Generation of hierarchically correlated multivariate symbolic sequences

We introduce an algorithm to generate multivariate series of symbols from a finite alphabet with a given hierarchical structure of similarities. The target hierarchical structure of similarities is arbitrary, for instance the one obtained by some hierarchical clustering procedure as applied to an empirical matrix of Hamming distances. The algorithm can be interpreted as the finite alphabet equivalent of the recently introduced hierarchically nested factor model (M. Tumminello et al. EPL 78 (3) 30006 (2007)). The algorithm is based on a generating mechanism that is different from the one used in the mutation rate approach. We apply the proposed methodology for investigating the relationship between the bootstrap value associated with a node of a phylogeny and the probability of finding that node in the true phylogeny.Comment: 7 pages, 6 figures, 1 tabl

Use of 24 hours esophageal pHmetry for detection of gastroesophageal reflux in infants : what is the ideal score and the optimal threshold? A receiver operating characteristic analysis

BACKGROUND: The search for the ideal score and best cut-off value to interpret the data from 24-hour continuous pH-monitoring interests both gastroenterologists with adult patients and paediatric gastroenterologists. AIMS: To evaluate 24-hour continuous pH monitoring as a discriminatory test in the diagnosis of gastro-oesophageal reflux disease in a paediatric population, using various pH-metry scores and cut-off values. PATIENTS: One hundred and one patients presenting gastro-oesophageal reflux disease (endoscopic diagnosis of oesophagitis or coincidence between apnoea and reflux episodes observed during pH-metry), median age 10 months, were studied, together with a control group of 84 subjects, median age 11 months. RESULTS: After plotting the receiver operating characteristic curves and calculating the area below them, the evaluation of the total percentage reflux time proved to have a higher capacity for distinguishing between the patients and controls than the Euler score (p < 0.05). The cut-off value of 5.2% for the total percentage reflux time had a sensitivity of 75% and was 88% specific. Using higher cut-off values according to age, a 95% specificity and a 49% sensitivity were obtained. The most sensitive score was the Jolley score: 96% with a cut-off of 64 and 90% with a cut-off of 100 (a value determining the maximum diagnostic accuracy); specificity, however, was low: 39-61%. In addition, the Jolley score was the most useful parameter in detecting patients with apnoeic episodes secondary to gastro-oesophageal reflux disease and allowed a correct diagnosis in 12/13 cases. CONCLUSIONS: a) The simple determination of total percentage reflux time, according to the methodology used, has a higher predictive capacity than the more complex pH-monitoring scores; b) the best cut-off value for total percentage reflux time is 5.2% as it combines a good specificity and sensitivity which are necessary for this test; c) age-dependent cut-off values are highly specific but sensitivity is much too low; d) the Jolley score is very sensitive and this was maintained even when the cut-off was raised to a value of 100; it is the best predictive score for episodes of gastro-oesophageal reflux-dependent apparent life-threatening events