Essentially, the effective annual return accounts for intrayear compounding, and the stated annual return does not.
The difference between these two measures is best illustrated with an example. Suppose the stated annual interest rate on a savings account is 10%, and say you put $1,000 into this savings account. After one year, your money would grow to $1,100. But, if the account has a quarterly compounding feature, your effective rate of return will be higher than 10%. After the first quarter, or first three months, your savings would grow to $1,025. Then, in the second quarter, the effect of compounding would become apparent: you would receive another $25 in interest on the original $1,000, but you would also receive an additional $0.63 from the $25 that was paid after the first quarter. In other words, the interest earned in each quarter will increase the interest earned in subsequent quarters. By the end of the year, the power of quarterly compounding would give you a total of $1,103.80. So, although the stated annual interest rate is 10%, because of quarterly compounding, the effective rate of return is 10.38%.
That difference of 0.38% may appear insignificant, but it can be huge when you're dealing with large numbers. 0.38% of $100,000 is $380! Another thing to consider is that compounding does not necessarily occur quarterly, or only four times a year, as it does in the example above. There are accounts that compound monthly, and even some that compound daily. And, as our example showed, the frequency with which interest is paid will have an effect on effective rate of return.
(To read more, see Projected Returns: Honing The Craft.)
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