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Was working on a different question to ask on here, and realized I might be completely wrong when I realized the historical data on Yahoo and Google are completely different. Therefore I have no idea which is true.
And to confirm I understood Google right I went into their actual Historical price lookup:
Does anyone know which is correct? Are they using two different calculations?
The difference is that Yahoo is showing the unadjusted price that the security traded for on that date, while google is adjusting for price splits. This means that Google is showing how much you would have had to pay to get what is now one share. Since 1979, JNJ has split 3-for-1 once, and 2-for-1 four times. 3x2x2x2x2 = 48. If you bought 1 share at that time, you would now have 48 shares today. Yahoo is showing a price of $66 for what was then 1 share. $66/48 = 1.375, which Google rounds to 1.38. You can see this if you get the prices from May 14-21, 1981. The stock split 3-for-1, and the price dropped from 108 to 36.38.
Yahoo's adjusted close column has not been accurate since they re-wrote the Finance website. It now just represents the closing price.
The other relevant field on Yahoo is the Adj. Close. This adjusts for splits, but also adjusts for dividends. Hence why this doesn't match either the Google or Yahoo numbers.
I work on a buy-side firm, so I know how these small data issues can drive us crazy. Hope my answer below can help you:
Reason for price difference:
1. Vendor and data source
Basically, data providers such as Google and Yahoo redistribute EOD data by aggregating data from their vendors. Although the raw data is taken from the same exchanges, different vendors tend to collect them through different trading platforms. For example, Yahoo, is getting stock data from Hemscott (which was acquired by Morningstar), which is not the most accurate source of EOD stocks. Google gets data from Deutsche Börse.
To make the process more complicated, each vendor can choose to get EOD data from another EOD data provider or the exchange itself, or they can produce their own open, high, low, close and volume from the actual trade tick-data, and these data may come from any exchanges.
2. Price Adjustment
For equities data, the re-distributor usually adjusts the raw data by applying certain customized procedures. This includes adjustment for corporate actions, such as dividends and splits. For futures data, rolling is required, and back-ward and for-warding rolling can be chosen. Different adjustment methods can lead to different price display.
3. Extended trading hours
Along with the growth of electronic trading, many market tends to trade during extended hours, such as pre-open and post-close trading periods. Futures and FX markets even trade around the clock. This leads to another freedom in price reporting: whether to include the price movement during the extended trading hours.
Conclusion
To cross-verify the true price, we should always check the price from the Exchange where the asset is actually traded. Given the convenience of getting EOD data nowadays, this task should be easy to achieve.
In fact, for professional traders and investors alike, they will never reply price on free providers such as Yahoo and Google, they will most likely choose Bloomberg, Reuters, etc. However, for personal use, Yahoo and Google should both be good choices, and the difference is small enough to ignore.
