*3.3. Population*

Participants were all adults diagnosed with ME/CFS (*n* = 564 total; sample size range *n* = 9–150). Six studies used the Center for Disease Control and Prevention (CDC) criteria for the diagnosis of their patients [28,39–41,45,51]. One study used the 2003 Canadian criteria [43]. One study used Oxford criteria [29], one study used both CDC and Oxford criteria [30] and three studies used a combination of CDC criteria with 2003 or 2005 versions of Canadian criteria and 2011 international consensus criteria [10,42,44].

The healthcare settings included outpatient settings [28,39], community [40,41,43,45], a university hospital clinic [38], department of psychosomatic medicine [10,42] and a specialist ME/CFS unit [30,44]. One study did not report the setting from which their patients were recruited [29].

Three studies were conducted in the United Kingdom [29,30,39], three in Japan [10,28,42],two were conducted in Hong Kong, China [40,41], and one each in Belgium [38], Norway, Sweden, and USA [43–45].
