Assessment of risk of bias in included randomised studies

• Random sequence generation

  • Low risk of bias e.g. participants assigned to treatments on basis of a computer‐generated random sequence or a table of random numbers.

  • High risk of bias e.g. participants assigned to treatments on basis of date of birth, clinic id‐number or surname, or no attempt to randomise participants.

  • Unclear risk of bias e.g. not reported, information unavailable.

• Allocation concealment

  • Low risk of bias e.g. where the allocation sequence could not be foretold.

  • High risk of bias e.g. allocation sequence could be foretold by patients, investigators or treatment providers.

  • Unclear risk of bias e.g. not reported.

• Blinding of outcomes assessors

  • Low risk of bias if outcome assessors were adequately blinded.

  • High risk of bias if outcome assessors were not blinded to the intervention that the participant received.

  • Unclear risk of bias if this was not reported or unclear.

Blinding of participants and personnel is usually impossible for surgical interventions.

• Incomplete outcome data

We will record the proportion of participants whose outcomes were not reported at the end of the study. We will code a satisfactory level of loss to follow‐up for each outcome as follows.

• • Low risk of bias, if fewer than 20% of patients were lost to follow‐up and reasons for loss to follow‐up were similar in both treatment arms.

  • High risk of bias, if more than 20% of patients were lost to follow‐up or reasons for loss to follow‐up differed between treatment arms.

  • Unclear risk of bias if loss to follow‐up was not reported.

• Selective reporting of outcomes

  • Low risk of bias e.g. review reports all outcomes specified in the protocol.

  • High risk of bias e.g. it is suspected that outcomes have been selectively reported.

  • Unclear risk of bias e.g. it is unclear whether outcomes had been selectively reported.

• Other bias

  • Low risk of bias if we do not suspect any other source of bias and the trial appears to be methodologically sound.

  • High risk of bias if we suspect that the trial was prone to an additional bias.

  • Unclear risk of bias if we are uncertain whether an additional bias may have been present.

Assessment of risk of bias in included non‐randomised studies

We will assess the risk of bias in non‐randomised controlled trials in accordance with four additional criteria concerning cohort selection comparability of treatment groups.

• Relevant details of criteria for assignment of patients to treatments

  • Low risk of bias e.g. yes.

  • High risk of bias e.g. no.

  • Unclear risk of bias.

• Representative group of patients who received the experimental intervention

  • Low risk of bias, if representative of women with mutations in BRCA1 or BRCA2.

  • High risk of bias, if groups of patients were selected.

  • Unclear, if selection of group was not described.

• Representative group of patients who received the comparison intervention

  • Low risk of bias, if drawn from the same population as the experimental cohort.

  • High risk of bias, if drawn from a different source.

  • Unclear risk of bias, if selection of group not described.

• No differences between the two groups or differences controlled for, in particular with reference to mutations (BRCA1 or BRCA2 or both), type of surgery, age, obesity, race, reproductive history, ovarian stimulation, menstrual history, use of the oral contraceptives, breastfeeding, estrogens therapy and pelvic inflammatory disease etc.

• • Low risk of bias, if the study authors reported at least six of these characteristics.

  • High risk of bias, if the six of these characteristics differed and differences were not controlled for.

  • Unclear risk of bias, if fewer than six of these characteristics were reported even if there were no other differences between the groups, and other characteristics were controlled for.