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Re: Negative Variance? [message #65771 is a reply to message #65770]

Thu, 19 March 2009 15:10

anniebryant@gmail.com is currently offline

anniebryant@gmail.com
Messages: 16
Registered: March 2009

Junior Member

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.

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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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