title: Standardisation of rates using logistic regression: a comparison with the direct method.
abstract: BACKGROUND
  Standardisation of rates in health services research is generally undertaken using the direct and indirect arithmetic methods. These methods can produce unreliable estimates when the calculations are based on small numbers. Regression based methods are available but are rarely applied in practice. This study demonstrates the advantages of using logistic regression to obtain smoothed standardised estimates of the prevalence of rare disease in the presence of covariates.
  
  
  METHODS
  Step by step worked examples of the logistic and direct methods are presented utilising data from BETS, an observational study designed to estimate the prevalence of subclinical thyroid disease in the elderly. Rates calculated by the direct method were standardised by sex and age categories, whereas rates by the logistic method were standardised by sex and age as a continuous variable.
  
  
  RESULTS
  The two methods produce estimates of similar magnitude when standardising by age and sex. The standard errors produced by the logistic method were lower than the conventional direct method.
  
  
  CONCLUSIONS
  Regression based standardisation is a practical alternative to the direct method. It produces more reliable estimates than the direct or indirect method when the calculations are based on small numbers. It has greater flexibility in factor selection and allows standardisation by both continuous and categorical variables. It therefore allows standardisation to be performed in situations where the direct method would give unreliable results.
authors.affiliation: Primary Care Clinical Sciences, School of Health and Population Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. a.k.roalfe@bham.ac.uk
authors.forename: Andrea K
authors.initials: AK
authors.lastname: Roalfe
authors.forename: Roger L
authors.initials: RL
authors.lastname: Holder
authors.forename: Sue
authors.initials: S
authors.lastname: Wilson
mesh_terms: D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D003430:Cross-Sectional Studies; D004739:England / Q000453:epidemiology; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D006980:Hyperthyroidism / Q000453:epidemiology*; D007037:Hypothyroidism / Q000453:epidemiology*; D016015:Logistic Models; D008297:Male; D015233:Models, Statistical; D015995:Prevalence; D035583:Rare Diseases / Q000453:epidemiology; D017678:Sex Distribution