title: Intra-Cluster Correlation Estimates for HIV-related Outcomes from Care and Treatment Clinics in Dar es Salaam, Tanzania.
abstract: BACKGROUND
  Researchers planning cluster-randomized controlled trials (cRCTs) require estimates of the intra-cluster correlation coefficient (ICC) from previous studies for sample size calculations. This paper fills a persistent gap in the literature by providing estimates of ICCs for many key HIV-related clinical outcomes.
  
  
  METHODS
  Data from HIV-positive patients from 47 HIV care and treatment clinics in Dar es Salaam, Tanzania were used to calculate ICCs by site of enrollment or site of ART initiation for various clinical outcomes using cross-sectional and longitudinal data. ICCs were estimated using linear mixed models where either clinic of enrollment or clinic of ART initiation served as the random effect.
  
  
  RESULTS
  ICCs ranged from 0 to 0.0706 (95% CI: 0.0447, 0.1098). For most outcomes, the ICCs were large enough to meaningfully affect sample size calculations. For binary outcomes, the ICCs for event prevalence at baseline tended to be larger than the ICCs for later cumulative incidences. For continuous outcomes, the ICCs for baseline values tended to be larger than the ICCs for the change in values from baseline.
  
  
  CONCLUSIONS
  The ICCs for HIV-related outcomes cannot be ignored when calculating sample sizes for future cluster-randomized trials. The differences between ICCs calculated from baseline data alone and ICCs calculated using longitudinal data demonstrate the importance of selecting an ICC that reflects a study's intended design and duration for sample size calculations. While not generalizable to all contexts, these estimates provide guidance for future researchers seeking to design adequately powered cRCTs in Sub-Saharan African HIV treatment and care clinics.
authors.affiliation: Department of Epidemiology, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Dale
authors.initials: D
authors.lastname: Barnhart
authors.affiliation: Department of Epidemiology, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Ellen
authors.initials: E
authors.lastname: Hertzmark
authors.affiliation: Department of Global Health and Population, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Enju
authors.initials: E
authors.lastname: Liu
authors.affiliation: Department of Global Health and Population, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Ester
authors.initials: E
authors.lastname: Mungure
authors.affiliation: Management and Development of Health, Mwai Kibaki Road, Dar es Salaam, Tanzania.
authors.forename: Aisa N
authors.initials: AN
authors.lastname: Muya
authors.affiliation: Department of Global Health and Population, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Management and Development of Health, Mwai Kibaki Road, Dar es Salaam, Tanzania.
authors.forename: David
authors.initials: D
authors.lastname: Sando
authors.affiliation: Management and Development of Health, Mwai Kibaki Road, Dar es Salaam, Tanzania.
authors.forename: Guerino
authors.initials: G
authors.lastname: Chalamilla
authors.affiliation: Department of Immunology and Infectious Diseases, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Management and Development of Health, Mwai Kibaki Road, Dar es Salaam, Tanzania.
authors.forename: Nzovu
authors.initials: N
authors.lastname: Ulenga
authors.affiliation: Department of Global Health and Population, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Wellcome Trust Africa Centre for Population Health, A2074 Road, Mtubatuba, KwaZulu-Natal 3935, South Africa.
authors.forename: Till
authors.initials: T
authors.lastname: Bärnighausen
authors.affiliation: Department of Epidemiology, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Department of Global Health and Population, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Department of Nutrition, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Wafaie
authors.initials: W
authors.lastname: Fawzi
authors.affiliation: Department of Epidemiology, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA; Department of Biostatistics, Harvard T.H. Chan School of Public Health, Huntington Avenue, Boston, Massachusetts 02115, USA.
authors.forename: Donna
authors.initials: D
authors.lastname: Spiegelman
keywords: Cluster Randomized Controlled; HIV Infections; Intra-cluster Correlation Coefficient; Multicenter Studies; Sample Size