title: Assessing subject-related variations of the Ocular Response Analyzer parameter calculation.
abstract: BACKGROUND
  The aim was to study the relationships between the output parameters of the Ocular Response Analyzer (ORA) and those calculated from the raw ORA data and to ascertain the subject-related variations of ORA parameter calculation procedures.
  
  
  METHODS
  Six subjects were recruited for a prospective study. Up to 32 measurements by ORA were performed in series on the dominant eye of each subject. A relationship was examined between Goldmann-correlated intraocular pressure values (IOPg) obtained from the standard ORA output and IOPg' calculated from raw ORA data with a custom-written procedure. The same analysis was carried out for the parameters of corneal hysteresis (CH and CH'). Data and statistical analysis included Epanechnikov kernel smoothing, orthogonal linear regression, hypothesis testing and bootstrap techniques.
  
  
  RESULTS
  The group average (mean ± standard deviation) IOPg and CH values were 11.6 ± 1.8 mmHg and 10.7 ± 1.7 mmHg, respectively. A strong correlation was found between IOPg and IOPg' and also between CH and CH' parameters. There was a significant (Behrens-Fisher test, p < 0.001) difference between subjects for both IOPg and CH calculations, in terms of the regression slope parameter.
  
  
  CONCLUSIONS
  Subject-related variations of ORA parameter calculation were demonstrated. This could indicate that currently employed estimators of IOP parameters include unreported algorithmic procedures that may lead to biased results.
authors.affiliation: Department of Biomedical Engineering, Wroclaw University of Technology, Wroclaw, Poland.
authors.forename: Marta E
authors.initials: ME
authors.lastname: Rogowska
authors.affiliation: Department of Biomedical Engineering, Wroclaw University of Technology, Wroclaw, Poland.
authors.forename: D Robert
authors.initials: DR
authors.lastname: Iskander
authors.affiliation: Department of Optics, Wroclaw University of Technology, Wroclaw, Poland. marta.rogowska@pwr.edu.pl.
authors.forename: Henryk T
authors.initials: HT
authors.lastname: Kasprzak
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D011446:Prospective Studies
keywords: air-puff tonometry; corneal hysteresis; glaucoma; intraocular pressure