work, the object of a movingaverage curve is not so much to get a smooth curve as to show the average for the preceding year or other period of time considered. Under these circumstances it seems permissible to plot the moving-average curve as done in Fig. 215 instead of following the accepted method shown in Fig. 91.