Matlab SSIM and Corr2 values

Hi, Ive built a simple program that checks images for similairty but after using the SSIM and corr2 methods, I have found that often I get negative values. What ould I be doing wring (i am using images of same size and format). Thanks in advance; any help will be much appreciated.

[Roger Stafford]

Correlation values can easily be negative. Their possible range is from -1 to +1.

[Mohammed Khan]

oh ok. so what does it mean if the value is negative. for example i get -0.3428 -> what does this mean. thanks