118 APPLYING EFFICIENCY PRINCIPLES
Amount and date and number of order of all material
ordered;
Amount and date of reception, and order number,
of all material received;
Amount of all material reserved, and dates on and
orders for which it is reserved;
Amount of all material delivered and date on and
order to which it is delivered; and
Amount of material available in stores on any
given date.
The usefulness of the perpetual inventory is very
much increased if it also states maximum and minimum
limits of the stock.
In order to be adapted to changes without any resulting
confusion, the inventory must be in loose leaf or
in card index form. It is next necessary to design a
form to contain the desired information, which shall
also be suitable to the filing arrangements available.
Fundamentally, the minimum limit of stock is the
product of the average rate of use of the material and
of the average time required to obtain a new supply,
both determined by previous experience. This product
may be called the theoretical minimum. The result
must be tested and corrected by general knowledge.
When the data are insufficient, or for other reasons the
result is in doubt, it is well to add to the result a safety
reserve, just as the engineer adds a constant to rational
formulas for the dimensions of parts subject to
corrosion. The final result may be called the actual
minimum, or low limit.
If the average rate of use of the material and the
time required to obtain a new supply were constant
and definitely known, there would be no need of a
safety reserve. If then a supply equal to the minimum