Why a model and not just a field experiment or a natural experiment The policy issue we are concerned with is longer term. It relates to the behaviour of future cohorts Even with a longer term perspective in mind we cannot envisage the existence of a control group How do changing incentives affect education choice We need to infer the impact of the policy from analogou effects implied by variations in the current environme To achieve our aim we need to specify structure and use observational dataWhy a model and not just a field experiment or a natural experiment? • The policy issue we are concerned with is longer term. It relates to the behaviour of future cohorts. • Even with a longer term perspective in mind we cannot envisage the existence of a control group • We need to infer the impact of the policy from analogous effects implied by variations in the current environment: How do changing incentives affect education choice? • To achieve our aim we need to specify structure and use observational data