Table 9 Categorizations of the Studies based on the outcomes and presented degree of evidence

ABC studies

41 (79%)

Positive

12 (23%)

(Bonde et al., 2014; Hakulinen et al., 2015; Hasegawa et al., 2015; Holman et al., 2013; Holman et al., 2015; Krause et al., 2015; Landers & Landers, 2015; Lehtonen et al., 2015; Nevin et al., 2014; Paiva et al., 2015; Pettit et al., 2015; Smith et al., 2014)

Negative

3 (6%)

(Attali & Arieli-Attali, 2015; Hanus & Fox, 2015; Long & Aleven, 2014)

Inconclusive

26 (50%)

(Amriani et al., 2014; Anderson et al., 2015; Auvinen et al., 2015; Barrio et al., 2015; Bernik et al., 2015; Boskic & Hu, 2015; Boticki et al., 2015; Christy & Fox, 2014; Codish & Ravid, 2015; Hew et al., 2016; Ibanez et al., 2014; Knutas et al., 2014b; Lambruschini & Pizarro, 2015; Laskowski & Badurowicz, 2014; Latulipe et al., 2015; Leach et al., 2014; Mekler et al., 2015; Morschheuser et al., 2014; Pedro et al., 2015b; Perry, 2015; Poole et al., 2014; Shi et al., 2014; Sillaots, 2014; Simoes et al., 2015; Su & Cheng, 2015; Utomo & Santoso, 2015)

Non-ABC

11 (21%)

(Barata et al., 2014; Chang & Wei, 2015; Codish & Ravid, 2014; Davis & Klein, 2015; Herbert et al., 2014; Holman et al., 2015; Knutas et al., 2014a; Pedro et al., 2015a; Sillaots, 2015; Tu et al., 2015; Tvarozek & Brza, 2014)