Product control means that controlling the quality of the product by critical examination through sampling inspection plans. It aims at a certain quality level to be guaranteed to the customers. It attempts to ensure that the product sold does not contain a large number of defective items.

Chance Causes are small variations which are natural and inherent in the manufacturing process. The variation occurring due to these causes is beyond the human control and cannot be prevented or eliminated under any circumstances. The minor causes which do not affect the quality of the products to an extent are called as Chance Causes or Random Causes. For example Rain, floods, power cuts, etc

Statistical Quality control is a powerful technique used to diagnose the lack of quality in any of the raw materials, processes, machines etc. It is essential that end products should possess the qualities that the consumer expects from the manufacturer.

In this method, the weights are calculated by multiplying prices and quantity of the base year.
Cost of living index number = \(\frac {\sum pV}{\sum V}\) where
P = \(\frac {p_{1}}{p_{0}} \times 100 \) is the price relative and
V = \(\sum p_{0}q_{0}\) is the value relative

The ratio between the total value of current period and total value of the base period is known as true value ratio.
(\( \frac{\sum P_{1} q_{1}}{\sum p_{0} q_{0}}\) is a true value tario)