Piezoresistive Coefficients

Hi

I want to find out the sensitivity of my piezoresitive cantilver beam. In order to do that, based on the equations, I should have the piezoresistive coefficient (Pi-1):

Delta-R/R=(PI-1)*maximum surface stress

In the materiel properties of polysilicon i have a matrix for piezoresitive coupling factor which I believe should give me the coefficient that I need to use in the equation. However, there is a "sigma" parameter in that I do not know what is its value, since i did not define that.
Can anyone help me with this?
here is the matrix that is defined by comsol :

{58.8e-11[1/Pa]/sigma0(nd[m^3])[S/m], -27.2e-11[1/Pa]/sigma0(nd[m^3])[S/m], 58.8e-11[1/Pa]/sigma0(nd[m^3])[S/m], -27.2e-11[1/Pa]/sigma0(nd[m^3])[S/m], -27.2e-11[1/Pa]/sigma0(nd[m^3])[S/m], 58.8e-11[1/Pa]/sigma0(nd[m^3])[S/m], 0, 0, 0, 85.9e-11[1/Pa]/sigma0(nd[m^3])[S/m], 0, 0, 0, 0, 85.9e-11[1/Pa]/sigma0(nd[m^3])[S/m], 0, 0, 0, 0, 0, 85.9e-11[1/Pa]/sigma0(nd[m^3])[S/m]}

Best Regards
Bahar