Define increasing returns to scale (IRS), constant returns to scale (CRS) and diminishing returns to scale (DRS).

Since all factors are variable in the long run the scale of production can easily be varied by altering the quantity of factors used in the production process.

There are three stages that the production goes through which are:
i. Increasing Returns to Scale ii. Constant Returns to Scale and iii. Diminishing Returns to Scale.

Increasing returns to scale refers to the first stage that the production function undergoes. Here since all inputs have increased the output is also rising at an increasing rate. Mathematically this means if inputs are doubled then the output increases more than double.

In constant returns to scale which is the second stage, here with the increase in the input the output also increases by the same measure. For instance, if the input is doubled the output also increases by double.

In diminishing returns to scale which is the third stage, here if the input is tripled the output only increases by double. The output hence is increasing at a proportion less than that of the input.