title: Measuring concurrency:  an empirical study of different methods in a large population-based survey and evaluation of the UNAIDS guidelines.
abstract: BACKGROUND
  Recent UNAIDS guidelines recommend measuring concurrency 6 months before the interview date, based on overlapping partnership dates. This has theoretical advantages, but little is known about how well it can be measured in practice.
  
  
  METHODS
  The assumptions underlying the UNAIDS measure were tested using data from a sexual behaviour survey conducted in rural northern Malawi. All resident adults aged 15-59 were eligible. Questions included self-reported concurrency and dates for all marital and nonmarital partnerships in the past 12 months.
  
  
  RESULTS
  A total of 6796 women and 5253 men were interviewed, 83 and 72% of those eligible, respectively. Since few women reported multiple partners, detailed analysis was restricted to men. Overall 19.2% [95% confidence interval (CI) 18.1-20.2] of men self-reported concurrent relationships in the past year (almost all of those with more than one partner). Using overlapping dates the estimate was 16.7% (15.7-17.7). Excluding partnerships which tied on dates (making overlap uncertain) or restricting the analysis to the three most recent partners gave similar results. The UNAIDS 6-month measure was 12.0% (11.1-12.9), and current concurrency was 11.5% (10.6-12.4). The difference between dates-based and self-reported 12-month measures was much larger for unmarried men: 11.1% (9.7-12.4) self-reported; 7.1% (6.9-8.2) on dates. Polygyny (15% of married men) and the longer duration of relationships stabilized the estimates for married men. Nonmarital partnerships were under-reported, particularly those starting longer ago.
  
  
  CONCLUSIONS
  The difficulties of recall of dates for relationships, and under-reporting of partners lead to underestimation of concurrency using date-based measures. Self-reported concurrency is much easier to measure and appears more complete.
authors.affiliation: Faculty of Epidemiology and Population Health, London School of Hygiene & Tropical Medicine, UK. judith.glynn@lshtm.ac.uk
authors.forename: Judith R
authors.initials: JR
authors.lastname: Glynn
authors.forename: Albert
authors.initials: A
authors.lastname: Dube
authors.forename: Ndoliwe
authors.initials: N
authors.lastname: Kayuni
authors.forename: Sian
authors.initials: S
authors.lastname: Floyd
authors.forename: Anna
authors.initials: A
authors.lastname: Molesworth
authors.forename: Fiona
authors.initials: F
authors.lastname: Parrott
authors.forename: Neil
authors.initials: N
authors.lastname: French
authors.forename: Amelia C
authors.initials: AC
authors.lastname: Crampin
mesh_terms: D000293:Adolescent; D000328:Adult; D006801:Humans; D008295:Malawi; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance / Q000379:methods*; D017410:Practice Guidelines as Topic; D015203:Reproducibility of Results; D012423:Rural Health; D012725:Sexual Behavior / Q000706:statistics & numerical data*; D012747:Sexual Partners; D013997:Time Factors; D055815:Young Adult