title: Ambulatory blood pressure monitoring: a critical review of the current methods to handle outliers.
abstract: OBJECTIVE
  To examine the methods to handle marginal readings in the analysis of ambulatory blood pressure.
  
  
  METHODS
  Data obtained from automatic ambulatory blood pressure monitoring include several 'outliers', i.e. readings at the frontier of physiologically acceptable ranges. Several methods have been used to handle these readings. We need to know whether using different methods to reject outliers leads to different results in the analysis of the data. If so, then it is important that a common method be used by different authors.
  
  
  METHODS
  Ten reported methods to handle outliers were selected and applied to a large set of unpublished blood pressure profiles (Novacor Diasys system). We compared the effects of data rejection by these methods on the mean values and standard deviations (calculated over 24 h, daytime and night-time) of the remaining data.
  
  
  RESULTS
  The different methods had quite different effects on the same data set. Depending on the method used, the discarded data varied from 1 to 17% of the total number of readings. Among the rejected data, readings that occurred in the daytime varied from 14 to 56%. Also, 'high-value' outliers varied from 1 to 60% of the rejected data. On average over the total sample, the rejection of outliers had only a small effect on the mean values of blood pressure. In contrast, it may strongly reduce the standard deviation of the readings.
  
  
  CONCLUSIONS
  The study emphasized the need to use a common method to handle outliers in the analysis of ambulatory blood pressure data.
authors.affiliation: Unité de Recherches Biomathématiques et Biostatistiques, INSERM U263, Université Paris 7, France.
authors.forename: L
authors.initials: L
authors.lastname: Berardi
authors.forename: N P
authors.initials: NP
authors.lastname: Chau
authors.forename: X
authors.initials: X
authors.lastname: Chanudet
authors.forename: J
authors.initials: J
authors.lastname: Vilar
authors.forename: P
authors.initials: P
authors.lastname: Larroque
mesh_terms: D000328:Adult; D000368:Aged; D001794:Blood Pressure; D001795:Blood Pressure Determination / Q000379:methods*; D015924:Blood Pressure Monitors; D003627:Data Interpretation, Statistical; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged