At the start of the study, group differences on demographic, health status and past stressful life events were assessed with chi-square and independent samples t-tests. Significant differences (p < .05) between Nature Group Walkers and Non-Group Walkers existed on age, gender, ethnicity, education, disability, past physical activity, and past stressful life events. These group differences could confound analyses of the influence of nature-based group walks on well-being. To adjust for confounders and make the groups comparable, propensity score matching (PSM) (Rosenbaum & Rubin, 1983) was used. PSM has been used in public health research investigations of outdoor physical exercise (Boer, Zheng, Overton, Ridgeway, & Cohen, 2007; Cohen et al., 2013; Hendriksen, Simons, Garre, & Hildebrandt, 2010). A propensity score is the conditional probability that a study participant belongs to the treatment group (i.e. Nature Group Walkers) given all the observed covariates. Propensity scores were estimated using logistic regression with group walk participation as the outcome variable and selected covariates (i.e. age, gender, ethnicity, education, disability, past physical activity, past stressful life events) as predictors. Participants were matched using 1:1 nearest neighbour matching with replacement, the recommended approach when there are fewer ‘control’ than ‘treated’ participants (Dehejia & Wahba, 1999). Each ‘control’ participant (Non-Group Walker) was thus matched to one or more ‘treated’ participant(s) (Nature Group Walkers) with the most similar propensity score. ‘Control’ participants “receive a frequency weight that reflects the number of times they were selected as a match” (Stuart, 2010 p. 13). To ensure ‘good’ matches, a caliper of .25 standard deviations of the logit of the propensity score was applied (Ho, Imai, King, & Stuart, 2007); ‘control’ participants not identified as a ‘good’ match for a ‘treated’ participant were ‘pruned’ from the sample (Stuart, 2010). The end result is a reduced sample of participants that have similar propensity scores. The PSM procedure was performed using the SPSS PSM plug-in ‘psmatching’ (Thoemmes, 2012).
Perceived stress and Negative affect Group walks in nature were significantly associated with less perceived stress and less negative affect, controlling for other predictors. Recent stressful life events were significantly associated with greater perceived stress and negative affect. Increased frequency of other nature walks and recent physical activity were both significantly associated with reduced perceived stress and negative affect. Recent stressful life events was the strongest predictor of more perceived stress (β =.21) and more negative affect (β =.19). Group walk participation was the second strongest predictor of perceived stress (β = -.15) and negative affect (β = in the opposite direction. Positive affect and Mental well-being Controlling for other predictors, Nature Group Walkers were significantly associated with greater mental well-being and positive affect compared to Non-Group Walkers. Greater frequency of other nature walks and recent physical activity were both significantly associated with greater positive affect and mental well-being. Recent stressful life events were significantly associated with less mental well-being and positive affect. Duration of other nature walks was significantly associated with greater positive affect only. Recent physical activity was the strongest predictor for both mental well-being (β = .13) and positive affect (β = .20), followed by group walk participation (β = .12, β = .14 respectively). Social support