In making comparisons between separate curves there is a great advantage in having a standard arrangement of the five-year cards so that there may be no danger of comparing two curves for different years when it is intended to compare for the same year. It is desirable, as seen in Fig. 215, that all five-year cards should have the years arranged by half-decades. One arrangement includes those years ending in one to five inclusive, and the other arrangement takes those years ending in six to ten inclusive. The person reading the curves can then pick up any two cards relating to the same half-decade and compare the curves instantly and correctly, simply by placing the cards so that the edges coincide. If this half-decade arrangement were not followed there would be five different positions in making curve comparisons, and there would be grave possibility of frequent

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error in that different years might be compared without the reader's noticing that he was making an error. In starting a new curve on a five-year card the curve should be started in the middle of the card if the first year plotted happens to fall in the middle of a half-decade. The half -decade arrangement should be carefully followed even though it does not leave a portion of a card unfilled.

The fine-line curve running through Fig. 215 represents a twelvemonths moving average of the points on the heavy curve. If the executive wishes to know the general trend of his costs, he refers at once to this fine line and sees what the cost has been for the last twelve months for which costs are known. In making up this moving average (as explained in Chapter VI), one month is dropped from the addition and another month is included in the addition, so that the twelve months added are always the most recent months for which figures are available.