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I need your help!  I am trying to figure out how to calculate a personal rate of return while dollar-cost averaging.  Here's the assumptions:

$100 invested in Vanguard's S&P 500 Index Fund on the last day of each month.  The last column has the 
weighted returns for each month's transaction.  At the end of the year, I get a portfolio return of 6.54%.

Does this look right or am I missing something?
             Purchase    Amount      Shares    Yearend   Yearend   Transaction   Portfolio    Weighted
               Price    Invested   Purchased    Price     Value        ROR       Weighting      ROR
1/31/2004      104.54       $100   0.9565716    111.64      $107         6.79%   0.08333333      0.57%
2/29/2004      105.98       $100   0.9435743    111.64      $105         5.34%   0.08333333      0.45%
3/31/2004      104.01       $100    0.961446    111.64      $107         7.34%   0.08333333      0.61%
4/30/2004      102.37       $100   0.9768487    111.64      $109         9.06%   0.08333333      0.75%
5/31/2004      103.76       $100   0.9637625    111.64      $108         7.59%   0.08333333      0.63%
6/30/2004      105.41       $100   0.9486766    111.64      $106         5.91%   0.08333333      0.49%
7/31/2004      101.92       $100   0.9811617    111.64      $110         9.54%   0.08333333      0.79%
8/31/2004      102.31       $100   0.9774216    111.64      $109         9.12%   0.08333333      0.76%
9/30/2004      102.99       $100   0.9709681    111.64      $108         8.40%   0.08333333      0.70%
10/31/2004     104.55       $100   0.9564802    111.64      $107         6.78%   0.08333333      0.57%
11/30/2004     108.78       $100   0.9192866    111.64      $103         2.63%   0.08333333      0.22%
12/31/2004     111.64       $100   0.8957363    111.64      $100         0.00%   0.08333333      0.00%
                          $1,200                          $1,278                                 6.54%
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