We targeted the general population aged 18–65 living in eight cities: Ghent–Belgium; Stuttgart–Germany; Athens–Greece; Budapest–Hungary; Porto–Portugal; Granada–Spain; Östersund–Sweden; London–United Kingdom (UK). Assuming an expected IPV prevalence of 15%1 and 3.0% of relative precision, size of samples was determined as 544 (272 women) for each centre. Samples were proportionally stratified according to age and sex, based on national Statistics Institutes data for resident population in 2008. Non-institutionalized national citizens or documented migrants residing in the participating cities were eligible.

Registry-based sampling was used in Spain, Belgium, Germany and Sweden and random-route was used in Greece and Hungary. In Portugal, two strategies were used: registry-based sampling and random-digit-dialling. The UK also resorted to two sampling strategies: registry-based and a via-public approach.

Participants selected through registries were sent an invitation letter with a project summary. Data collection took approximately 9 months and was completed in May 2011.

Random sample lists were obtained through city's municipality registries in Belgium (n=2720), Spain (n=2176) and Germany (n=3077), through electoral registry in Portugal (n=1990) and UK (n=4720) and through state person address registry in Sweden (n=1996).

Additionally, in Porto we used random-digit dialling of Porto city landlines (n=10623 calls) and in the UK participants were approached in public settings (n=1280).

In Belgium, Portugal and Germany, after sending invitation letters, participants were called to schedule an interview. In Greece, random route sampling was based on stratification of four major regions of the Greater Municipality Area of Athens according to geographical proximity of municipalities and similar socioeconomic structure. In Hungary, streets were selected from localities in Budapest. An adapted Leslie Kish Key was used for participant selection.

The assessment tool comprised a range of existing validated scales and questions designed specifically for this study. It included information on socio-demographics, intimate relationships, physical and mental health, use of medication, past-year health care use. The following scales were included: WHO-AUDIT – Alcohol Use Disorders Identification Test,2 Short Form (SF-36) Health-Related Quality of Life Questionnaire,3 Hospital Anxiety and Depression Scale (HADS),4 Multidimensional Scale of Perceived Social Support (MSPSS),5 and Post-Traumatic Stress Symptoms Scale.6 IPV was assessed with the Revised-Conflict Tactics Scales (CTS2),7 and violence-associated factors were examined with the Controlling Behaviours Scale–Revised8 and seven items assessing exposure to child abuse.

The tool was piloted using convenience samples in each city (n=89 total pilot sample) and the study protocol was approved by local Research Ethics Committees.

Questionnaires were administered by face-to-face interviewing for all sections, except for the IPV sections, which was self-administered for ethical reasons. As a last alternative option, questionnaires could be mailed in all countries if participants were otherwise unreachable. The only variation of administration occurred in Sweden, where questionnaires were posted to identified participants with a pre-paid envelope for return as per this ethics committee's request.

The WHO ethical and safety guidelines9 for the conduct of this type of research were considered by all centres and a study manual was produced in accordance. Interviewer training included presentation of the projects’ aims, detailed explanation of survey tool, role-playing involving scenarios related to introducing the interview, dealing with difficult participants and sensitive situations, research ethics and safety during field work including handling of reported/witnessed IPV incidents and a crisis-intervention protocol. The voluntary character of participation was emphasized and, although written informed consent was obtained by all face-to-face interviewed participants, no link between signed consents and questionnaires existed.

Necessary steps were taken by interviewers to ensure that the interview took place in a confidential and safe manner, meaning that only the trained interviewer and interviewee were present in the private setting during the completion of the questionnaire. In case a third person was present and refused to leave, the interviewer would have explained that, according with the study's objectives, he/she could not carry out the interview and would have tried to re-schedule it to another day and/or place. Questionnaires were administered at participants’ home (Greece, Hungary), university premises (Belgium) or either places (Portugal, Germany). In the UK, university premises and pre-selected public locations (with private spaces) were used.

To assess national samples representativeness, chi-square goodness of fit tests were used to compare the proportions of participants with each city population. Also, Population Fractions (PFs) by age and sex were computed for each country, using the corresponding reference city population provided by the national statistics institutions for 2008. PF was defined as the number of persons responding in each age-sex group divided by the number of persons with the same characteristics according to the available data. Population fraction ratios (PFRs, ratio of men’ to women’ PF) were estimated for each country. PFRs greater than 1 indicate an “excess” of men in the sample, while an excess of women is indicated by PFRs lower than 1.

Participants’ educational level was categorized to match International Standard Classification of Education (ISCED) into two categories: primary to secondary corresponding to ISCED levels 0–4 (pre-primary, primary, basic, secondary and post-secondary non-tertiary education), and university corresponding to ISCED levels 5 to 6 (tertiary). These were compared with the corresponding reference country population as available in Eurostat10 for 2009. PFs by age and education were computed for each country, so as PFRs for education. An “excess” of participants with education level university is indicated by PFRs lower than 1, compared to the country's distribution in that age strata.

A within-country comparison of the resulting samples according to age, sex and education was conducted for Portugal and UK. Chi square tests and Student's t-tests were used when appropriate. The Statistical Package for Social Sciences version 18 was used.