MathGroup Archive 2006

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Testing for a non-constant variance


Hello all,

I made a post a while back asking about testing a set of data for
stationarity.  While there are statistical tests that can be used to
detect a trend in a set of data, I am not aware of any that can detect
a non-constant variance.

One way of doing this would be to calculate the local standard
deviation for a series of points throughout the time series and compare
these to either the standard deviation of the whole data set, or the
average of these local standard deviations.  I am not sure exactly how
to implement this in an optimal way, ie what kind of cut-offs to use.

Does anyone know of any standard statistical tests that determine if a
data set has a non-constant variance, possibly ones that are
implemented in mathematica?  Or does anyone have any ideas on what a
good way to quantify my above method would be, in the general case?
Any thoughts would be greatly appreciated.


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