kathryn.hubbard@kcl.ac.uk emma.c.palmer@kcl.ac.uk

1. Control Sampling and Selection Bias in Case-Control Studies of PsychosisKathryn Hubbard*, Emma Palmer*, Simona Stilo*, Charlotte Gayer-Anderson*, Adanna Onyejiaka*, Susana Borges*, Chelsea Gardner*, Tjasa Velikonja**, Anisa Kurti*, Arune Keraite*, Sophie Sowden*, Francois Bourque*, Helen Fisher*, Valeria Mondelli*, Marta Di Forti*, Robin M Murray*, Carmine Pariante*, Paola Dazzan*, Rowena Handley*, Craig Morgan**Institute of Psychiatry, King’s College London, **University College London kathryn.hubbard@kcl.ac.uk emma.c.palmer@kcl.ac.uk • Results • Analyses indicate different recruitment methods attract a different subset of participants across socio- demographic variables. • Gender (fig 1) Age (fig 2) • Ethnicity (fig 3)Psychiatric History (fig 4) • Background • Case control designs are often used in psychosis research but are prone to selection and information biases. • Previous reports have highlighted the issue of recruiting ‘well’ controls based on: • Opportunistic samples • Student participants • Exclusion of non-psychotic disorders • Papers often provide detailed descriptions of case recruitment but not control recruitment. • Could lead to exaggerated differences and false effects reported. • Aim • We sought to investigate the impact of recruitment methods on the sample characteristics. • χ2(82,N=119)=84.84,p= 0.393, ns • χ2(2,N=119)= 8.52,p=0.014 Fig 1. shows % gender distribution across methods Fig 2. shows age distribution across methods • Method • Analyses are based on samples of controls recruited for on going studies of first-episode psychosis in south London (including the EU-GEI incidence and case-control study). • The aim was to recruit a representative sample of controls from South East London for comparison against a first episode psychosis sample from the same area. • To date 119 participants have been recruited using 3 distinct methods (table 1). • χ2(4,N=119)= 7.39, p=0.117, ns. • χ2(20,N=119) =29.419, p=0.08, ns Fig 3. shows % ethnicity distribution across methods. Fig 4. shows previous psychological treatment across methods % % • Discussion • Results demonstrate that different control recruitment methods generate samples differing in certain characteristics, suggesting sampling bias is likely whatever method is used. • Analyses indicate significant differences in the gender of participants recruited by different methods, and trends towards differences in ethnicity and previous psychological problems. • These findings point to the importance of paying attention to recruitment methods and the potential for selection bias in studies of psychosis. • Future work will aim to compare this sample with a study conducted in the same area that relied on opportunistic sampling. % % Table 1. Description of the three recruitment methods used and proportions recruited using each. Acknowledgements This study is funded by the Wellcome Trust (Grant Number: WT087417), the European Union (European Community’s Seventh Framework Program (grant agreement No. HEALTH-F2-2009-241909) (Project EU-GEI) and the UK Department of Health via the National Institute for Health Research (NIHR) Specialist Biomedical Research Centre for Mental Health award to the South London and Maudsley NHS Foundation Trust (SLaM) and the Institute of Psychiatry, King’s College London.