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I use Yahoo Finance to download stock price data in CSV. In the data of one particular ticker - ARM.L (ARM Holdings PLC @ LSE) I see the prices swinging between 2p and 3000-5000 GBP in the first 2 years since entering the market in April 1998. Have a look at this link to see what I mean: http://finance.yahoo.com/q/hp?s=ARM.L&d=0&e=7&f=2016&g=d&a=3&b=17&c=1998&z=66&y=4554

Ie.:

Date            Open        High        Low Close   Volume      Adj Close*
Jun 25, 1998    357,000.00  357,000.00  357,000.00  357,000.00  22,400  15,923.17
Jun 24, 1998    362,000.00  362,000.00  362,000.00  362,000.00  16,900  16,146.18
Jun 23, 1998    2.24    2.24    2.24    2.24    43,551,200  0.10
Jun 22, 1998    370,000.00  370,000.00  358,000.00  362,000.00  11,800  16,146.18
Jun 19, 1998    2.40    2.40    2.40    2.40    37,146,400  0.11
Jun 18, 1998    344,000.00  344,000.00  344,000.00  344,000.00  8,200   15,343.33
...
Jun 10, 1998    350,000.00  350,000.00  346,000.00  350,000.00  8,200   15,610.95
Jun 9, 1998 337,000.00  352,000.00  337,000.00  352,000.00  5,900   15,700.15
Jun 8, 1998 312,000.00  318,000.00  312,000.00  318,000.00  200 14,183.66
Jun 5, 1998 1.96    1.96    1.96    1.96    1,478,000   0.09
Jun 4, 1998 1.92    1.92    1.92    1.92    21,899,600  0.09

Note that the prices are in GBp, so ie. 1.96 = 1.96GBp = 0.0196GBP, 337000 = 3370GBP. This 'anomaly' screws up my price charts and confuses me as I couldn't find any explanation. I downloaded data of other LSE stocks and did not find any similar price movements in that period. I would greatly appreciate it if someone could explain why is the data recorded like that and how to normalize it.

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    Interestingly, the volume is nearly 0 (in some cases exactly 0) on the days where the stock's price is over 100,000. I wonder if these numbers represent some artificially assigned price when the volume is too low? – barrycarter Jan 8 '16 at 4:20
  • @barrycarter I let Yahoo know about this issue using the 'Report an Issue' link on the page with the stock data. We'll see if they get back to me. – mac13k Jan 8 '16 at 11:05
  • The question to Yahoo was posted here. Please vote it up to get it answered quicker. – mac13k Jan 11 '16 at 10:05
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This is just a shot in the dark but it could be intermarket data. If the stock is interlisted and traded on another market exchange that day then the Yahoo Finance data feed might have picked up the data from another market. You'd have to ask Yahoo to explain and they'd have to check their data.

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