title: Different conceptual constructs for modelling sedentary behaviour and physical activity: the impact on the correlates of behaviour.
abstract: BACKGROUND
  Research on the correlates of physical activity (PA) and sedentary behaviour (SB) to date has used independent prediction equations for each behaviour, without considering that they are both part of the same continuum of movement. This assumption of independence might lead to inaccurate estimates because common underlying latent variables may simultaneously influence the propensity to engage in PA and SB. This study tests empirically the interdependent nature of PA and SB by comparing independent equations (current approach in the literature), and joint estimators (a novel but unexplored approach). Using Health Survey for England 2008 data, accelerometry-accessed PA and SB were separately modelled (using ordinary least squared regressions-OLS) and then jointly (using seemingly unrelated regressions-SUR). We tested for diagonality, specification, and goodness of fit.
  
  
  RESULTS
  The best fit models were the ones that allowed for interdependence of the two movement-related behaviours (rho=-0.156; p<0.001). The SUR showed more favourable properties compared to OLS models; producing lower standard errors and more consistent and efficient coefficients. The efficiency gain was more pronounced in the SB equation (Chi2=92.75; p<0.001).
  
  
  CONCLUSIONS
  Evidence from a large national population-wide accelerometry study suggests that accounting for the interdependent nature of PA and SB in prediction equations leads to more efficient modelling estimates. Further research using different samples is, however, required to fully understand the magnitude of efficiency gains accruable from using the joint estimators.
authors.affiliation: Health Economics Research Group (HERG), Brunel University, Uxbridge, Middlesex, London UB8 3PH, UK. nana.anokye@brunel.ac.uk.
authors.forename: Nana Kwame
authors.initials: NK
authors.lastname: Anokye
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Stamatakis
mesh_terms: D000328:Adult; D000368:Aged; D004739:England; D005260:Female; D015438:Health Behavior; D006306:Health Surveys; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D009043:Motor Activity / Q000502:physiology*; D012307:Risk Factors; D057185:Sedentary Behavior