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I've been playing with stock data and I've discovered that in 10 years apple's stock price passed from roughly $3 to $100. However, this gain isn't due to intraday price variation dayN_close - dayN_open but the overnight variation dayN_open - dayN-1_close.

In fact when you sum up the intraday variation since 10years you get roughly -48$ drop in price (starting at 3$ in 2004) while when you sum up the overnight variation you get +148$ augmentation in price (148-48=100$ nowadays).

This means that the reason why apple stock price went from 3 to 100 in 10 years is the overnight variation in price. This is quite unexpected, if there was no overnight variation the stock price would have died a long time ago... Why is that?

Why do intraday traders close their position at then end of day while most gains can be done overnight (buy just before the market close and sell just after it opens). Is this observation true for other companies or is it specific to apple ?

Edit : Well I performed some additional analysis and it turns out that the gain due to overnight variation is because over 10 years the frequency of the o/n variation being positive is by far superior to it being negative. As for the intraday variation the probability of it going downwards is slightly superior to it going upwards. However the variations that are high (>3% let's say) are more frequent intradaily. o/n variation tends to be small but steadily positive basically. As for the data I took it from nasdaq.com so I hope it isn't glitchy.

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    Check for whether those are synched with product announcements and the like. I'll bet dollars to donuts they are, and that the fact that it appears to be overnight is an artifact of the data rather than reality. – keshlam Sep 21 '14 at 15:43
  • well I performed some additional analysis and it turns out that the gain due to overnight variation is because over 10 years the frequency of the o/n variation being positive is by far superior to it being negative. As for the intraday variation the probability of it going downwards is slightly superior to it going upwards. However the variations that are high (>3% let's say) are more frequent intradaily. o/n variation tends to be small but steadily positive basically. As for the data I took it from nasdaq.com so I hope it isn't glitchy. – Wicelo Sep 21 '14 at 16:24
  • This is an invitation to a discussion rather than about an issue of personal finance that the OP is facing. As such, it is off-topic for money.SE and I vote to close this question. – Dilip Sarwate Sep 21 '14 at 16:49
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    If you found the answer to your question, you should post it as an answer, not as an edit to your question thanks! – serakfalcon Sep 22 '14 at 7:52
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I'll answer this question:

"Why do intraday traders close their position at then end of day while most gains can be done overnight (buy just before the market close and sell just after it opens). Is this observation true for other companies or is it specific to apple ?"

Intraday traders often trade shares of a company using intraday leverage provided by their firm. For every $5000 dollars they actually have, they may be trading with $100,000, 20:1 leverage as an example. Since a stock can also decrease in value, substantially, while the markets are closed, intraday traders are not allowed to keep their highly leveraged positions opened.

Probabilities fail in a random walk scenario, and only one failure can bankrupt you and the firm.

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