title: Representativeness of the participants in the smoking Cessation in Pregnancy Incentives Trial (CPIT): a cross-sectional study.
abstract: The limited representativeness of trial samples may restrict external validity. The aim of this study was to ascertain the representativeness of the population enrolled in the Cessation in Pregnancy Incentives Trial (CPIT), a therapeutic exploratory study to examine the effectiveness of financial incentives for smoking cessation during pregnancy.
  
  
  
  CPIT participants (n = 492) were compared with all self-reported smokers at maternity booking who did not participate in the trial (n = 1982). Both groups were drawn from the National Health Service (NHS) Greater Glasgow and Clyde area over a 1-year trial enrolment period. Variables used for comparison were age, area-based deprivation index, body mass index, gestation, and carbon monoxide (CO) breath test level. Chi-square and Mann-Whitney U tests were used to compare groups.
  
  
  
  From January to December 2012, 2474/13,945 (17.7 %) women, who booked for maternity care, self-reported as current smokers (at least one cigarette in the last week). Seven hundred and fifty-two were ineligible for trial participation because of a CO breath test level of less than 7 parts per million (ppm) used as a biochemical cut-off to corroborate self-report of current smoking. At telephone consent 301 could not be contacted, 11 had miscarried, 16 did not give consent and 3 opted out after randomisation, leaving 492 participants for analysis. There were no differences in demographic or clinical characteristics between trial participants, and self-reported smokers not enrolled in the trial in terms of CO breath test (as a measure of smoking level for those with a CO level of 7 ppm or higher), material deprivation (using an area-based measure), maternal age and maternal body mass index. Gestation at booking was statistically significantly lower for participants.
  
  
  
  To ensure that all trial participants were smokers, biochemical validation excluded self-reported smokers with a CO level of less than 7 ppm from taking part in the trial, which excluded 30 % of self-reported smokers who were 'lighter' smokers. The efficacy of financial incentives would not have been likely to decrease if 'lighter' smokers had been included in the trial population. Trial participants were slightly earlier in their pregnancy at maternity booking, but this difference would not clinically affect the provision of financial incentives if provided routinely. Overall, the trial population was representative of all self-reported smokers with regard to available routinely collected data. Appropriate comparison of trial and target populations, with detailed reporting of exclusion criteria would contribute to the understanding of the wider applicability of trial results.
  
  
  
  Current Controlled Trials ISRCTN87508788 . Registered/Assigned on 1 September 2011.
authors: World Health Organization, Section of Expanded Programme on Immunization, Ground Floor, UNECA building, Box 3069, Addis Ababa, Ethiopia.
authors: Barnabas
authors: B
authors: Bessing
authors: Centre for Tobacco and Alcohol Studies, School of Health Sciences, University of Stirling, Stirling, FK9 4LA, UK.
authors: Linda
authors: L
authors: Bauld
authors: Centre for Tobacco and Alcohol Studies, School of Health Sciences, University of Stirling, Stirling, FK9 4LA, UK.
authors: Lesley
authors: L
authors: Sinclair
authors: Institute of Health and Wellbeing, Public Health, University of Glasgow, 1 Lilybank Gardens, Glasgow, G12 8RZ, Scotland, UK.
authors: Daniel F
authors: DF
authors: Mackay
authors: School of Medicine, University of Glasgow, 1 Lilybank Gardens, Glasgow, G12 8RZ, Scotland, UK.
authors: William
authors: W
authors: Spence
authors: Section of Child Health, School of Medicine, Glasgow University, Scottish Cot Death Trust, West Glasgow Ambulatory Hospital, Yorkhill, Glasgow, G3 8SJ, UK. david.tappin@glasgow.ac.uk.
authors: David M
authors: DM
authors: Tappin
mesh_terms: D000328:Adult; D016009:Chi-Square Distribution; D003430:Cross-Sectional Studies; D004467:Economics; D005260:Female; D015438:Health Behavior; D007722:Health Knowledge, Attitudes, Practice; D006801:Humans; D007182:Income; D009035:Mothers / Q000523:psychology*; D009042:Motivation; D011247:Pregnancy; D011248:Pregnancy Complications / Q000209:etiology / Q000517:prevention & control*; D011295:Prenatal Care / Q000379:methods*; D012201:Reward; D012307:Risk Factors; D012606:Scotland; D000074285:Smokers / Q000523:psychology*; D012907:Smoking / Q000009:adverse effects / Q000191:economics / Q000523:psychology*; D000074606:Smoking Prevention / Q000191:economics / Q000379:methods*; D012959:Socioeconomic Factors; D016896:Treatment Outcome; D055815:Young Adult
keywords: Clinical markers; Cross-sectional studies; Data collection; Demography; Treatment effectiveness