Propensity Score Methods in Nursing Research: Take Advantage of Them but Proceed With Caution.

Intervention research on health outcomes is important to advancing the nursing science. Using randomized controlled trials (RCTs) to estimate intervention (or treatment) effects is optimal for this purpose. Unfortunately, for practical or ethical reasons, RCTs are often not feasible, and thus, researchers often rely on observational or non-RCT data to estimate treatment effects. Such practice poses a threat to the validity of treatment effect estimation, because the lack of randomization introduces selection bias in observational and non-RCT data. Since the inception of Rosenbaum and Rubin’s (1983) seminal work on propensity score methods for reducing selection bias, numerous studies have been published in the social, behavioral, and medical literature in applying propensity score methods to observational studies and non-RCTs. A propensity score is defined as the conditional probability of a subject (e.g., patient) being assigned to a treatment group given a set of observed covariates (or confounders) such as age, gender, race/ethnicity, and health status (Rosenbaum & Rubin, 1985). Propensity score methods are a set of statistical procedures for using propensity scores to balance the distributions of observed covariates between the treatment and control (or comparison) groups with the aim of reducing selection bias. Such covariate balance enables a direct comparison between the treatment and control groups, similar to RCTs; therefore, applying propensity score methods can increase the validity of treatment effect estimation when observational or non-RCT data are used (Bai, 2011b; Pan & Bai, 2015b; Rubin, 2008). Propensity score methods usually consist of four basic steps: (a) selecting covariates; (b) estimating propensity scores; (c) matching, weighting, or stratifying subjects on/using propensity scores; and (d) conducting the intended outcome analysis (Pan & Bai, 2015a). Although propensity score methods are widely used in social, behavioral, and medical research, few publications related to propensity score methods are found in nursing research. For example, a search in Web of Science using “propensity score” as a topic keyword showed that, of the 14,561 articles related to propensity scores published since 1983 in Web of Science categorized journals, only 48 of them (0.33%) were published in nursing journals. The dearth of applications of propensity score methods to nursing research suggests that propensity score methods may not be well known to nursing