topic Re: Impossible correlation coefficient values in ArcGIS Spatial Analyst Questions

Impossible correlation coefficient values

CorentinBohl — Thu, 27 Oct 2011 16:29:51 GMT

Hi,

I just computed the correlation and covariance matrices for a set of rasters using the "Band Collection Statistics" tool. The correlation matrix includes several values that are below -1 which should be impossible and does not make sense statistically.

Has anyone encountered this issue before? Any idea where the problem could come from?

Thanks,

Corentin

Re: Impossible correlation coefficient values

SarahPene — Tue, 27 Dec 2011 23:36:24 GMT

Hi Corentin - did you find a solution yet? I am having the same problem - I'm running principal components on 19 worldclim layers (for an area in W. Europe), and I get numbers in my correlation matrix greater than 1, and less than -1. When i run it on just 2 layers i get a different number in my correlation matrix than between the same two layers when i run all 19 together. Surely that shouldn't be the case, since the correlation between 2 variables should be the same regardless of the other variables. If anyone has any idea what I'm doing wrong, I'd be really grateful for some advice!

Sarah

Re: Impossible correlation coefficient values

PyrrosTelionis — Mon, 27 Apr 2020 01:31:55 GMT

Almost ten years later I am having the same problem. I'm getting a -1.31 correlation between two BioClim layers. I think I have found the problem though!

This only happens when I am using modified BioClim data that I had converted to floating point. I was making sample data for students and I wanted to them to be able to see real temperatures in C. So I simply divided Bio10 and Bio11 by 10.0 in Raster Calculator. This resulted in floating point rasters. Meanwhile my elevation and rainfall data (also from BioClim) remain as signed integers. The combination confuses the Band Collection Statistics tools in both ArcMap 10.6 and ArcGIS Pro 2.2.4. In both cases resulting in the same impossible correlation values. If I rerun the exact analysis with only the original signed integer data from WorldClim, it works normally.

So I'd guess Esri needs to add a warning about using different pixel type rasters in the tool.