Marginal cost can either be thought of as the cost of producing the last unit of output or the cost of producing the next unit of output. Because of this, it's sometimes helpful to think of marginal cost as the cost associated with going from one quantity of output to another, as shown by q1 and q2 in the equation above. To get a true reading on marginal cost, q2 should be just one unit larger than q1.
For example, if the total cost of producing 3 units of output is $15 and the total cost of producing 4 units of output is $17, the marginal cost of the 4th unit (or the marginal cost associated with going from 3 to 4 units) is just ($17-$15)/(4-3) = $2.
