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Publicações

Publicações por LIAAD

2007

Radon variability at the Elat granite, Israel: Heteroscedasticity and nonlinearity

Autores
Barbosa, SM; Steinitz, G; Piatibratova, O; Silva, ME; Lago, P;

Publicação
GEOPHYSICAL RESEARCH LETTERS

Abstract
The basic statistical features of radon time series from continuous radon monitoring at the Elat granite, Israel are analysed. A similar analysis is carried out for ancillary and possibly related geophysical parameters for the Elat area. The results show that air temperature, precipitable water and longwave radiation time series exhibit constant variance over the analyzed period, while radon time series, atmospheric pressure, short-wave radiation and total electron content exhibit heteroscedasticity. Furthermore, for radon and shortwave radiation the variability is associated with the overall mean level, while for atmospheric pressure such an association is not present. The analyzed radon time series not only are non-stationary but also nonlinear, reflecting the complex dynamics of radon emanation and transport in natural subsurface systems.

2007

Scale-based comparison of sea level observations in the North Atlantic from satellite altimetry and tide gauges

Autores
Barbosa, SM; Fernandes, MJ; Silva, ME;

Publicação
DYNAMIC PLANET: MONITORING AND UNDERSTANDING A DYNAMIC PLANET WITH GEODETIC AND OCEANOGRAPHIC TOOLS

Abstract
A comparative study is carried out for sea level observations in the North Atlantic from tide gauges and satellite altimetry. Monthly tide gauge records from 12 stations in both sides of the North Atlantic from January 1993 to December 2003 and monthly time series of sea level anomalies derived from TOPEX measurements are considered. The degree of association between tide gauge and altimetry observations is analysed for different scales by computing the correlation between the sea level components resulting from a multiresolution analysis based on the maximal overlap discrete wavelet transform. A similar correlation analysis is carried out to assess the relationship between the sea level observations and climate variables: sea surface temperature, precipitation rate and wind speed. The results show that altimetry and tide gauge observations are strongly correlated, as expected, but also that the relation is scale dependent, with covariability driven by the seasonal signal for most stations. For all variables the obtained correlation patterns exhibit significant spatial variability reflecting the diversity of local conditions affecting coastal sea level.

2007

Long-Range Dependence in Heart Rate Variability Data: ARFIMA Modelling vs Detrended Fluctuation Analysis

Autores
Leite, A; Rocha, AP; Silva, ME; Gouveia, S; Carvalho, J; Costa, O;

Publicação
COMPUTERS IN CARDIOLOGY 2007, VOL 34

Abstract
Heart rate variability (HRV) data display non-stationary characteristics and exhibit long-range correlation (memory). Detrended fluctuation analysis (DFA) has become a widely-used technique for long memory estimation in non-stationary HRV data. Recently, we have proposed an alternative approach based on fractional integrated autoregressive moving average (ARFIMA) models. ARFIMA models, combined with selective adaptive segmentation may be used to capture and remove long-range correlation, leading to an improved description and interpretation of tire components in 24 hour HRV recordings. In this work estimation of long memory by DFA and selective adaptive ARFIMA modelling is carried out in 24 hour HRV recordings of 17 healthy subjects of two age groups. The two methods give similar information on long-range global characteristics. However ARFIMA modelling is advantageous, allowing the description of long-range correlation in reduced length segments.

2006

Visual interactive subgroup discovery with numerical properties of interest

Autores
Jorge, AM; Pereira, F; Azevedo, PJ;

Publicação
DISCOVERY SCIENCE, PROCEEDINGS

Abstract
We propose an approach to subgroup discovery using distribution rules (a kind of association rules with a probability distribution on the consequent) for numerical properties of interest. The objective interest of the subgroups is measured through statistical goodness of fit tests. Their subjective interest can be assessed by the data analyst through a visual interactive subgroup browsing procedure.

2006

Improving SVM-linear predictions using CART for example selection

Autores
Moreira, JM; Jorge, AM; Soares, C; de Sousa, JF;

Publicação
FOUNDATIONS OF INTELLIGENT SYSTEMS, PROCEEDINGS

Abstract
This paper describes the study on example selection in regression problems using mu-SVM (Support Vector Machine) linear as prediction algorithm. The motivation case is a study done on real data for a problem of bus trip time prediction. In this study we use three different training sets: all the examples, examples from past days similar to the day where prediction is needed, and examples selected by a CART regression tree. Then, we verify if the CART based example selection approach is appropriate on different regression data sets. The experimental results obtained are promising.

2006

Distribution rules with numeric attributes of interest

Autores
Jorge, AM; Azevedo, PJ; Pereira, F;

Publicação
KNOWLEDGE DISCOVERY IN DATABASES: PKDD 2006, PROCEEDINGS

Abstract
In this paper we introduce distribution rules, a kind of association rules with a distribution on the consequent. Distribution rules are related to quantitative association rules but can be seen as a more fundamental concept, useful for learning distributions. We formalize the main concepts and indicate applications to tasks such as frequent pattern discovery, sub group discovery and forecasting. An efficient algorithm for the generation of distribution rules is described. We also provide interest measures, visualization techniques and evaluation.

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