the parameters are overidentified in terms of the moments of (y, x1, x2, z1, z2). This does not mean that there is now simply a second estimator. If z1 and z2 are both exogenous and relevant, then any linear combination of them, z* = a1z1 + a2z2, would also be a valid instrument. More than one IV estimator means an infinite number of possible estimators. Overidentification is qualitatively different from exact identification. The methods examined in this section are usable for overidentified models.