			Home About us Contact

Bias Reduction (bias + reduction)

Distribution by Scientific Domains

Mathematics and Statistics	50%
Life Sciences	33%

Selected Abstracts

Substantial Gains in Bias Reduction from Matching with a Variable Number of Controls

BIOMETRICS, Issue 1 2000
Kewei Ming
Summary. In observational studies that match several controls to each treated subject, substantially greater bias reduction is possible if the number of controls is not fixed but rather is allowed to vary from one matched set to another. In certain cases, matching with a fixed number of controls may remove only 50% of the bias in a covariate, whereas matching with a variable number of controls may remove 90% of the bias, even though both control groups have the same number of controls in total. An example of matching in a study of surgical mortality is discussed in detail. [source]

Non-participation and adjustment for bias in case,control studies of periodontitis

EUROPEAN JOURNAL OF ORAL SCIENCES, Issue 5 2008
Rodrigo López
Periodontal researchers frequently use case,control studies, but information on participation rates and the reasons for participation are often missing in the publications, thus hindering the assessment of the validity of those studies. A nested case,control study based on a well-defined population was used to (i) describe the patterns of participation; (ii) show how some associations can be biased; and (iii) illustrate how inverse probability weights can be applied to reduce bias. Differential subject participation was quantified using the ratio between participation for each level and the overall participation. Possible biased associations were illustrated using the odds ratios found for eligible and participant subjects. Finally, we used the estimated probability that an individual participates in the case,control study conditional on that individual's covariate pattern, as observed in the screening study to attempt bias reduction. Considerable differential participation was observed for selected factors, including age, annual tuitions and fees, parental income, and parental education. The strategy used for adjustment of bias resulted in some degree of bias reduction. These findings challenge the inferential validity of many studies on periodontitis. The design and conduct of these studies should aim to improve subject participation and must consider and minimize this potential source of bias. [source]

Use of longitudinal data in genetic studies in the genome-wide association studies era: summary of Group 14

GENETIC EPIDEMIOLOGY, Issue S1 2009
Berit Kerner
Abstract Participants analyzed actual and simulated longitudinal data from the Framingham Heart Study for various metabolic and cardiovascular traits. The genetic information incorporated into these investigations ranged from selected single-nucleotide polymorphisms to genome-wide association arrays. Genotypes were incorporated using a broad range of methodological approaches including conditional logistic regression, linear mixed models, generalized estimating equations, linear growth curve estimation, growth modeling, growth mixture modeling, population attributable risk fraction based on survival functions under the proportional hazards models, and multivariate adaptive splines for the analysis of longitudinal data. The specific scientific questions addressed by these different approaches also varied, ranging from a more precise definition of the phenotype, bias reduction in control selection, estimation of effect sizes and genotype associated risk, to direct incorporation of genetic data into longitudinal modeling approaches and the exploration of population heterogeneity with regard to longitudinal trajectories. The group reached several overall conclusions: (1) The additional information provided by longitudinal data may be useful in genetic analyses. (2) The precision of the phenotype definition as well as control selection in nested designs may be improved, especially if traits demonstrate a trend over time or have strong age-of-onset effects. (3) Analyzing genetic data stratified for high-risk subgroups defined by a unique development over time could be useful for the detection of rare mutations in common multifactorial diseases. (4) Estimation of the population impact of genomic risk variants could be more precise. The challenges and computational complexity demanded by genome-wide single-nucleotide polymorphism data were also discussed. Genet. Epidemiol. 33 (Suppl. 1):S93,S98, 2009. © 2009 Wiley-Liss, Inc. [source]

Habitat-specific normal and reverse diel vertical migration in the plankton-feeding basking shark

JOURNAL OF ANIMAL ECOLOGY, Issue 4 2005
DAVID W. SIMS
Summary 1Megaplanktivores such as filter-feeding sharks and baleen whales are at the apex of a short food chain (phytoplankton,zooplankton,vertebrate) and are sensitive indicators of sea-surface plankton availability. Even though they spend the majority of their time below the surface it is still not known how most of these species utilize vertical habitat and adapt to short-term changes in food availability. 2A key factor likely to control vertical habitat selection by planktivorous sharks is the diel vertical migration (DVM) of zooplankton; however, no study has determined whether specific ocean-habitat type influences their behavioural strategy. Based on the first high-resolution dive data collected for a plankton-feeding fish species we show that DVM patterns of the basking shark Cetorhinus maximus reflect habitat type and zooplankton behaviour. 3In deep, well-stratified waters sharks exhibited normal DVM (dusk ascent,dawn descent) by tracking migrating sound-scattering layers characterized by Calanus and euphausiids. Sharks occupying shallow, inner-shelf areas near thermal fronts conducted reverse DVM (dusk descent,dawn ascent) possibly due to zooplankton predator,prey interactions that resulted in reverse DVM of Calanus. 4These opposite DVM patterns resulted in the probability of daytime-surface sighting differing between these habitats by as much as two orders of magnitude. Ship-borne surveys undertaken at the same time as trackings reflected these behavioural differences. 5The tendency of basking sharks to feed or rest for long periods at the surface has made them vulnerable to harpoon fisheries. Ship-borne and aerial surveys also use surface occurrence to assess distribution and abundance for conservation purposes. Our study indicates that without bias reduction for habitat-specific DVM patterns, current surveys could under- or overestimate shark abundance by at least 10-fold. [source]

Kernel matching scheme for block bootstrap of time series data

JOURNAL OF TIME SERIES ANALYSIS, Issue 2 2004
Tae Yoon Kim
Abstract., The block bootstrap for time series consists in randomly resampling blocks of the original data with replacement and aligning these blocks into a bootstrap sample. Recently several matching schemes for the block bootstraps have been suggested to improve its performance by reduction of bias [Bernoulli 4 (1998), 305]. The matching schemes are usually achieved by aligning with higher likelihood those blocks which match at their ends. The kernel matching scheme we consider here takes some of the dependence structure of the data into account and is based on a kernel estimate of the conditional lag one distribution. In this article transition probabilities of the kernel matching scheme are investigated in detail by concentrating on a simple case. Our results here discuss theoretical properties of the transition probability matrix including ergodicity, which shows the potential of the matching scheme for bias reduction. [source]