title: Accurate Ensembles for Data Streams: Combing Restricted Hoeffding Trees using Stacking

authors: Bifet, Albert

subjects: Science & Technology

abstract: The success of simple methods for classification shows that it is often not necessary to model complex attribute interactions to obtain good classification accuracy on practical problems. In this paper, we propose to exploit this phenomenon in the data stream context by building an ensemble of Hoeffding trees that are each limited to a small subset of attributes. In this way, each tree is restricted to model interactions between attributes in its corresponding subset. Because it is not known a priori which attribute subsets are relevant for prediction, we build exhaustive ensembles that consider all possible attribute subsets of a given size. As the resulting Hoeffding trees are not all equally important, we weight them in a suitable manner to obtain accurate classifications. This is done by combining the log-odds of their probability estimates using sigmoid perceptrons, with one perceptron per class. We propose a mechanism for setting the perceptrons’ learning rate using the ADWIN change detection method for data streams and also use ADWIN to reset ensemble members (i.e., Hoeffding trees) when they no longer perform well. Our experiments show that the resulting ensemble classifier outperforms bagging for data streams in terms of accuracy when both are used in conjunction with adaptive naive Bayes Hoeffding trees, at the expense of runtime and memory consumption.

text: Accurate Ensembles for Data Streams: Combing Restricted Hoeffding Trees using Stacking The success of simple methods for classification shows that it is often not necessary to model complex attribute interactions to obtain good classification accuracy on practical problems. In this paper, we propose to exploit this phenomenon in the data stream context by building an ensemble of Hoeffding trees that are each limited to a small subset of attributes. In this way, each tree is restricted to model interactions between attributes in its corresponding subset. Because it is not known a priori which attribute subsets are relevant for prediction, we build exhaustive ensembles that consider all possible attribute subsets of a given size. As the resulting Hoeffding trees are not all equally important, we weight them in a suitable manner to obtain accurate classifications. This is done by combining the log-odds of their probability estimates using sigmoid perceptrons, with one perceptron per class. We propose a mechanism for setting the perceptrons’ learning rate using the ADWIN change detection method for data streams and also use ADWIN to reset ensemble members (i.e., Hoeffding trees) when they no longer perform well. Our experiments show that the resulting ensemble classifier outperforms bagging for data streams in terms of accuracy when both are used in conjunction with adaptive naive Bayes Hoeffding trees, at the expense of runtime and memory consumption.

year: 2010-01-01

title: Pacific climate reflected in Waipuna Cave drip water hydrochemistry

authors: Nava-Fernandez, Cinthya

abstract: Cave microclimate and geochemical monitoring is vitally important for correct interpretations of proxy time series from speleothems with regard to past climatic and environmental dynamics. We present results of a comprehensive cave-monitoring programme in Waipuna Cave in the North Island of New Zealand, a region that is strongly influenced by the Southern Westerlies and the El Niño–Southern Oscillation (ENSO). This study aims to characterise the response of the Waipuna Cave hydrological system to atmospheric circulation dynamics in the southwestern Pacific region in order to assure the quality of ongoing palaeo-environmental reconstructions from this cave. Drip water from 10 drip sites was collected at roughly monthly intervals for a period of ca. 3 years for isotopic (δ18O, δD, d-excess parameter, δ17O, and 17Oexcess) and elemental (Mg∕Ca and Sr∕Ca) analysis. The monitoring included spot measurements of drip rates and cave air CO2 concentration. Cave air temperature and drip rates were also continuously recorded by automatic loggers. These datasets were compared to surface air temperature, rainfall, and potential evaporation from nearby meteorological stations to test the degree of signal transfer and expression of surface environmental conditions in Waipuna Cave hydrochemistry. Based on the drip response dynamics to rainfall and other characteristics, we identified three types of discharge associated with hydrological routing in Waipuna Cave: (i) type 1 – diffuse flow, (ii) type 2 – fracture flow, and (iii) type 3 – combined flow. Drip water isotopes do not reflect seasonal variability but show higher values during severe drought. Drip water δ18O values are characterised by small variability and reflect the mean isotopic signature of precipitation, testifying to rapid and thorough homogenisation in the epikarst. Mg∕Ca and Sr∕Ca ratios in drip waters are predominantly controlled by prior calcite precipitation (PCP). Prior calcite precipitation is strongest during austral summer (December–February), reflecting drier conditions and a lack of effective infiltration, and is weakest during the wet austral winter (July–September). The Sr∕Ca ratio is particularly sensitive to ENSO conditions due to the interplay of congruent or incongruent host rock dissolution, which manifests itself in lower Sr∕Ca in above-average warmer and wetter (La Niña-like) conditions. Our microclimatic observations at Waipuna Cave provide a valuable baseline for the rigorous interpretation of speleothem proxy records aiming at reconstructing the past expression of Pacific climate modes.

text: Pacific climate reflected in Waipuna Cave drip water hydrochemistry Cave microclimate and geochemical monitoring is vitally important for correct interpretations of proxy time series from speleothems with regard to past climatic and environmental dynamics. We present results of a comprehensive cave-monitoring programme in Waipuna Cave in the North Island of New Zealand, a region that is strongly influenced by the Southern Westerlies and the El Niño–Southern Oscillation (ENSO). This study aims to characterise the response of the Waipuna Cave hydrological system to atmospheric circulation dynamics in the southwestern Pacific region in order to assure the quality of ongoing palaeo-environmental reconstructions from this cave. Drip water from 10 drip sites was collected at roughly monthly intervals for a period of ca. 3 years for isotopic (δ18O, δD, d-excess parameter, δ17O, and 17Oexcess) and elemental (Mg∕Ca and Sr∕Ca) analysis. The monitoring included spot measurements of drip rates and cave air CO2 concentration. Cave air temperature and drip rates were also continuously recorded by automatic loggers. These datasets were compared to surface air temperature, rainfall, and potential evaporation from nearby meteorological stations to test the degree of signal transfer and expression of surface environmental conditions in Waipuna Cave hydrochemistry. Based on the drip response dynamics to rainfall and other characteristics, we identified three types of discharge associated with hydrological routing in Waipuna Cave: (i) type 1 – diffuse flow, (ii) type 2 – fracture flow, and (iii) type 3 – combined flow. Drip water isotopes do not reflect seasonal variability but show higher values during severe drought. Drip water δ18O values are characterised by small variability and reflect the mean isotopic signature of precipitation, testifying to rapid and thorough homogenisation in the epikarst. Mg∕Ca and Sr∕Ca ratios in drip waters are predominantly controlled by prior calcite precipitation (PCP). Prior calcite precipitation is strongest during austral summer (December–February), reflecting drier conditions and a lack of effective infiltration, and is weakest during the wet austral winter (July–September). The Sr∕Ca ratio is particularly sensitive to ENSO conditions due to the interplay of congruent or incongruent host rock dissolution, which manifests itself in lower Sr∕Ca in above-average warmer and wetter (La Niña-like) conditions. Our microclimatic observations at Waipuna Cave provide a valuable baseline for the rigorous interpretation of speleothem proxy records aiming at reconstructing the past expression of Pacific climate modes.

year: 2020

title: Images of Pakeha-Māori: A Study of the Representation of Pakeha-Māori by Historians of New Zealand From Arthur Thomson (1859) to James Belich (1996)

authors: Bentley, Trevor William

subjects: Pakeha-Maori

abstract: This thesis investigates how Pakeha-Māori have been represented in New Zealand non-fiction writing during the nineteenth and twentieth centuries. The chronological and textual boundaries range from Arthur Thomson's seminal history The Story of New Zealand (1859) to James Belich's Making Peoples (1996). It examines the discursive inventions and reinventions of Pakeha-Māori from the stereotypical images of the Victorian era to modern times when the contact zone has become a subject of critical investigation and a sign of changing intellectual dynamics in New Zealand and elsewhere.

This thesis is about the history of attitudes to culture-crossers in New Zealand, the use of the term 'Pakeha-Māori', and the images that underlie the thinking of Britons and Pakeha about them. It explores the motives and backgrounds of specific authors and the ways in which they frame New Zealand history. It elucidates the ambiguous and contradictory perspectives of Pakeha-Māori in the literature and analyses its impact on changing public perceptions about them. The study critiques the literature with emphasis on theoretically informed research, historical analysis, and literary insights. Discussion is confined to published texts, with the aim of exploring the multiplicity of Pakeha-Māori images and the processes that gave rise to them.

This study is essentially an investigation into how and why historians and other scholars try to draw boundaries between cultures in order to create a satisfactory metanarrative or myth of the 'settlement' of New Zealand and thus to forge a sense of New Zealandness. The cultural and racial categories of 'Māori' and 'Pakeha' are very unstable, however, and a consideration of the 'in-between' or 'culture-crossing' category of 'Pakeha-Māori' can reveal the way in which 'Māori' and 'Pakeha' and a sense of New Zealand and New Zealanders have been constructed. More particularly, consideration of representations of those culture-crossers or race-crossers called Pakeha-Māori can reveal the hopes and fears of Pakeha writers regarding Pakeha, Māori and New Zealand and how Pakeha-Māori have frequently been a barometer or litmus test of public perceptions of relations between Māori and Pakeha in different historical periods.

text: Images of Pakeha-Māori: A Study of the Representation of Pakeha-Māori by Historians of New Zealand From Arthur Thomson (1859) to James Belich (1996) This thesis investigates how Pakeha-Māori have been represented in New Zealand non-fiction writing during the nineteenth and twentieth centuries. The chronological and textual boundaries range from Arthur Thomson's seminal history The Story of New Zealand (1859) to James Belich's Making Peoples (1996). It examines the discursive inventions and reinventions of Pakeha-Māori from the stereotypical images of the Victorian era to modern times when the contact zone has become a subject of critical investigation and a sign of changing intellectual dynamics in New Zealand and elsewhere.

This thesis is about the history of attitudes to culture-crossers in New Zealand, the use of the term 'Pakeha-Māori', and the images that underlie the thinking of Britons and Pakeha about them. It explores the motives and backgrounds of specific authors and the ways in which they frame New Zealand history. It elucidates the ambiguous and contradictory perspectives of Pakeha-Māori in the literature and analyses its impact on changing public perceptions about them. The study critiques the literature with emphasis on theoretically informed research, historical analysis, and literary insights. Discussion is confined to published texts, with the aim of exploring the multiplicity of Pakeha-Māori images and the processes that gave rise to them.

This study is essentially an investigation into how and why historians and other scholars try to draw boundaries between cultures in order to create a satisfactory metanarrative or myth of the 'settlement' of New Zealand and thus to forge a sense of New Zealandness. The cultural and racial categories of 'Māori' and 'Pakeha' are very unstable, however, and a consideration of the 'in-between' or 'culture-crossing' category of 'Pakeha-Māori' can reveal the way in which 'Māori' and 'Pakeha' and a sense of New Zealand and New Zealanders have been constructed. More particularly, consideration of representations of those culture-crossers or race-crossers called Pakeha-Māori can reveal the hopes and fears of Pakeha writers regarding Pakeha, Māori and New Zealand and how Pakeha-Māori have frequently been a barometer or litmus test of public perceptions of relations between Māori and Pakeha in different historical periods.

year: 2007