The impact of additional instruction time has been identified by researchers who have used quasi-experimental methods to examine the causal relationship between school days and student achievement. Naturally occurring and exogenous variation in amount of instructional time - due, for example, to school cancellations resulting from bad weather, or teacher absenteeism - has been shown to have a substantial impact on student achievement (Hayes and Gershenson, 2015). For example, Fitzpatrick et al.