Need help with monte carlo analysis

hello,
I am just a beginner in this field. I am trying to learn how to use monte carlo analysis. I created an inverter by using symbol nmos4 and pmos4. Then i wrote a .scs file based on mos1. I don't care if the parameter variation(vto) is reasonable. I just want to see the spreading of the output curve.
After simulating I found the vto changed by checking the iteration versus value,but the there was no output spreading. Anyone can help me?

simulator lang=spectre

parameters vto = 0.75
statistics{
process{
vary vto dist=gauss std=10
}
}


model nmos6012 mos1
+type = n +l = 1.5e-6 +w = 3.0e-6
+kp = 100e-6 +lambda = 7e-2 +phi = 0.7
+gamma=0.6 +tox = 1.5e-8 +cj = 1e-4 +cjsw = 5e-10 +pb = 0.9
+lds = 4.5e-6 +ldd = 4.5e-6 +ad = 1.35e-11
+ps = 1.2e-5 +pd = 1.2e-5 +nrs = 1.5 +nrd = 1.5


model pmos6012 mos1
+type = p +l = 1.5e-6 +w = 6.0e-6
+kp = 50e-6 +lambda = 7e-2 +phi = 0.7
+gamma=0.6 +tox = 1.5e-8 +cj = 3e-4
+cjsw = 3.5e-10 +pb = 0.9 +lds = 4.5e-6
+ldd = 4.5e-6 +as = 2.7e-11 +ad = 2.7e-11 +ps = 1.5e-5
+pd = 1.5e-5 +nrs = 1.5 +nrd = 1.5

See the followings.
The Designer's Guide Community Forum - Mismatch Parameters don't change in MonteCarlo of Cadence Spectre
The Designer's Guide Community Forum - Size Dependent Mismatch Variation's Description don't work in Agilent RFDE2006U3