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Would investing equally in all 30 companies which comprise the DJIA net the same performance as the DJIA?
DJIA is a price weighted index (as in the amount of each component company is weighted by its price) and the constituents change occasionally (51 times so far). With these two effects you would not get anything like the same return by equally weighting your holdings and would have to rebalance every so often. Note that your premise was most obviously flawed thinking the number of near bankruptcies there have been in that time. More details of the differing make-ups of the index are available on Wikipedia. When you ask about the "average investment" you would have to be a lot more specific; is it limited just to US shares, to shares, to shares and fixed income securities, should I include all commodities, etc. see also What's the justification for the DJIA being share-price weighted?