Suppose you had 10,000 on Jan 1, 2000. Would you have been better with a lump sum investment into an no load S&P 500 index or would you have been better off investing at the start of each month with 10000/number of months=117, which equals $85.47
If you invested in lumb sum on Jan 1, 2000, you could have bought 7.171 shares.
If you invested each once a month, you would have 8.672 shares.
Date Adj Close
9/1/2009 998.04 0.085637936
8/3/2009 1020.62 0.083743299
7/1/2009 987.48 0.086553738
6/1/2009 919.32 0.092970984
5/1/2009 919.14 0.092989191
4/1/2009 872.81 0.09792519
3/2/2009 797.87 0.107122821
2/2/2009 735.09 0.116271593
1/2/2009 825.88 0.103489715
12/1/2008 903.25 0.09462506
11/3/2008 896.24 0.095365176
10/1/2008 968.75 0.088227185
9/2/2008 1166.36 0.073279335
8/1/2008 1282.83 0.066626198
7/1/2008 1267.38 0.067438405
6/2/2008 1280 0.066773504
5/1/2008 1400.38 0.061033495
4/1/2008 1385.59 0.061684976
3/3/2008 1322.7 0.064617892
2/1/2008 1330.63 0.064232796
1/2/2008 1378.55 0.061999989
12/3/2007 1468.36 0.058207855
11/1/2007 1481.14 0.057705609
10/1/2007 1549.38 0.055164056
9/4/2007 1526.75 0.055981716
8/1/2007 1473.99 0.057985526
7/2/2007 1455.27 0.058731428
6/1/2007 1503.35 0.056853085
5/1/2007 1530.62 0.055840173
4/2/2007 1482.37 0.057657727
3/1/2007 1420.86 0.06015377
2/1/2007 1406.82 0.060754102
1/3/2007 1438.24 0.059426859
12/1/2006 1418.3 0.060262346
11/1/2006 1400.63 0.061022601
10/2/2006 1377.94 0.062027436
9/1/2006 1335.85 0.063981798
8/1/2006 1303.82 0.065553593
7/3/2006 1276.66 0.066948197
6/1/2006 1270.2 0.067288683
5/1/2006 1270.09 0.067294511
4/3/2006 1310.61 0.065213973
3/1/2006 1294.87 0.066006692
2/1/2006 1280.66 0.066739092
1/3/2006 1280.08 0.066769331
12/1/2005 1248.29 0.068469735
11/1/2005 1249.48 0.068404525
10/3/2005 1207.01 0.070811415
9/1/2005 1228.81 0.069555168
8/1/2005 1220.33 0.070038502
7/1/2005 1234.18 0.069252528
6/1/2005 1191.33 0.071743417
5/2/2005 1191.5 0.071733181
4/1/2005 1156.85 0.073881735
3/1/2005 1180.59 0.072396078
2/1/2005 1203.6 0.071012035
1/3/2005 1181.27 0.072354403
12/1/2004 1211.92 0.070524528
11/1/2004 1173.82 0.072813622
10/1/2004 1130.2 0.075623859
9/1/2004 1114.58 0.076683671
8/2/2004 1104.24 0.077401729
7/1/2004 1101.72 0.077578773
6/1/2004 1140.84 0.074918556
5/3/2004 1120.68 0.076266272
4/1/2004 1107.3 0.077187831
3/1/2004 1126.21 0.075891783
2/2/2004 1144.94 0.074650275
1/2/2004 1131.13 0.075561682
12/1/2003 1111.92 0.076867118
11/3/2003 1058.2 0.080769312
10/1/2003 1050.71 0.081345077
9/2/2003 995.97 0.085815924
8/1/2003 1008.01 0.08479091
7/1/2003 990.31 0.086306394
6/2/2003 974.5 0.087706604
5/1/2003 963.59 0.088699639
4/1/2003 916.92 0.093214332
3/3/2003 848.18 0.100768806
2/3/2003 841.15 0.101610991
1/2/2003 855.7 0.099883236
12/2/2002 879.82 0.097144968
11/1/2002 936.31 0.091283961
10/1/2002 885.76 0.096493503
9/3/2002 815.28 0.104835253
8/1/2002 916.07 0.093300824
7/1/2002 911.62 0.093756264
6/3/2002 989.82 0.08634912
5/1/2002 1067.14 0.080092664
4/1/2002 1076.92 0.079365306
3/1/2002 1147.39 0.074490875
2/1/2002 1106.73 0.077227585
1/2/2002 1130.2 0.075623859
12/3/2001 1148.08 0.074446106
11/1/2001 1139.45 0.075009948
10/1/2001 1059.78 0.080648895
9/4/2001 1040.94 0.082108561
8/1/2001 1133.58 0.075398371
7/2/2001 1211.23 0.070564703
6/1/2001 1224.38 0.069806829
5/1/2001 1255.82 0.068059185
4/2/2001 1249.46 0.06840562
3/1/2001 1160.33 0.073660153
2/1/2001 1239.94 0.068930824
1/2/2001 1366.01 0.062569151
12/1/2000 1320.28 0.064736333
11/1/2000 1314.95 0.064998734
10/2/2000 1429.4 0.059794379
9/1/2000 1436.51 0.059498427
8/1/2000 1517.68 0.056316276
7/3/2000 1430.83 0.059734619
6/1/2000 1454.6 0.05875848
5/1/2000 1420.6 0.060164779
4/3/2000 1452.43 0.058846268
3/1/2000 1498.58 0.057034049
2/1/2000 1366.42 0.062550377
1/3/2000 1394.46 0.061292605
Sum =8.67201014
Data taken from yahoo finance