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Re: Negative Variance? [message #65770 is a reply to message #65769]

Thu, 19 March 2009 15:35

Jean H.
Messages: 472
Registered: July 2006

Senior Member

anniebryant@gmail.com wrote:
> On Mar 19, 4:08 pm, Paolo <pgri...@gmail.com> wrote:
>> anniebry...@gmail.com wrote:
>>> I am using the VARIANCE comand in IDL and am getting negative values.
>>> The description in the HELP menu for IDL doesn't provide much
>>> information, but from what I know, I thought variance was either an
>>> absolute value or a deviation from a squared number, neither of which
>>> could yield a negative value.
>>> I am looking at the variance for 2 different bands in an AVIRIS
>>> image. I use the same .pro, just change the band I am looking at.
>>> The results for Band 1 (lets call it) make sense: all positive
>>> values. For Band 2, I get a lot of negatives.?! How is this
>>> possible.
>>> These are the values for Band 1:
>>> integer mean std dev minimum maximum n_elements
>>> 2633.3 2059.3 0.0000 9367.0 (160686) =
>>> 160686
>>> These are the values for Band2:
>>> integer mean std dev minimum maximum n_elements
>>> 1167.5 18269. -32768. 32767. (160686) =
>>> 160686
>> I don't see any variance here. The only negative number is the
>> minimum.
>>
>> Ciao,
>> Paolo
>
> Those are the values of the vector AFTER the variance was computed, it
> should read Variance_Band1 and Variance_Band2.

How can you get multiple variances for a band?
Jean

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[Message index]

		Re: Negative Variance? By: Homeyer on Thu, 19 March 2009 20:11
		Re: Negative Variance? By: wlandsman on Thu, 19 March 2009 19:43
		Re: Negative Variance? By: anniebryant@gmail.com on Thu, 19 March 2009 18:47
		Re: Negative Variance? By: Jean H. on Thu, 19 March 2009 15:35
		Re: Negative Variance? By: anniebryant@gmail.com on Thu, 19 March 2009 15:10
		Re: Negative Variance? By: pgrigis on Thu, 19 March 2009 15:08

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