Goodall et al. BMC Public Health 2012, 12:747 http://www.biomedcentral.com/1471-2458/12/747 Statistical methodology A power calculation prior to commencing the study sug- gested that a sample size of 500 would allow estimation of rates of condom use, knowledge and errors to within a standard error of not more than 2%. It would give 80% power to detect as significant differences of the order of 13% in the rates of condom variables between approxi- mately equal-sized groups based on demography. Review of STI rates in MSM attending clinics over the previous 2 years showed that these were unlikely to exceed 15% even for all infections combined, so power would be considerably lower for detecting associations between condom use and incident rates of STI. Major reductions of the order of 40% in STI rates in those using condoms relative to those not doing so would be detectable with high power. An additional measure of self-reported STI was included to ensure that STI data was available on both clinic and community samples and to increase the probability of detecting associations between demo- graphic and behavioral variables and STI acquisition. Data were analysed using SPSS version 18.0. Univariate associations between independent variables and STI diagnosis were tested by chi-squared tests for categorical variables and Mann–Whitney tests for continuous vari- ables. Multiple logistic regression was used to control for the effects of multiple factors on STI diagnosis. Lo- gistic regressions were conducted separately for clinic and community samples since factors significantly asso- ciated with STI diagnosis differed between these two samples. Factor analysis was carried out by principal component analysis. Condom proficiency and condom problem scales Questions on condom proficiency, knowledge and pro- blems were developed from those showing the most fre- quent positive responses in published studies [7,13], those reported by Scottish MSM [11] and questions used in one or more validated MRC bar survey tools [3,4]. New questions were tested for face validity and revised. Responses were combined and tested for reliability using factor analysis to produce a scale of condom proficiency and a scale of condom problems. The condom proficiency scale combined responses to 3 questions on ease of having safer sex, confidence in using condoms and ease of negotiating condom use with part- ners for anal sex. The answers were all on 4-point ordinal scales (Table 1). In a factor analysis, the first principal component had approximately equal coefficients for each of the questions, suggesting that the sum of these would be a suitable composite scale with a range of 0–9 and a mean of 1.42 for the 757 cases for which all 3 items were scored by respondents. Cronbach’s alpha for this scale was 0.71, indicating acceptable internal consistency (Cronbach’s alpha scores greater than 0.5 may indicate Table 1 Questions combined to assess condom proficiency Question How easy is it for you to have safer sex? (tick one) How confident are you at using condoms? (tick one) How easy is it for you to negotiate condom use with partners? (tick one) Page 3 of 8 Possible responses Very easy Quite easy Quite difficult Very difficult Not at all confident Slightly confident Confident Very confident Very easy Quite easy Quite difficult Very difficult some degree of internal consistency with higher scores, closer to 1, indicating greater internal consistency [15]). The condom problem scale was a cumulative score cre- ated by adding the number of condom problems reported by individuals in response to questions on hav- ing difficulty finding condoms that fit and experience of problems with condom use, both over the last 12 months and at last sexual encounter (Table 2). A higher value was associated with a higher condom problem score. This scale had a total of 23 ‘yes/no’ responses. Factor analysis again suggested that the sum of these would be a suitable composite scale with a range of 0–23 and a mean of 1.47 for the 764 cases in which all questions were answered by respondents. Cronbach’s alpha for the condom problem scale was 0.66. Results 459 clinic and 333 community questionnaires were returned. Response rates were 70% in both clinic and community samples. 666 MSM attended clinic during the study period, 268 of whom were known to be HIV positive (40%). 98% (652) of all MSM attending clinic during the study period were offered questionnaires. 22.2% of men in the clinic sample and 15.1% of men in the community sample reported being diagnosed with an STI over the previous 12 months. In the clinic sam- ple, 333 men consented to questionnaire linkage with STI results. 42 STIs were diagnosed in 38 men. The mean age of men was 36 (range 17–74) in the clinic sample and 38 in the community sample (range 17–73). The majority of participants in both samples were of white UK ethnicity; 79% of clinic and 81% of community sample; and most were employed; 72% of clinic and 76% of community sample. Men in both sam- ples tended to be highly educated; 58% of the clinic and