Program membership : PPi ¼ 1 ½l0
PPZiPP þ vipp > 0 ð4Þ
ZiPP contains an additional identification variable for program
membership. This is specified as a dummy variable indicating
whether 5 years ago (before the start of the program) the
household was member of an agricultural association that
was not selected as program association. This variable is considered
to be exogenous since it concerns a pre-treatment variable.
It is also relevant since it is negatively correlated with
program association membership (R2 = 0.09, p = 0.06). Because
correcting for selection bias and for endogenous explanatory
variables at the same time is difficult when outcome
variables are binary (Wooldridge, 2010), we perform the bivariate
probit analysis with the restricted samples for the tryout
and adoption equations (and with the full sample for the
awareness equation). Due to a lack of selection variables for
different program interventions, we can only estimate the
bivariate probit models for program membership as main
dependent variable.