Table 6 Summary of volunteer characteristics

 White et al. 2012 [111]

57

93%

–

Heterogeneous

 Philippi et al. 2015 [108, 109, 112, 130]

35

85.7%

61.2(9.7)

Mainly no

 Haider et al. 2017 [113,114,115,116, 127]

–

–

50+

Mainly no

 Etkin et al. 2006 [117]

103

89%

53.2(−)

–

 Stolee et al. 2012 [119]

59

–

–

Heterogeneous

 Connelly 2008 [118]

113

90%

50.9(20.7)

–

 Wang et al. 2013 [120]

11

55%

45.3(6.6)

Yes

 Sales 2013 [121]

6

–

–

Yes