282 research outputs found
Flexible constrained sampling with guarantees for pattern mining
Pattern sampling has been proposed as a potential solution to the infamous
pattern explosion. Instead of enumerating all patterns that satisfy the
constraints, individual patterns are sampled proportional to a given quality
measure. Several sampling algorithms have been proposed, but each of them has
its limitations when it comes to 1) flexibility in terms of quality measures
and constraints that can be used, and/or 2) guarantees with respect to sampling
accuracy. We therefore present Flexics, the first flexible pattern sampler that
supports a broad class of quality measures and constraints, while providing
strong guarantees regarding sampling accuracy. To achieve this, we leverage the
perspective on pattern mining as a constraint satisfaction problem and build
upon the latest advances in sampling solutions in SAT as well as existing
pattern mining algorithms. Furthermore, the proposed algorithm is applicable to
a variety of pattern languages, which allows us to introduce and tackle the
novel task of sampling sets of patterns. We introduce and empirically evaluate
two variants of Flexics: 1) a generic variant that addresses the well-known
itemset sampling task and the novel pattern set sampling task as well as a wide
range of expressive constraints within these tasks, and 2) a specialized
variant that exploits existing frequent itemset techniques to achieve
substantial speed-ups. Experiments show that Flexics is both accurate and
efficient, making it a useful tool for pattern-based data exploration.Comment: Accepted for publication in Data Mining & Knowledge Discovery journal
(ECML/PKDD 2017 journal track
Mercury concentrations in plaice, flounder and dab from Belgian continental shelf waters (1971-1990)
Results obtained during a survey of 20 years on plaice (Pleuronectes platessa) and 15 years on flounder (Platichthys flesus) and dab (Limanda limanda) are reported. With flounder, two periods could be distinguished. In 1975-1981, higher mercury concentrations (0.38 mg/kg on average) with rather large variations were noted. From 1982 on, concentrations fluctuated around the same mean value (0.23 mg/kg). A gradual decrease of mercury levels in plaice was observed during the twenty years' monitoring period. The mean concentration in 1990 (0.06 mg/kg) was only one third of that of 1971 (0.18 mg/kg). In contrast with flounder and plaice, mercury concentrations in dab remained practically at the same level (0.09-0.13 mg/kg)
Evolutie van de gehalten aan zware metalen in geselecteerde bodemorganismen van het Belgisch Continentaal Plat (1981-1996)
Trace metals in the Belgian dumping area for acid wastes from the titanium dioxide industry (1985-89)
During the period of investigation, two stations in the dumping site and six reference stations were monitored. In water and sediments only the iron content, a key parameter, was determined. In benthos, Fe, Cr, Ni, Zn, Pb, Cu and Cd were assessed. The iron content was very similar in both the dumping area and in five reference stations (0.06-0.08 mg/1). In the station nearest to the coast however, the concentrations were 2 to 3 times higher, showing the influence of river inputs. There was no temporal trend. A lack of dissolved oxygen was never observed and no marked differences between the eight areas were noted indicating that the oxidation of Fe++, the main component of the waste stream, had no measurable consequences in the water column. Abnormally low pH-values were not observed. In the sediments, no temporal trend in iron concentrations could be detected. No higher amounts were noted in the dumping area. The concentration of iron did not appear to be linked directly to the disposal of titanium dioxide waste but to input sources in general. The distance to the coast seems to play an important role. The iron content in the total mud fraction (<63 µm) decreased seawards (from ca 4.2 to 0.12 g/kg) up to about 20 nautical miles. In sea star (Asterias rubens), hermit crab (Pagurus bernhardus), swimming crab (Macropipus holsatus) brittle star (Ophiura texturata) and cut trough shell (Spisula subtruncata), there was neither a clear temporal trend nor evidence of accumulation of metals due to the dumping of titanium dioxide waste. Nevertheless, large amounts of heavy metals associated with particulate matter can be carried over long distances before being deposited. For this reason, all dumpings were banned since 1990
La teneur en métaux lourds dans les soles pêchées en Mer du Nord et en Mer d'Irlande : 10 années de recherches (1973-1982) = Het gehalte aan zware metalen in tong uit de Noordzee en de Ierse Zee: 10 jaar onderzoek (1973-1982)
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