title: Intraclass correlations among physiologic measures in children and adolescents.
abstract: BACKGROUND
  Cluster designs have become increasingly popular in many research areas. Adopting these designs requires special considerations because individuals within the same cluster may be correlated. Therefore, appropriate sample size calculations in these settings require the incorporation of additional information regarding intraclass or intracluster correlations (ICCs), which measure the degree of similarity between individuals within the same cluster or class.
  
  
  OBJECTIVE
  To discuss the importance of the ICC in cluster-designed studies and present ICC estimates for selected physiologic characteristics in children and adolescents.
  
  
  METHODS
  The ICCs for body mass index (BMI), sum of skinfolds (SSF), blood pressure, glucose, insulin, and lipids were determined using baseline data from the Cardiovascular Health in Children and Youth Studies (CHIC II and III). The ICC estimates presented were obtained through the use of mixed models for the entire data set and separately by age groups, gender, and ethnicity.
  
  
  RESULTS
  The estimated ICCs ranged from .0104 for BMI to .1657 for glucose. The differences in the estimated ICCs among the three age groups were most marked for blood pressure, SSF, and glucose and were small for aerobic power, insulin, and lipids. The greatest difference in ICC by gender was in BMI and SSF: The ICC for SSF in girls was 8.2 times larger than that for boys. Caucasians had higher ICC estimates for insulin than did African Americans and other ethnic groups.
  
  
  CONCLUSIONS
  The magnitude of the ICC varied by the outcome of interest, and factors such as age, gender, and ethnicity also influenced the magnitude of the ICC. The presence of ICCs should be assumed when using cluster designs, and ICCs should be considered when conducting sample size calculations for such studies.
authors.affiliation: Department of Statistics, Federal University of Bahia, Salvador, Brazil.
authors.forename: Leila D
authors.initials: LD
authors.lastname: Amorim
authors.forename: Shrikant I
authors.initials: SI
authors.lastname: Bangdiwala
authors.forename: Robert G
authors.initials: RG
authors.lastname: McMurray
authors.forename: Dana
authors.initials: D
authors.lastname: Creighton
authors.forename: Joanne
authors.initials: J
authors.lastname: Harrell
mesh_terms: D000293:Adolescent; D041923:Adolescent Development / Q000502:physiology*; D000704:Analysis of Variance; D001794:Blood Pressure; D015992:Body Mass Index; D001837:Body Weights and Measures / Q000379:methods / Q000451:nursing*; D002648:Child; D002657:Child Development / Q000502:physiology*; D016000:Cluster Analysis; D016001:Confidence Intervals; D005260:Female; D005947:Glucose; D006801:Humans; D007328:Insulin; D008055:Lipids; D008297:Male; D015404:Nursing Methodology Research; D009765:Obesity; D012885:Skinfold Thickness
chemical_list: D007328:Insulin; D008055:Lipids; D005947:Glucose