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P J Rousseeuw

Publications and source records attributed to P J Rousseeuw.

5 recordsLinked to original sources
PubMed1998

On the calculation of a robust S-estimator of a covariance matrix.

An S-estimator of multivariate location and scale minimizes the determinant of the covariance matrix, subject to a constraint on the magnitudes of the corresponding Mahalanobis distances. The relationship between S-estimators and w-estimators of multivariate location and scale can be used to calculate robust estimates of covariance matrices. Elemental subsets of observations are generated to derive initial estimates of means and covariances, and the w-estimator equations are then iterated until convergence to obtain the S-estimates. An example shows that converging to a (local) minimum from the initial estimates from the elemental subsets is an effective way of determining the overall minimum. None of the estimates gained from the elemental samples is close to the final solution.

Biometry↗
PubMed1998

The two-factorial symptom structure of post-traumatic stress disorder: depression-avoidance and arousal-anxiety.

UNLABELLED: The first part of this study showed that the DSM-III-R symptom structure of post-traumatic stress disorder (PTSD), i.e. criteria B (reexperience), C (avoidance-numbing), and D (arousal), and, consequently the diagnosis of PTSD, could not be validated in fire and car-accident victims. The aims of this study were to: (i) determine the factors as well as their structure in the symptoms of PTSD; and (ii) develop a new classification or typology of PTSD. Exploratory and confirmatory factor analyses and cluster analyses were employed to: (i) examine the factors in PTSD symptomatology; and (ii) find and validate adequate diagnostic criteria for PTSD. The Composite International Diagnostic Interview (CIDI), PTSD Module, was used between 7 and 9 months after the traumatic event in a study group of 185 victims of two different traumatic events, i.e. 130 fire and 55 car-accident victims. Our findings support the existence of two factors, i.e. a first labeled 'depression-avoidance (DAV) dimension', as it contains items reminiscent of depression and avoidance, and a second labeled 'the anxiety-arousal (AA) dimension', as it contains symptoms reminiscent of anxiety and increased arousal. Cluster analysis yielded two clusters, i.e. a cluster of subjects with PTSD cases and another with non-cases. Our PTSD algorithm was significantly less restrictive than the DSM-III-R diagnosis of PTSD. There are only quantitative, but no qualitative, differences between the cluster analytically derived classes. IN CONCLUSION: PTSD is not a well-delineated clinical entity, as there is a clinical continuum from PTSD non-cases to cases with less and more severe DAV and AA symptoms. It is more appropriate to express PTSD in terms of general severity of PTSD and severity of the DAV and AA dimensions.

Accidents, Traffic↗
PubMed1998

Epidemiologic and phenomenological aspects of post-traumatic stress disorder: DSM-III-R diagnosis and diagnostic criteria not validated.

The aim of this cohort study was: (i) to validate the diagnostic criteria for post-traumatic stress disorder (PTSD) of the DSM-III-R; and (ii) to examine the incidence rate of PTSD in a study population exposed to two different traumatic events, i.e. a fire in a hotel ball-room and a multiple collision car-crash on a Belgian highway. One hundred and eighty-five victims (130 fire and 55 car accident victims) were assessed between 7 and 9 months after the traumatic event using the Composite International Diagnostic Interview (CIDI), PTSD Module, a fully structured diagnostic interview for the assessment of PTSD according to DSM-III-R criteria. Twenty-three percent of the study population met DSM-III-R criteria for PTSD. By means of unsupervised and supervised multivariate statistical analyses we were unable to validate the three-factorial structure, i.e. criteria B, C and D, of the DSM-III-R PTSD diagnosis. The latter relies heavily on the C diagnostic criteria, which appear to be too restrictive. Women were more likely to develop symptoms of reexperience (B) and arousal (D) than men. There was a significantly higher incidence of criteria B, C and D, but not of PTSD, in fire than in car-accident victims. Between 42 and 57% of the victims developed the first PTSD symptoms on the day of the trauma; within the next week these incidence rates increased to 77.1, 57.8 and 73.5% for criteria B, C and D, respectively. In conclusion, this study was unable to demonstrate the validity of the diagnostic criteria for PTSD according to DSM-III-R. The present cohort study has defined a number of factors that may predict new occurrences of PTSD symptoms after a traumatic event, i.e. gender, type of trauma and time delay between the trauma and the assessment of the diagnostic criteria.

Accidents, Traffic↗
PubMed1992

LMSMVE: a program for least median of squares regression and robust distances.

The program LMSMVE performs robust regression analysis by using the method of the least median of squares. It also computes robust distances to locate leverage points, that is, outliers with respect to the set of independent variables. LMSMVE constructs plots of least median of squares residuals against robust distances. Both methods can tolerate up to half the data being outliers before they fail to give results that describe the bulk of the data. A complete system that operates directly on SYSTAT files is available for the IBM PC and compatibles; it includes a utility that converts ASCII files to SYSTAT format.

Animals↗