title: Global Visit Effects in Point-Wise Longitudinal Modeling of Glaucomatous Visual Fields.
abstract: OBJECTIVE
  One of the difficulties in modeling visual field (VF) data is the sometimes large and correlated measurement errors in the point-wise sensitivity estimates. As these errors affect all locations of the same VF, we propose to model them as global visit effects (GVE). We evaluate this model and show the effect it has on progression estimation and prediction.
  
  
  METHODS
  Visual field series (24-2 Full Threshold; 15 biannual VFs per patient) of 125 patients with primary glaucoma were included in the analysis. The contribution of the GVE was evaluated by comparing the fitting and predictive ability of a conventional model, which does not contain GVE, to such a model that incorporates the GVE. Moreover, the GVE's effect on the estimated slopes was evaluated by determining the absolute difference between the slopes of the models. Finally, the magnitude of the GVE was compared with that of other measurement errors.
  
  
  RESULTS
  The GVE model showed a significant improvement in both the model fit and predictive ability over the conventional model, especially when the number of VFs in a series is limited. The average absolute difference in slopes between the models was 0.13 dB/y. Lastly, the magnitude of the GVE was more than three times larger than the measureable factors combined.
  
  
  CONCLUSIONS
  By incorporating the GVE in the longitudinal modeling of VF data, better estimates may be obtained of the rate of progression as well as of predicted future sensitivities.
authors.affiliation: Rotterdam Ophthalmic Institute Rotterdam, The Netherlands 2Department of Biostatistics, Erasmus MC, Rotterdam, The Netherlands.
authors.forename: Susan R
authors.initials: SR
authors.lastname: Bryan
authors.affiliation: Department of Biostatistics, Erasmus MC, Rotterdam, The Netherlands.
authors.forename: Paul H C
authors.initials: PH
authors.lastname: Eilers
authors.affiliation: Department of Biostatistics, Erasmus MC, Rotterdam, The Netherlands 3KU Leuven, L-Biostat, Leuven, Belgium.
authors.forename: Emmanuel M E H
authors.initials: EM
authors.lastname: Lesaffre
authors.affiliation: Glaucoma Service, Rotterdam Eye Hospital, Rotterdam, The Netherlands.
authors.forename: Hans G
authors.initials: HG
authors.lastname: Lemij
authors.affiliation: Rotterdam Ophthalmic Institute Rotterdam, The Netherlands.
authors.forename: Koenraad A
authors.initials: KA
authors.lastname: Vermeer
mesh_terms: D018450:Disease Progression; D005901:Glaucoma / Q000175:diagnosis / Q000503:physiopathology*; D006801:Humans; D008962:Models, Theoretical; D058609:Visual Field Tests; D014794:Visual Fields / Q000502:physiology*