Pa = the summation of utility values describing Product A

Pb = the summation of utility values describing Product B

...

Pn = the summation of utility values describing Product N

Share of Preference is:

=EXP(Pa) / (EXP(Pa) + EXP(Pb) + ... +EXP(Pn) )

=EXP(Pb) / (EXP(Pa) + EXP(Pb) + ... +EXP(Pn) )

...

=EXP(Pn) / (EXP(Pa) + EXP(Pb) + ... +EXP(Pn) )

Generally, what is the SOP for including a "None of these" option in this calculation? My assumption would be that we would take an EXP() function of a single None value as opposed to the EXP() of the summation of values for a given product. Is this correct?