Fig. 2

Example of an inappropriate subgroup comparison, leading to differential attrition of >20%. This triage comparison trial compared a strategy of only undertaking diagnostic laparoscopy in women who had failed first-line intrauterine insemination (IUI) rather than undertaking laparoscopy in all women prior to fertility treatment [59]. The primary outcome was the proportion of women experiencing a change in fertility treatment from IUI. The published analysis used the proportion of participants undergoing diagnostic laparoscopy as the denominator rather than the number randomized in each arm. The authors analysis reported a non-significant small increase (experimental 13/23 (56%), control 31/64 (48%); OR = 1.4 [95%CI: 0.5–3.6]). However when the full study population is used a significant decrease in the proportion of women receiving a change in treatment is observed (experimental 13/77 (17%), control 31/77 (40%); OR = 0.3 [95%CI: 0.14–0.64]). Excluding participants who did not receive a laparoscopy (70% of experimental group participants, and 17% of comparator arm participants) all experimental group patients who became pregnant during intrauterine insemination treatment were excluded from the effectiveness measurement introducing selection bias