between option can be to implement some general screening for characteristics associated with higher migration likelihoods, such as focusing on young men in high migration areas.

One complementary suggestion is for researchers to make sure they collect detailed information on the whole set of steps needed for migration to occur. For example, an intervention to increase migration may first raise an individuals desire or intention to migrate, then lead them to take steps to look for job opportunities in different places, then lead them to get their appropriate documentation in order and to apply for jobs, and finally to get them to move. Even if studies lack sufficient statistical power to detect impacts on this final stage of the migration process, they may find impacts on these migration steps, and in doing so, learn about additional constraints that bind once individuals have decided they want to migrate.

The rareness of migration is also an issue when attempting to conduct experiments with immigrants, since it can be difficult and expensive to collect a representative sample of immigrants. McKenzie and Mistiaen (2009) show how intercept-point sampling can be used to achieve reasonably representative samples at a cost much lower than census-based sampling. Intercept-point sampling involves sampling individuals during set time periods at a pre-specified set of locations where households in the target group are likely to congregate, and then re-weighting the sample so that individuals who go to more of the locations (and are hence more likely to be sampled) receive less weight. An alternative has been to use convenience samples of customers of remittance organizations, clients of an NGO working with immigrants, or other such approaches.

_Measurement of outcomes can be challenging when people are moving._ It is costly and challenging to track individuals who move. In many randomized experiments, migration therefore manifests itself as attrition, and researchers try and assure the reader that it is limited and balanced across treatment groups. But when the focus is on migration as an outcome, or mobile individuals as the subjects of interest, much more care and effort needs to be devoted to tracking individuals. Of particular concern is the possibility that all of the treatment effect occurs among attritors. For example, consider an experiment designed to deter irregular migration, perhaps through an information campaign. If researchers return a year later and re-interview 80% of individuals, they might find very low rates of irregular migration in both the treatment and control groups. But the 20% of individuals who cannot be found might be precisely the ones who are most mobile and most likely to be affected by the treatment. There are seldom easy approaches to dealing with this issue, but rather the need for a range of methods to be used. These can include collecting lots of contact information at the beginning for friends and family members who may be able to report on individuals who have moved and provide assistance relocating them, trying to use technologies such as Whatsapp or other online communication methods that migrants may retain when they move, attempting to track individuals and interview them in their new 9