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If I have adj.close and close values of a stock, and I want to calculate adj.open from open, should I just multiply the open prices with the percentage change (adj.close/close) or should I add the difference (close - adj.close) to each open price?

Thank you.

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    $\begingroup$ There isn't enough information to do this. A stock split would be a multiplicative adjustment, while a dividend would be additive. $\endgroup$
    – msitt
    Commented Apr 10, 2017 at 17:29
  • $\begingroup$ I see. This is unfortunate lol. $\endgroup$
    – moondra
    Commented Apr 10, 2017 at 19:30
  • $\begingroup$ Is there any further information you require? I see you haven't accepted an answer. $\endgroup$ Commented Apr 25, 2017 at 18:48
  • $\begingroup$ Sorry, about that. I was meaning to get back to it, as I had to move onto another project. $\endgroup$
    – moondra
    Commented Apr 25, 2017 at 19:10
  • $\begingroup$ @Moondra. Thank you very much! Sorry to bother! I am happy to assist further if possible. $\endgroup$ Commented Apr 25, 2017 at 19:42

2 Answers 2

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Your first answer will get close enough for most practical applications; multiplicative factors are generally how adj. closes are calculated.

If you have dividend and split data, you can actually get a more granular answers as follows:

$P_{adj,\,t-1} = P_{adj, \,t} (1 + (\frac{P_{t-1}}{S} - P_t -D)\frac{1}{P_t}) $

where $P$ is the actual price

$P_{adj}$ is the adjusted price

$S$ is the split ratio when $t$ is a split ex-date. Otherwise it is $1$.

and, $D$ is dividends per share if $t$ is a dividend ex-date. $D = 0$ when $t$ is not a dividend ex-date.

I hope this helps. I tested the formula and it appears to work. It may get more complicated if there are other kinds of corporate actions which affect share price. But then again, these corporate actions will be reflected in the adjusted close prices as published within most data sources which leads us back to the start: multiplication.

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Factor = Close / Adjusted Close
Adjusted Open = Open / Factor

It's important to remember that precision is lost since dividends are often given with 4 decimals and over multiple adjustments, the effect is significant. Many providers only give 2 decimals for prices which is the wrong approach when splits are taken into consideration.

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