[turc1656]

While it's true the Dow doesn't take into account market cap because it is a price weighted index, it is not true that removing a $5 security and adding a $105 one jumps the index $100 in value.

Every index has a divisor, which acts to preserve the return. The general calculation for an index level is Level=MCAP/Divisor. With the Dow, the MCAP is simply the sum of the prices instead of the sum of all MCAPS for the individual securities. When you drop one security and add another, the divisor is calculated such that it offsets the change in MCAP. In this way, when the index opens the next day it opens at the exact same level as the previous close and then the real time feeds kick in and update based on the gap up/down of each security in the Dow. The same basic logic applies to all top level indices. The calc changes for gross/net returns, currency variants, hedge calcs, etc. But the same principal applies that there must be a way to preserve the day over day return so that adding and removing securities doesn't throw off the actual index levels and returns.

Your 401k and other investment accounts that report performance do the same thing behind the scenes - particularly 401k. With a 401k you are consistently adding money to your investment pool and placing small trades to obtain more shares of a pool of securities. This influx of money doesn't artificially inflate your personal return for the year because the way the calculation is done is very similar under the hood to how an index is calculated.