Again, there is a question of the length of each rest period.  It has been proven that in many classes of work, and especially in those where the work is interrupted periodically by reason of its peculiar nature, or by reason of inefficient performance in one of the same sequence of dependent operations, alternate working and resting periods are best.  There is to be considered in this connection, however, the recognized disadvantage of reconcentrating the attention after these rest periods.  Another thing to be considered is that the rate of output does not decline from the beginning of the day, but rather the high point of the curve representing rate of production is at a time somewhat later than at the starting point.  The period before the point of maximum efficiency is known as “warming up” among ball players, and is well recognized in all athletic sports.

As for the point of minimum efficiency, or of greatest fatigue, this varies for “morning workers,” and “night workers.”  This exemplifies yet another variable.

The minuteness of the sub-fields that demand observation, is shown by an entry in the Psychological Index:  “1202.  Benedict, F.G.  “Studies in Body—­Temperature.” 1.  Influence of the Inversion of the Daily Routine; the Temperature of Night Workers."[14]

SELECTION OF BEST UNIT OF MEASUREMENT NECESSARY AND IMPORTANT.—­ Selecting the unit of measurement that will of itself reduce costs is a most important element in obtaining maximum efficiency.[15] This is seldom realized.[16] Where possible, several units of measurements should be used to check each other.[17] One alone may be misleading, or put an incentive on the workers to give an undesirable result.

The rule is,—­always select that unit of output that will, of itself, cause a reduction in costs.

For example:—­In measuring the output of a concrete gang, counting cement bags provides an incentive to use more cement than the instruction card calls for.  Counting the batches of concrete dumped out of the mixer, provides an incentive to use rather smaller quantities of broken stone and sand than the proportions call for,—­and, furthermore, does not put the incentive on the men to spill no concrete in transportation, neither does it put an incentive to use more lumps for Cyclopean concrete.

Measuring the quantity actually placed in the forms puts no incentive to watch bulging forms closely.

While measuring outputs by all these different units of measurements would be valuable to check up accuracy of proportions, accuracy of stores account, and output records, the most important unit of measurement for selection would be, “cubic feet of forms filled,” the general dimensions to be taken from the latest revised engineer’s drawings.

NECESSITY FOR CHECKING ERRORS.—­Dr. Stratton says,—­“No measurements, whether they be psychic or physical, are exact beyond a certain point, and the art of using them consists largely in checks and counter checks, and in knowing how far the measurement is reliable and where the doubtful zone begins."[18]