them, which they believed had helped boost sales. Berge et al. (2011) suggest their training had impacts on male clients through an expansion effect, since profit margins did not change whereas sales did.

Such spillovers have possible implications for both internal and external validity. If the increased sales come mainly as a result of taking business from the control group firms, then this a violation of the stable unit treatment value assumption (SUTVA), and the experimental estimate will no longer give the average impact of training for the sample population. If the increased sales come mainly from other firms not in the sample, the results of the experiment could be misleading with respect to the gain to society from scaling up the training program. It should be noted that spillovers might also be positive, if control or non-sample firms copy some of the techniques or new products introduced by firms which have gone through training. Indeed this possibility is often given as one of the main justifications for public subsidies of matching grant programs which subsidize the purchase of business development services by small and medium enterprises. These issues are part of the broader question of how competition responds to newly trained firms – we do not know whether this deters some new firms from entering the industry, causes others to exit, or causes the incumbents who remain in business to make other changes to the way they run their businesses.

To investigate this issue, a much larger sample is needed. Experimental variation in the intensity of the treatment within different geographical areas could then be used to test for and measure these spillovers. A first attempt in this direction is found in Calderon et al. (2012), which randomly assigned 17 villages into 7 treatment villages and 10 control villages, with half the individuals in the treated villages then assigned to training. Their preliminary analysis surprisingly finds little evidence for spillovers, despite working in remote villages with 1500 or fewer households and where people mostly make or sell goods for local consumption. However, it is unclear how much power there is to examine these spillovers given the relatively small number of villages included in the study.

# _Do larger impacts emerge over time?_

As noted above, most of the studies take a single snapshot of the impact of the training at a relatively short interval after training has ended. Two studies which have traced trajectories of impacts suggest that effects can indeed vary a lot over time. In de Mel et al. (2012) the impacts on business start-up fade with time, as control firms catch up. Bloom et al. (2012) find that introducing management practices in larger firms shows immediate effects on quality, then slowly leads to changes in inventory levels, output and productivity, and it is only after several years of using these practices that impacts start to show in terms of employment generation (through new plants being opened). Given the large interest of many governments in employment creation, studies which look only within a year or so of treatment ending may miss effects that take some time to be realized – or conversely, we may find that effects which look very promising in the short-term dissipate over time.

# _If it is so helpful, why don’t firms purchase it?_

It is noticeable that all the business training studies reviewed here offer the training for free, as do two out of the three consulting experiments with the other offering a 90 percent subsidy. In part this is done for research purposes – in order to ensure sufficiently high take-up and to provide evidence on how 21