Using Capture-Recapture to Estimate Population Size: A Case Study Assessing Homelessness in Kittitas County, WA, and the Applicability of the Capture-Recapture Method to Primate Species Estimates

Capture-recapture (CRC) is a method of population estimation pioneered in biology in the late 19th century. CRC allows the surveyor to approximate the proportion of individuals that evaded the sampling efforts (i.e., captures) based on successful repeated samples from the same population. Hidden populations, such as individuals who experience homelessness, are suspected to elude conventional sampling methods likely because of the social stigma commonly associated with homelessness. By using a local community action agency’s homeless outreach program and cold weather shelter as two distinct sampling sources to capture data, the current study sought to estimate the homeless population in Kittitas County, Washington, as a means of evaluating the utility and ease of CRC methodology. Four months of data collection between the two sampling sources yielded an estimate of 115 homeless individuals in the county. A single day enumeration also called the point-in-time (PIT) count conducted in the same year sampled 61 individuals, supporting the idea that cross-sectional enumeration (i.e., PIT counts) have a history of underestimating the population in question. We propose that CRC enumeration serves to supplement information provided by annual PIT counts that otherwise neglects to capture homelessness as periodic events rather than static states of being