... A total of 56 studies used proxy-reported outdoor play [33-35, 40, 41, 47, 50, 52-54, 59-61, 63, 64, 68, 69, 73, 75, 77, 79, 80, 82, 83, 85-87, 89-93, 95-98, 101, 106, 111-113, 116-118, 122, 123, 125-129, 133, 135-138] and 15 studies used self-reported outdoor play [39,51,57,62,70,71,74,83,99,100,103,107,108,114,130], of which one observation used both proxy-and selfreported data [102]. Ten studies used accelerometry or pedometry (i.e., device-based measures of physical activity that took place outdoors) [34,35,48,72,77,86,113,117,119,122] and another five studies were based on direct observation [30,43,45,108,109]. Average duration of outdoor play reported from 26 studies ranged between 60.0 to 165.4 min/d (mean or median) or 2.0 to 10.3 h/week. ...