Function effect

Calculates the effective annual interest rate, given the nominal annual interest rate and the number of compounding periods per year.

Remarks:

• npery is truncated to an integer.
• If either argument is nonnumeric, an error is thrown.
• If nominalRate <= 0 or if npery < 1, an error is thrown.
• Rate is calculated as follows: (1 + nominalRate / npery)^npery - 1
• This function is related to nominal() through effectiveRate = (1 + (nominalRate / npery)) * npery - 1.