Table 2. Assessment of Risk of Bias.

Study	Sequence generation	Allocation concealment	Blinding	Incomplete outcome data	Selective reporting	Other sources of bias
Ferrari et al. (1985)	Unclear – “10 female volunteers were instructed to randomly use one of two strategies” p. 792	Researchers aware of allocation to groups	Only participants blind to allocation	Adequate	Adequate	Adequate “no difference existed between (…) conditions in sex and donation history”
Reich et al. (2006)	Unclear – “each donor had an equal chance of being randomized into groups” p. 1091	Recruitment staff aware of allocation to groups	Only participants blind to allocation	Adequate	Adequate	No baseline comparison
Mellström & Johannesson (2008)	Unclear – “participants were randomly allocated into three groups” p. 848	Unclear	Unclear - participants blinded to allocation	Adequate	Adequate	Adequate
Goette & Stutzer (2008)	Quasi - cluster randomization per donation center and per day of week	Adequate. Allocation concealed to staff and participants	Adequate. Staff and participants blinded to allocation	Adequate	Adequate	Adequate control of baseline differences
Goette et al. (2009) Study 1	Unclear – “treatments were randomly assigned to mail orders” p. 527	Unclear	Participants blinded to allocation	Adequate	Adequate	Adequate control of baseline differences
Goette et al. (2009) Study 2	Unclear – “randomly invited” p. 527	Unclear	Participants blinded to allocation	Adequate	Adequate	Adequate control of baseline differences
Lacetera et al. (2012)	Unclear – “randomly selected” p. 17	Researchers aware of allocation, unclear for staff	Participants blinded to allocation	Adequate	Adequate	Adequate control of baseline differences