Dear NBS' users,

I am running an analysis with two groups (HC and patients) where I want to control for age and gender. 
This is a short example of the matrix I am using to do so:

1 1 0 -1 0.5
1 1 0 1 -05
1 1 0 -1 0.5
1 0 1 1 -05
1 0 1 1 -05
1 0 1 -1 0.5

Where the first column is the intercept, the second column the first group, 3rd the second group, 4th column is gender and 5th age (demeaned).

My contrast would be [0 1 -1 0 0] running an F-test.

Now, when I run the analysis with these settings MATLAB gives me the following warning: 
Warning: Rank deficient, rank = 4, tol = 1.154632e-14.

Is it possible this is caused by the sample size being too small (HC=26, patients=26)?
What else could the problem be? How is it possible to solve this?

Thank you so much!

Best, 

Giulia