I've found errors for almost every security I've looked at in Yahoo's historical data. As far as CH, here's a copy of the latest message I sent to Yahoo on the subject:
Based on comparisons with other sources, such as the NYSE closing prices, and S&P Annual Dividend records, there appear to be serious errors in your historical prices for CH. Beyond the missing dates I pointed out in previous messages (e.g. 09-Apr-02, 01-Feb-02, etc.) the dividends are incorrect for several dates:
The open/high/low/closing prices are inccrrect for all dates prior to 13-Dec-99. Your prices have subtracted dividends paid on each ex-dividend date from the prices for each day prior (thus making the prices more inaccurate as they go back in time with each dividend).
Yahoo's historical data definitely requires cleansing to be useful, however, it can be purified with only a modest amount of effort.
You have a good example in CH of the problem, however it is fixable with the following actions.
1. Copy & paste every page of each quote sheet to an excel sheet. The yahoo download feature does not work for cefs, where cg divvy pieces may occur. Yahoo historicals generally provide accurate mkt quotes for every day, the download divvy adjustments are not correct.
2. Go to etfconnect.com to get the real divvy values. Etfconnect is 98% accurate on divvys and this info is available for most cefs.
3. Join the divvy values to the yahoo unadjusted mkt quotes, via Access.
4. Calculate the adjustments on your own excel sheet.
5. NAVs can be obtained, via the download as they are not adjusted for divvys. Then do a similar adjustment for them to get NAV performance.
Both yahoo and etfconnect services are free so you get what you pay for, but the data provided is sufficient to allow one to work with it and come out with credible results.
I've found the blend of Yahoo and etfconnect to be quite useful.