Average, Marginal and Total Product

In the short-run, at least one input is fixed and technology is unchanged during the period. The fixed input(s) may be used to refer to the "size of a plant." Here K is used to represent capital as the fixed input. The relationship between the variable input (here L is used for "labour") and the output (Q) can be viewed from several perspectives.

The short-run production function will take the form Q = f (L), K and technology are fixed or held constant A change in any of the fixed inputs or technology will alter the short-run production function.

In the short run, the relationship between the physical inputs and output can be describes from several perspectives. The relationship can be described as the total product, the output per unit of input (the average product, AP) or the change in output that is attributable to a change in the variable input (the marginal product, MP).

Total product (TP or Q) is the total output. Q or TP = f(L) given a fixed size of plant and technology.

Average product (APL) is the output per unit of input. AP = TP/L (in this case the output per worker). APL is the average product of labour.

Marginal product(MPL) is the change in output "caused" by a change in the variable input (L), so MPL = ∆Q/∆L.

Over the range of inputs there are four possible relationships between Q and L

1) TP or Q can increase at an increasing rate. MP will increase, (In Figure this range is from O to L_A.)

2) TP may pass through an inflection point, in which case MP will be a maximum. (In Figure, this is point A at L_A amount of input) TP may increase at a constant rate over some range of output. In this case, MP will remain constant in this range.

3) TP might increase at a decreasing rate. This will cause MP to fall. This is referred to as "diminishing MP." In Figure, this is shown in the range from L_A to L_B.

4) If "too many" units of the variable input are added to the fixed input, TP can decrease, in which case MP will be negative. Any addition of L beyond L_B will reduce output; the MP of the input will be negative. It would be foolish to continue adding an input (even if it were "free") when the MP is negative.

The average product (AP) is related to both the TP and MP. Construct a ray (OR in Figure) from the origin to a tangent point (H) on the TP. This will locate the level of input where the AP is a maximum, LH. Note that at LH level of input, APL is a maximum and is equal to the MPL. When the MP is greater than the AP, MP "pulls" AP up. When MP is less than AP, it "pulls" AP down. MP will always intersect the AP at the maximum of the AP.