: Should returns from my S&P Index fund (SWPPX) match the S&P 500 index? I have been purchasing ~ 0 worth of SWPPX twice per month since March 2015. My annualized return appears to be 8.22%. I used the annual returns for

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Because you're buying at different times.

Total annual return looks at the value today of $X invested on Jan 1 2015. But you don't have $X invested on Jan 1, 2015, you have:

$Y on Jan 1, 2015
$Y on Jan 15, 2015
$Y on Feb 1, 2015
etc. where the sum of the Y values is X that you're trying to compare

You have a different average unit cost and that impacts your return. Your first contribution will match that return very closely less the expense fee and possibly dividends. Your second contribution hasn't enjoyed all of the time and price appreciation as your first contribution and will have an appropriately different return; in fact the return on your most recent contribution is certainly not 8.22%, it's probably negative.

This is a chart that I made for a different answer but it applies here because it illustrates that each contribution has it's own rate of return. This is a monthly 0 deposit in to VOO

Orange - Your very first 0 (green got lost in the blue)
Yellow - A deposit that went in and immediately lost value
Red - Your most valuable 0. (this 0 was contributed at the lowest unit cost of the bunch)