A large number of analyses in the social sciences are concerned with the determinants of the probability that a particular event will occur. For example, one may be interested in the forces which motivate commuters to use their cars to go to work as against some form of mass transport. Time, cost, and other considerations will play a role. Let p be the probability that a car is used, and hence 1 — p the probability that mass transport is preferred; then one may want to apply a linear specification of the form $$p = a + b{{{T_1}} \over {{T_2}}} + c{{{C_1}} \over {{C_2}}} + {d_1}{Q_2} + {d_2}{Q_2} + {d_3}{Q_3},$$ where T 1 and T 2 are the times (minutes per day) needed for the two competing modes of transportation, C 1 and C 2 their costs (dollars per day), and Q l, Q 2, and Q 3 commuter characteristics which are considered to be relevant to the choice (income, family size, etc.).