There are many business activities and events which affect the quantity and value of stock.
When company buys the material , it increase the quantity and value of stock.
When company returns the goods to supplier , it reduces quantity and value of stock.
When company sells the goods to buyers then it reduces quantities and value of stock.
When our customers returns us the goods at this time our quantity and value of stock will increase .
Today is most important effect is the effect of of inflation and deflation . It affect only on the value of stock . But there is no change the value of stock.
There are different method of calculating of stock can affect the value of stock . Calculating the value of stock with FIFO will differ the calculated stock with LIFO method .