Hi Saly,
Your compute "Pslab" finds the sum of the three vectors c_PRESS[1], c_PRESS[4] and c_PRESS[5]. But the result of this compute will be a vector with three components, will it not?
Have you tried using "variable PT equal (c_Pslab[2]^2+c_Pslab[3]^2)^0.5" instead? I would assume that c_Pslab only contains items with index 1, 2 or 3. (Or I might be completely wrong, but it's worth a shot.)
Regards,
Christer H. Ersland.