What is the empirical basis for converting banded ordinal data on numbers of sex partners among MSM into a continuous scale level variable? A secondary analysis of 13 surveys across 17 countries.

BACKGROUND: To provide empirically based guidance for substituting partner number categories in large MSM surveys with mean numbers of sexual and condomless anal intercourse (CAI) partners in a secondary analysis of survey data. METHODS: We collated data on numbers of sexual and CAI partners reported in a continuous scale (write-in number) in thirteen MSM surveys on sexual health and behaviour across 17 countries. Pooled descriptive statistics for the number of sexual and CAI partners during the last twelve (N = 55,180) and 6 months (N = 31,759) were calculated for two sets of categories commonly used in reporting numbers of sexual partners in sexual behaviour surveys. RESULTS: The pooled mean number of partners in the previous 12 months for the total sample was 15.8 partners (SD = 36.6), while the median number of partners was 5 (IQR = 2-15). Means for number of partners in the previous 12 months for the first set of categories were: 16.4 for 11-20 partners (SD = 3.3); 27.8 for 21-30 (SD = 2.8); 38.6 for 31-40 (SD = 2.4); 49.6 for 41-50 (SD = 1.5); and 128.2 for 'more than 50' (SD = 98.1). Alternative upper cut-offs: 43.4 for 'more than 10' (SD = 57.7); 65.3 for 'more than 20' (SD = 70.3). Self-reported partner numbers for both time frames consistently exceeded 200 or 300. While there was substantial variation of overall means across surveys, the means for all chosen categories were very similar. Partner numbers above nine mainly clustered at multiples of tens, regardless of the selected time frame. The overall means for CAI partners were lower than those for sexual partners; however, such difference was completely absent from all categories beyond ten sexual and CAI partners. CONCLUSIONS: Clustering of reported partner numbers confirm common MSM sexual behaviour surveys' questionnaire piloting feedback indicating that responses to numbers of sexual partners beyond 10 are best guesses rather than precise counts, but large partner numbers above typical upper cut-offs are common.