StATS: What is a purposive sample?
A purposive sample is a sample selected in a deliberative and non-random fashion to achieve a certain goal. In a focus group, for example, you may want to consciously seek out respondents at both ends of a spectrum (as well as some in the middle) to insure that all viewpoints are adequately represented. You might also preferentially recruit subjects who have the best knowledge and experience in an area.
In addition to focus group studies, purposive samples are often used in pilot studies. A purposive sample share the same weaknesses as a convenience sample and you will have difficulty making strong quantitative inferences from such a sample.
Example
A study of rehabilitation after stroke collected a small sample for a focus group of patients, care givers, and health care providers with unique expertise.
In order to achieve experimental diversification, participants were identified by purposive selection. The patients (n = 4) and the caregivers (n = 5) were recruited through social workers from the local community service centers, daycare centers and also chosen from a data bank of participants to previous studies. Health care providers (n = 9) were solicited through directors of professional services and coordinators of rehabilitation or home-based services. They worked in different fields of healthcare and services, in rural and urban areas. Administrators (n = 7) were recruited through hospital managers who would identify which one was more familiar with the study clientele and worked in various rural and urban areas. All participants were recruited because of their critical abilities and their experience with needs related to the stroke process from onset to reintegration into the community. Identification of rehabilitation needs after a stroke: an exploratory study. Talbot LR, Viscogliosi C, Desrosiers J, Vincent C, Rousseau J, Robichaud L. Health Qual Life Outcomes 2004: 2(1); 53. [Medline] [Abstract] [Full text] [PDF]
In a study of barriers to adequate prenatal care, researchers sought feedback from homeless women, women with substance abuse problems, partners of these women, and members of communities known to have inadequate prenatal care.
The findings represent the views of 169 people with characteristics as presented in Table 1. The participants tended to be older since participants were purposively chosen for experience with the system and for characteristics (drug-dependence, homelessness) that develop over time. Teenagers were not excluded. Perceptions about prenatal care: views of urban vulnerable groups. Milligan R, Wingrove BK, Richards L, Rodan M, Monroe-Lord L, Jackson V, Hatcher B, Harris C, Henderson C, Johnson AA. BMC Public Health 2002: 2(1); 25. [Medline] [Abstract] [Full text] [PDF]
Further reading
This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Definitions, Category: Research designs.