sorry about that. actually i want to varies the TOX parameter. according tu paper, the variation of oxide thickness in the range of 10%-20% from the nominal value. So assumption is made the the variation of oxide thinkness follows normal distribution. that is why the term 3σ=20%. means that the most variation to occur is 20%.
However the MOS model that i want to modified is based on BSIM4 model which the oxide thickness is model to TOXE, TOXP, and DTOX which
DTOX=TOXE-TOXP
if i want to model the variation in TOXE to use in monte carlo i need to include the variation+the nominal value so that it can be used in monte carlo simulation. However, i'm not sure how to include the variation in the spice model parameter. Hope my explanation clear something here.