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I have a data frame with prices/dates, is there a method that checks if the variance is homogeneous during all the series? I know there are many test like fligner.test, bartlett.test but I need to pass to those methods few groups....I don't have groups I have only one series and I need to know if the variance is constant.

Thank you

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  • $\begingroup$ Do you have several prices for a given date? $\endgroup$ – ocram Nov 7 '11 at 13:24
  • $\begingroup$ Is this a time series, in which you wish to check if the variance remains constant over time? $\endgroup$ – onestop Nov 7 '11 at 13:31
  • $\begingroup$ @ocram, no ONE price and ONE date fro about 700 days. $\endgroup$ – Dail Nov 7 '11 at 13:58
  • $\begingroup$ @onestop, Exactly, That's what I am looking for. $\endgroup$ – Dail Nov 7 '11 at 13:58
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What you're looking for is a test for homoskedasticity. I haven't need to use this myself yet, so I can't offer much in-depth advice, but the wikipedia page on homoskedasticity lists a couple of tests, including the Breusch–Pagan test, which assumes normality in the data, the Koenker–Basset test which generalises the Breusch–Pagan test for non-normal data.

the bptest function in the lmtest R package implements the both tests (set studentize=true for the Koenker–Basset test). Looks like it's only for linear models though, so it might not work for your data.

There's a few interesting questions about these test on this site. Check out charlie's answer on Why does the Breusch-Pagan test fail? in particular.

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