The net periodic pension cost is a smoothed version of the economic pension cost.
For determining net periodic pension cost, all non-recurring or unusual components
of economic pension cost (e.g., actuarial gain/loss, prior service cost, excess of
actual plan return over expected return) are deferred and amortized using a complex
corridor method. The rationale for this smoothing mechanism is that the economic
pension cost is very volatile. Including this in income would cause income to be very
volatile and also hide the true operating profitability of the firm.