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I've been checking historical prices for some stocks at Google finance and Yahoo finance but something doesn't quite add up.

Taking stock splits into account, Google finance "says" that Microsoft's highest intraday high was 59.97 USD, which would mean that the company hit a market capitalization of 503 billion USD in 30 December, 1999:

US$59.97 * 8,390.77 shares = US$503,194.477 million

Yahoo finance has a 119.94 on the same day (which matches Google's value of 59.97 after the 2:1 split), but they say the adjusted value is 45.57 for the close (117.62), which would mean that the adjusted value would make it's market cap value even lower.

Some other sources point that Microsoft hit US$604 billion market cap once. Even Wikipedia shows the market cap for Microsoft was US$586 billion in 31 March, 2000 citing Financial Times as it's source.

My question is: What am I missing here?

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up vote 7 down vote accepted

In common with many companies, Microsoft has been engaging in share buyback programmes, where it buys its own shares in the market and then cancels them. It's often a more tax-efficient way to distribute profits to the shareholders than paying a dividend. So there were more Microsoft shares in circulation in 1999 than there are now. See here for information.

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yes there were a few times where MS dropped very low and the company took advantage to institute buy backs. The number may also be Inflation Adjusted. – user4127 Feb 29 '12 at 21:00

Adjustments can be for splits as well as for dividends. From

Historical prices stored on some public websites, such as Yahoo! Finance, also adjust the past prices of the stock downward by the dividend amount.

Thus, that could also be a possible factor in looking at the old prices.

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