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Re: Negative Variance? [message #65767]

Thu, 19 March 2009 20:11

Homeyer
Messages: 9
Registered: March 2009

Junior Member

On Mar 19, 5:00 pm, "anniebry...@gmail.com" <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 am not sure how you are doing your calculations.. but I think you
may have a conversion wrong in some process. If for some reason you
are converting a variance (should be a FLOAT or DOUBLE) to an INT,
which by the way maximizes at 32767 and minimizes at -32768, of course
you will have negative values. If you are converting to INTs, that is
your first problem. Variance should always be a FLOAT or DOUBLE.

Cheers,
Cameron Homeyer

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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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