title: A brief conceptual tutorial on multilevel analysis in social epidemiology: investigating contextual phenomena in different groups of people.
abstract: OBJECTIVE
  (1) To provide a didactic and conceptual (rather than mathematical) link between multilevel regression analysis (MLRA) and social epidemiological concepts. (2) To develop an epidemiological vision of MLRA focused on measures of health variation and clustering of individual health status within areas, which is useful to operationalise the notion of "contextual phenomenon". The paper shows how to investigate (1) whether there is clustering within neighbourhoods, (2) to which extent neighbourhood level differences are explained by the individual composition of the neighbourhoods, (3) whether the contextual phenomenon differs in magnitude for different groups of people, and whether neighbourhood context modifies individual level associations, and (4) whether variations in health status are dependent on individual level characteristics.
  
  
  METHODS
  Simulated data are used on systolic blood pressure (SBP), age, body mass index (BMI), and antihypertensive medication (AHM) ascribed to 25 000 subjects in 39 neighbourhoods of an imaginary city. Rather than assessing neighbourhood variables, the paper concentrated on SBP variance between individuals and neighbourhoods as a function of individual BMI.
  
  
  RESULTS
  The variance partition coefficient (VPC) showed that clustering of SBP within neighbourhoods was greater for people with a higher BMI. The composition of the neighbourhoods with respect to age, AHM use, and BMI explained about one fourth of the neighbourhood differences in SBP. Neighbourhood context modified the individual level association between BMI and SBP. Individual level differences in SBP within neighbourhoods were larger for people with a higher BMI.
  
  
  CONCLUSIONS
  Statistical measures of multilevel variations can effectively quantify contextual effects in different groups of people, which is a relevant issue for understanding health inequalities.
authors.affiliation: Department of Community Medicine, Lund University Hospital, S-205 02 Malmö, Sweden. juan.merlo@smi.mas.lu.se
authors.forename: Juan
authors.initials: J
authors.lastname: Merlo
authors.forename: Min
authors.initials: M
authors.lastname: Yang
authors.forename: Basile
authors.initials: B
authors.lastname: Chaix
authors.forename: John
authors.initials: J
authors.lastname: Lynch
authors.forename: Lennart
authors.initials: L
authors.lastname: Råstam
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D000959:Antihypertensive Agents; D001794:Blood Pressure; D015992:Body Mass Index; D016000:Cluster Analysis; D004812:Epidemiologic Methods; D006304:Health Status; D006801:Humans; D008875:Middle Aged; D015233:Models, Statistical; D012044:Regression Analysis; D012111:Residence Characteristics / Q000706:statistics & numerical data*
chemical_list: D000959:Antihypertensive Agents