A number of empirical studies across the world have shown that irrigation has a positive impact on
poverty (Dillon, 2007; Gebregziabher and Namara, 2009; Mangisoni, 2008; Omilola, 2009). Most irrigation
programs are designed as irrigation schemes targeting specific groups of farmers. Many studies that have
evaluated the impact of these schemes have not controlled for sample selection biases that may arise due
to systematic differences between participants and non-participants in those irrigation schemes. For instance,
farmers with prior experience in irrigation might be motivated to participate in irrigation and therefore
earn more net annual agricultural income than their less experienced counterparts. Failure to control for
selection bias in estimating the outcome of interest arising fromunobserved characteristics could potentially
result in overstated or understated estimates of the impact of the irrigation schemes on household income.