Estimating numbers of unsheltered homeless people through plant-capture and postcount survey methods.

Objectives: We sought to increase the accuracy of New York City's estimates of its unsheltered homeless population.
Methods: We employed 2 approaches to increasing count accuracy: a plant-capture strategy in which embedded decoys (or "plants") were used to estimate the proportion of visible homeless people missed by enumerators and a postcount survey of service users designed to estimate the proportion of unsheltered homeless people who were not visible.
Results: Plants at 17 sites (29%) reported being missed in the count, because counters either did not visit those sites or did not interview the plants. Of 293 homeless service users who were not in shelters, 31% to 41% were in locations deemed not visible to counters.
Conclusions: Both plant-capture estimation and postcount surveys are feasible approaches that can increase the accuracy of estimates of unsheltered homeless populations.