http://www.tsptalk.com/mb/showpost.php?p=171243&postcount=667
I tried to answer the question you posed. Click the above link.
Thanks squalebear,
I read this article. The equations I'm using match with the first one in the article. I then used the last equation to take time weighting of contributions into account and the result was the same +/-0.02%. I'm pretty sure I'm doing the calculations correctly (they are based on Tom's spreadsheet
)
The returns starting Jun 2003 are still showing radical difference between the returns calculated using the monthly fund returns and allocation percentage compared the the returns that use the dollar amount of my account at the beginning of each month, contributions for that month and the dollar amount at the beginning of the following month. I can not understand why there is the difference. Below are the equation used. I will try to post the picture of the spreadsheet shortly.
Monthly returns from Monthly Fund Returns and allocation percentages:
(100*(1+((S1*AP1)+(S2*AP2))) - 100)/100
where S1 and S2 are the fund monthly return (ex. If you where in C&S then, S1=-1.47%, S2=-2.02% for 1/1/2002.
AP1 and AP2 are the allocation percentages for the funds you are invested in (ex. account has 50% in C and 50% in S, so AP1=50% and AP2=50%
(100*(1+((S1*AP1)+(S2*AP2))) - 100)/100 = -1.745%
Monthly returns from Dollar amount at the beginning of the following month, total contributions made the month under consideration, and the dollar amount at the beginning of the month under consideration:
(((DA2-(C1))-DA1)/DA1)*100%
DA1 is the dollar amount of your tsp investment as of the 1st of the month you are calculating (ex. Jan 1st 2002 dollar amount from your account)
DA2 is the dollar amount of your tsp investment as of the 1st of the next month. (ex. Feb 1st 2002 dollar amount from your account)
C1 is the total contributions (15% plus matching) made the month you are calculating (ex. total contribution into your account for the month of Jan 2002)
NOTE: The +/-0.07 difference between the monthly returns from dollar amount and the returns from the Monthly Fund Returns is due to the the formula not using the net cash flow weighted by the days invested.
Investment time weighting equation:
(((DA2-(TC))-DA1)/(DA1+((C1*D1/Mday)+(C2*D2/Mday))))*100%
TC = total contributions for the month of Jan 2002
C1 = first contribution invested on day D1 in the month with Mday number of days in that month (ex $300 invested on the 5th and 19th day in a month with 30 days gives C1 = 300, D1=5, Mday = 30)
C2 = second contribution invested on day D2 in the month with Mday number of days in that month (C2 = 300, D2=19, Mday = 30)
This difference can not account for the radical change starting in June 2003.