title: Regression to the mean and alcohol consumption: a cohort study exploring implications for the interpretation of change in control groups in brief intervention trials.
abstract: BACKGROUND
  Reductions in drinking among individuals randomised to control groups in brief alcohol intervention trials are common and suggest that asking study participants about their drinking may itself cause them to reduce their consumption. We sought to test the hypothesis that the statistical artefact regression to the mean (RTM) explains part of the reduction in such studies.
  
  
  METHODS
  967 participants in a cohort study of alcohol consumption in New Zealand provided data at baseline and again six months later. We use graphical methods and apply thresholds of 8, 12, 16 and 20 in AUDIT scores to explore RTM.
  
  
  RESULTS
  There was a negative association between baseline AUDIT scores and change in AUDIT scores from baseline to six months, which in the absence of bias and confounding, is RTM. Students with lower baseline scores tended to have higher follow-up scores and conversely, those with higher baseline scores tended to have lower follow-up scores. When a threshold score of 8 was used to select a subgroup, the observed mean change was approximately half of that observed without a threshold. The application of higher thresholds produced greater apparent reductions in alcohol consumption.
  
  
  CONCLUSIONS
  Part of the reduction seen in the control groups of brief alcohol intervention trials is likely to be due to RTM and the amount of change is likely to be greater as the threshold for entry to the trial increases. Quantification of RTM warrants further study and should assist understanding assessment and other research participation effects.
authors.affiliation: Faculty of Public Health and Policy London School of Hygiene and Tropical Medicine, 15-17 Tavistock Place, WC1H 9SH, London. Electronic address: Jim.McCambridge@lshtm.ac.uk.
authors.forename: Jim
authors.initials: J
authors.lastname: McCambridge
authors.affiliation: Centre for Clinical Epidemiology and Biostatistics School of Medicine and Public Health University of Newcastle, Australia.
authors.forename: Kypros
authors.initials: K
authors.lastname: Kypri
authors.affiliation: Centre for Clinical Epidemiology and Biostatistics School of Medicine and Public Health University of Newcastle, Australia.
authors.forename: Patrick
authors.initials: P
authors.lastname: McElduff
mesh_terms: D000293:Adolescent; D000428:Alcohol Drinking / Q000453:epidemiology* / Q000628:therapy*; D015331:Cohort Studies; D035061:Control Groups; D061345:Early Medical Intervention / Q000639:trends*; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009520:New Zealand / Q000453:epidemiology; D012044:Regression Analysis; D013334:Students / Q000523:psychology; D011795:Surveys and Questionnaires; D055815:Young Adult
keywords: Alcohol; Brief intervention; Regression to the mean; Research participation; Student