IV-like weighting function for OLS specification 2.
sWald(z, p.to, p.from, e.to, e.from)
vector, the value of the instrument.
P[Z = z'], where z' is value of the instrument the agent is switching to.
P[Z = z], where z is the value of the instrument the agent is switching from.
E[D | Z = z'], where z' is the value of the instrument the agent is switching to.
E[D | Z = z], where z is the value of the instrument the agent is switching from.
scalar.