impact. They may be a good investment in some school systems; but given the partial coverage of each review, it would be difficult to claim conclusively that they are the very best investments.

A further limitation of these reviews extends from a limitation of most underlying studies: The reviews focus on effectiveness but say less about the cost-effectiveness of various intervention types, due to the fact that most of the studies they review do not report sufficiently detailed and comparable cost data (Evans and Popova 2014; McEwan 2014). Varying costs can lead certain interventions to have lower benefits but much higher benefit-per-dollar than others, and policy makers make investment decisions based on costs as well as impacts. Kremer, Brannen, & Glennerster (2013) do provide cost-effectiveness results for a subsample of 18 studies. They find pedagogical interventions that match teaching to students’ learning levels, contract teachers, and the provision of earnings information to be the most cost-effective. Informing the expensive end of the spectrum, McEwan (2014) combines his effect sizes with Kremer, Brannen, & Glennerster’s (2013) cost estimates for intersecting studies to find that interventions focusing on computer-assisted learning and class size reduction may be less cost-effective than others. However, these are based on a small sample (less than ten percent) of the 227 learning studies included in this review; much additional work is needed.

Similarly, the reviewers acknowledge that – due again to the underlying studies - these reviews focus largely on short-term learning impacts. For example, McEwan (2014) highlights that for his sample of studies, the average follow-up is conducted after 9-13 months of program exposure, with only about 10 percent of follow-ups occurring at least one month after the conclusion of the intervention. Across lowand high-income countries, it has been observed that educational gains are sometimes not sustained over time (Andrabi et al. 2011; Jacob, Lefgren, & Sims 2010; Evans, Kremer, & Ngatia 2014). Thus, a clear shortcoming of this literature is its inability to inform the trajectory of longer-term learning impacts.

Future reviews will benefit from combining methodologies, for example performing meta-analysis – which allows a highly systematic analysis – accompanied with narrative review – which can explore heterogeneity within categories and the apparent mechanisms behind effective programs. Furthermore, using narrative review will allow the inclusion of studies that are excluded from meta-analyses. Given the high observed level of heterogeneity within classes of interventions, the most useful reviews are likely to use low levels of aggregation, identifying specific characteristics of interventions that are effective rather than broad classes of interventions. Future reviews will also be most useful if they are careful to search out unpublished studies: Less than two-thirds of studies included in the six reviews were published journal articles.

Taken together, the reviews do identify certain key messages: Both student learning interventions and teacher training interventions will be most effective when tailored to the student or teacher involved. Pedagogical interventions must change students’ learning experiences and be adapted to individual student learning levels. Teacher training may be most effective when it is repeated and linked to a specific pedagogical method or tool. Increasing accountability can also improve student learning.

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