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The following is quoted from StockCharts.com:

Historical Price Data is Adjusted for Splits, Dividends and Distributions

We adjust our historical price data to remove the effects of stock splits, dividends and distributions.

When something "artificial" changes the price that a stock appears to trade at, we adjust all of that stock's historical data so that it "matches" the new prices.

For example, if a stock splits 2-for-1, the price is suddenly half of what it used to be. That creates a huge break in the chart. If you didn't know about the split, you might think that something really bad happened to the underlying company. What's worse, all of the technical indicators will suddenly give "Sell" signals because of the big "drop" in prices. To counter this effect, we divide all of the historical prices by 2 (and multiply all of the volume by 2) so that things "match up" smoothly on the charts.

While this is a HUGE benefit for people that do technical analysis, it causes problems for people that are trying to determine the price that they bought or sold a stock at on a certain date in the past. Do not use our historical data for determining actual buy or sell prices in the past.

We apply this same philosophy to all of our data - stocks and mutual funds - regardless of how big or small the adjustment is. We post all of our data adjustment activity on our Data Adjustments page.

The following is added for further clarification:

MUTUAL FUNDS

Mutual funds receive periodic dividends from some of the securities held in their portfolios, and they may also have capital gains from securities that have been sold. As these dividends and capital gains accumulate, the NAV (Net Asset Value) of the fund's shares increases by the amount of the dividends and capital gains being held. (NAV is the total assets owned by the fund divided by the shares outstanding.) Periodically (anywhere from monthly to annually, depending on the fund) the fund will make a "distribution" of these capital gains and dividends to the fund's shareholders, meaning that they are paid to shareholders.

On the day of the distribution the money being distributed is no longer a part of the fund's assets and therefore has to be deducted from the NAV. From the shareholder's point of view it is a "net zero" transaction, because the amount of the distribution remains in his/her account, and the value of the account doesn't change, BUT the fund's NAV will be reduced by the amount of the distribution -- the shareholder now owns the distributed assets, not the mutual fund.

For example, on 12/10/93 the NAV of Fidelity Magellan dropped by $4.18 to $69.36. The closing NAV was the net result of a $0.15 increase in the value of the stock held by the fund (added to NAV), and a distribution of $4.33 (subtracted from NAV). The account value of a shareholder owning one share of Fidelity Magellan on the day before the distribution was $73.54, which was the NAV of one share. On the day of the distribution, that same account was worth $73.69, which was the NAV of one share ($69.36) plus the $4.33 distribution. The shareholder  normally has the option of either having the distribution paid directly to him/her or having it reinvested into the fund, in which case the number of shares in the account would increase from 1.000 to 1.062.

If you are charting a mutual fund, you have to adjust previous data so that the chart makes sense. Otherwise, a day like the one described above would show Fidelity Magellan making a major down move, penetrating support and flashing warning signals. To adjust mutual fund data for distributions, multiply prior data by a factor that will reduce it proportionately. To calculate this adjustment factor, we subtract the amount of the distribution from the prior day's NAV, and divide the result by the prior day's NAV.

Example: Prior Day NAV = 10.00; Distribution = 1.00.

(10.00-1.00)/10.00 = Adjustment Factor of 0.9

In this example all prior data will be multiplied by 0.9.

Unfortunately, mutual fund distribution amounts are not available in the data feed, and StockCharts.com does not make historical data adjustments for mutual funds EXCEPT for Rydex and Fidelity Select mutual funds, which we track manually.

STOCKS/ETFs

What about stocks? A dividend is actually a distribution of company assets and has the same effect on the price of a share of company stock as a distribution has on the NAV of a mutual fund share - in the newspaper you will notice that the amount of the dividend has been subtracted from the share price on the ex-dividend date. Do we also adjust all historical data for the stock? StockCharts.com, who supplies our data and charts, does; however, many charting sites do not. You adjust historical stock data for distributions the same way you do with mutual funds (described above).

Finally, stock split is also a form of distribution -- additional shares are distributed to shareholders. Normally this is done in a straight forward manner, such as a 2 for 1 split. In this case the shareholder receives one additional share of stock for each share held. The effect is that the value of all shares is reduced by half (multiply historical data by 0.5). To determine the adjustment factor for a stock split, divide the old number of shares by the new number of shares. Example: The factor for a 3 for 2 split is:

2 / 3 = 0.66666

How to Get Information About Distributions

Now that you know how to handle distributions, your biggest problem will be in actually finding out that a distribution has occurred.

StockCharts.com reports all distributions of which they are aware on their Data Adjustments page.