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Downloaded all historical data for Oslo Stock Exchange from Yahoo Finance (yfinance), and picked a random stock to check if the data corresponds with what I see on different newssites.

If I look at NAS.OL from Yahoo at 2015-10-21, the close price is about 19911 with a volume of 4232.

But if I look at the data from Euronext (the owner of the exchange) the close price is about 9330 with a volume of 10122? This is also what I see in the different newssites here.

(Was just one action in 2020-12-18, a stock split of 0.01, according to Yahoo)

If I plot the graphs they look similar, but why is the difference in numbers so big?

(Edit: This has been marked as a duplicate, but the link to the other question is over 10 years old)

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    $\begingroup$ Unfortunately, the Yahoo! data is famously unreliable. $\endgroup$
    – Bob Jansen ♦
    Sep 27, 2021 at 17:33
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    $\begingroup$ You can try this question. I don't know about the EU rules. I'm pretty sure they are not required to provide extensive history though because in that case you would be able to easily get it. $\endgroup$
    – Bob Jansen ♦
    Sep 27, 2021 at 17:49
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    $\begingroup$ The answer is: 1) Yahoo! data is not a source professionals or academics use because the quality is bad. 2) We can only guess why a particular time series is bad and it will differ for each series so it is not generalizable and so does not make a good question for StackExchange. 1 and 2 combined make this a bad fit. If you don't agree I invite you to post on meta and make your case why question on the quality of free data is on-topic here. $\endgroup$
    – Bob Jansen ♦
    Sep 27, 2021 at 19:10
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    $\begingroup$ If you look at Yahoo, you will notice adj. close and close is identical throughout. If you download the history, you will see that the first big break occurs on Monday 4 May 2020. According to euronext, A total of 2,978,402,828 new shares* will be issued .... The total number of shares in the Company after the debt conversion will hence be 3,141,775,135**.... $\endgroup$
    – AKdemy
    Sep 27, 2021 at 21:48
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    $\begingroup$ Quick comment to yahoo finance by Graham Giller. Data cleaning is very important but trusted sources (which also have issues) are Bloomberg, Reuters and the like. It does not matter how old a link is, as long as nothing changed. $\endgroup$
    – AKdemy
    Sep 27, 2021 at 21:54

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