Dear all

I'm trying to make a two sample t-test with two further regressors.

my model is

1 0 a b

1 0 c d

.  .. . . . . . 

 . .  . . . . 

0 1 r t

0 1 y z

my contrasts are 

1 -1 0 0

-1 1 0 0 

when I plot the design matrix I see the weird pattern displayed in the attached image. on the left the t-test with two further regressors, on the right the correct t-test without any other regressors.

Is my design wrong or is the nbs's representation?

thanks in advance

Alberto