A firm's average cost decreases in the long-run because of

In the long run, all factors of production are variable, and a firm can change its scale of operation (plant size, workforce, equipment). A firm's average cost decreases in the long run because of increasing returns to scale, which is the source of economies of scale.

Increasing returns to scale means that when a firm increases all its inputs by a certain proportion, its output increases by a greater proportion. For example, if a firm doubles all its inputs and output more than doubles, the cost per unit of output falls. This happens because of advantages such as:

• Technical economies - larger machines and production lines operate more efficiently.
• Managerial economies - specialist managers can be employed for different functions.
• Purchasing economies - bulk buying of raw materials at discounted prices.
• Financial economies - larger firms can borrow at lower interest rates.

Decreasing marginal returns and diminishing average returns are short-run concepts related to adding more of one variable factor while others are fixed; they cause costs to rise, not fall. Decreasing average fixed cost is a short-run phenomenon (spreading fixed costs over more output) and does not explain the long-run decline in average cost, which requires a change in the scale of all inputs.