title: Influence of population versus convenience sampling on sample characteristics in studies of cognitive aging.
abstract: OBJECTIVE
  We examined whether differences in findings of studies examining mild cognitive impairment (MCI) were associated with recruitment methods by comparing sample characteristics in two contemporaneous Australian studies, using population-based and convenience sampling.
  
  
  METHODS
  The Sydney Memory and Aging Study invited participants randomly from the electoral roll in defined geographic areas in Sydney. The Australian Imaging, Biomarkers and Lifestyle Study of Ageing recruited cognitively normal (CN) individuals via media appeals and MCI participants via referrals from clinicians in Melbourne and Perth. Demographic and cognitive variables were harmonized, and similar diagnostic criteria were applied to both samples retrospectively.
  
  
  RESULTS
  CN participants recruited via convenience sampling were younger, better educated, more likely to be married and have a family history of dementia, and performed better cognitively than those recruited via population-based sampling. MCI participants recruited via population-based sampling had better memory performance and were less likely to carry the apolipoprotein E ε4 allele than clinically referred participants but did not differ on other demographic variables.
  
  
  CONCLUSIONS
  A convenience sample of normal controls is likely to be younger and better functioning and that of an MCI group likely to perform worse than a purportedly random sample. Sampling bias should be considered when interpreting findings.
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia; Dementia Collaborative Research Centre, University of New South Wales, Sydney, Australia. Electronic address: h.brodaty@unsw.edu.au.
authors.forename: Henry
authors.initials: H
authors.lastname: Brodaty
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia.
authors.forename: Annu
authors.initials: A
authors.lastname: Mothakunnel
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia.
authors.forename: Melissa
authors.initials: M
authors.lastname: de Vel-Palumbo
authors.affiliation: National Ageing Research Institute, Parkville, VIC, Australia; University of Melbourne Academic Unit for the Psychiatry of Old Age, Kew, VIC, Australia.
authors.forename: David
authors.initials: D
authors.lastname: Ames
authors.affiliation: National Ageing Research Institute, Parkville, VIC, Australia; University of Melbourne Academic Unit for the Psychiatry of Old Age, Kew, VIC, Australia; Mental Health Research Institute, Parkville, VIC, Australia.
authors.forename: Kathryn A
authors.initials: KA
authors.lastname: Ellis
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia.
authors.forename: Simone
authors.initials: S
authors.lastname: Reppermund
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia; Neuropsychiatric Institute, Prince of Wales Hospital, Randwick, NSW, Australia.
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Kochan
authors.affiliation: Department of Psychology and ARC Centre of Excellence in Cognition and its Disorders, Macquarie University, Sydney, NSW, Australia.
authors.forename: Greg
authors.initials: G
authors.lastname: Savage
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia; Department of Developmental Disability Neuropsychiatry, University of New South Wales, Sydney, Australia.
authors.forename: Julian N
authors.initials: JN
authors.lastname: Trollor
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia.
authors.forename: John
authors.initials: J
authors.lastname: Crawford
authors.affiliation: Centre for Healthy Brain Ageing, University of New South Wales, Sydney, Australia; Neuropsychiatric Institute, Prince of Wales Hospital, Randwick, NSW, Australia.
authors.forename: Perminder S
authors.initials: PS
authors.lastname: Sachdev
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology / Q000523:psychology*; D000544:Alzheimer Disease / Q000175:diagnosis / Q000235:genetics / Q000523:psychology; D053327:Apolipoprotein E4; D001315:Australia; D016022:Case-Control Studies; D060825:Cognitive Dysfunction / Q000175:diagnosis / Q000235:genetics / Q000523:psychology*; D015340:Epidemiologic Research Design; D016021:Epidemiologic Studies; D005260:Female; D015577:Geriatric Assessment / Q000379:methods; D006801:Humans; D008297:Male; D008568:Memory; D009483:Neuropsychological Tests; D018579:Patient Selection; D011159:Population Surveillance / Q000379:methods; D012307:Risk Factors; D015983:Selection Bias
chemical_list: D053327:Apolipoprotein E4
keywords: Aging; Apolipoprotein E4; Epidemiologic research design; Epidemiologic studies; Mild cognitive impairment; Neuropsychological tests; Patient selection; Selection bias