title: Methods for testing theory and evaluating impact in randomized field trials: intent-to-treat analyses for integrating the perspectives of person, place, and time.
abstract: Randomized field trials provide unique opportunities to examine the effectiveness of an intervention in real world settings and to test and extend both theory of etiology and theory of intervention. These trials are designed not only to test for overall intervention impact but also to examine how impact varies as a function of individual level characteristics, context, and across time. Examination of such variation in impact requires analytical methods that take into account the trial's multiple nested structure and the evolving changes in outcomes over time. The models that we describe here merge multilevel modeling with growth modeling, allowing for variation in impact to be represented through discrete mixtures--growth mixture models--and nonparametric smooth functions--generalized additive mixed models. These methods are part of an emerging class of multilevel growth mixture models, and we illustrate these with models that examine overall impact and variation in impact. In this paper, we define intent-to-treat analyses in group-randomized multilevel field trials and discuss appropriate ways to identify, examine, and test for variation in impact without inflating the Type I error rate. We describe how to make causal inferences more robust to misspecification of covariates in such analyses and how to summarize and present these interactive intervention effects clearly. Practical strategies for reducing model complexity, checking model fit, and handling missing data are discussed using six randomized field trials to show how these methods may be used across trials randomized at different levels.
authors.affiliation: Department of Epidemiology and Biostatistics, College of Public Health, University of South Florida, 13201 Bruce B Downs Blvd., Tampa, FL 33612, United States. hbrown@health.usf.edu
authors.forename: C Hendricks
authors.initials: CH
authors.lastname: Brown
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Sheppard G
authors.initials: SG
authors.lastname: Kellam
authors.forename: Bengt O
authors.initials: BO
authors.lastname: Muthén
authors.forename: Hanno
authors.initials: H
authors.lastname: Petras
authors.forename: Peter
authors.initials: P
authors.lastname: Toyinbo
authors.forename: Jeanne
authors.initials: J
authors.lastname: Poduska
authors.forename: Nicholas
authors.initials: N
authors.lastname: Ialongo
authors.forename: Peter A
authors.initials: PA
authors.lastname: Wyman
authors.forename: Patricia
authors.initials: P
authors.lastname: Chamberlain
authors.forename: Zili
authors.initials: Z
authors.lastname: Sloboda
authors.forename: David P
authors.initials: DP
authors.lastname: MacKinnon
authors.forename: Amy
authors.initials: A
authors.lastname: Windham
mesh_terms: D000293:Adolescent; D000328:Adult; D019958:Attention Deficit and Disruptive Behavior Disorders / Q000628:therapy*; D015142:Baltimore; D001521:Behavior Therapy / Q000706:statistics & numerical data*; D002648:Child; D003627:Data Interpretation, Statistical; D006801:Humans; D008137:Longitudinal Studies; D015233:Models, Statistical; D017063:Outcome Assessment, Health Care / Q000706:statistics & numerical data; D016032:Randomized Controlled Trials as Topic / Q000706:statistics & numerical data*; D012107:Research Design; D012574:Schools; D012951:Socialization; D019966:Substance-Related Disorders / Q000517:prevention & control*; D014754:Violence / Q000517:prevention & control*