| 1. |
- Lu, Xin, et al.
(författare)
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Respondent-driven sampling on directed networks
- 2013
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Ingår i: Electronic Journal of Statistics. - 1935-7524. ; 7, s. 292-322
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Tidskriftsartikel (refereegranskat)abstract
- Respondent-driven sampling (RDS) is a widely used method for generating chain-referral samples from hidden populations. It is an extension of the snowball sampling method and can, given that some assumptions are met, generate unbiased population estimates. One key assumption, not likely to be met, is that the acquaintance network in which the recruitment process takes place is undirected, meaning that all recruiters should have the potential to be recruited by the person they recruit. Using a mean-field approach, we develop an estimator which is based on prior information about the average indegrees of estimated variables. When the indegree is known, such as for RDS studies over internet social networks, the estimator can greatly reduce estimate error and bias as compared with current methods; when the indegree is not known, which is most common for interview-based RDS studies, the estimator can through sensitivity analysis be used as a tool to account for uncertainties of network directedness and error in self-reported degree data. The performance of the new estimator, together with previous RDS estimators, is investigated thoroughly by simulations on networks with varying structures. We have applied the new estimator on an empirical RDS study for injecting drug users in New York City.
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| 2. |
- Hansson, Disa, et al.
(författare)
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Estimating individual action dispositions using binary and frequency egocentric sexual network data
- 2020
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Ingår i: Statistica Neerlandica. - : Wiley. - 0039-0402 .- 1467-9574. ; 74:3, s. 242-260
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Tidskriftsartikel (refereegranskat)abstract
- The degree distribution of nodes in a sexual network has been under thorough investigation, as has its implications for the spread of sexually transmitted infections. However, not only the structure of the network is of importance in regulating the propagation of an infection. Two nodes connected by an edge may take actions that reduce the transmission probability through that edge. Condom use is one such action. In this article, we derive models for individual action dispositions, and how they together generate an outcome on the edge connecting two nodes. We derive two main models: One where two connected nodes generate one outcome together (suitable for casual sex partners), and one where they generate several outcomes together (suitable for steady sex partners). We model different disposition distributions and different rules on how the dispositions generate outcomes, using an egocentric network dataset on condom use behavior.
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| 3. |
- Tsirpitzi, Renata Eirini, 1991-, et al.
(författare)
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Robust optimal designs using a model misspecification term
- 2023
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Ingår i: Metrika (Heidelberg). - : Springer Science and Business Media LLC. - 0026-1335 .- 1435-926X. ; :86, s. 781-804
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Tidskriftsartikel (refereegranskat)abstract
- Much of classical optimal design theory relies on specifying a model with only a small number of parameters. In many applications, such models will give reasonable approximations. However, they will often be found not to be entirely correct when enough data are at hand. A property of classical optimal design methodology is that the amount of data does not influence the design when a fixed model is used. However, it is reasonable that a low dimensional model is satisfactory only if limited data is available. With more data available, more aspects of the underlying relationship can be assessed. We consider a simple model that is not thought to be fully correct. The model misspecification, that is, the difference between the true mean and the simple model, is explicitly modeled with a stochastic process. This gives a unified approach to handle situations with both limited and rich data. Our objective is to estimate the combined model, which is the sum of the simple model and the assumed misspecification process. In our situation, the low-dimensional model can be viewed as a fixed effect and the misspecification term as a random effect in a mixed-effects model. Our aim is to predict within this model. We describe how we minimize the prediction error using an optimal design. We compute optimal designs for the full model in different cases. The results confirm that the optimal design depends strongly on the sample size. In low-information situations, traditional optimal designs for models with a small number of parameters are sufficient, while the inclusion of the misspecification term lead to very different designs in data-rich cases.
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| 4. |
- Rekić, Dinko, 1984, et al.
(författare)
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In silico prediction of efavirenz and rifampicin drug-drug interaction considering weight and CYP2B6 phenotype
- 2011
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Ingår i: British Journal of Clinical Pharmacology. - : Wiley. - 0306-5251 .- 1365-2125. ; 71:4, s. 536-43
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Tidskriftsartikel (refereegranskat)abstract
- Aims: To test whether a pharmacokinetic simulation model could extrapolate non-clinical drug data to predict human efavirenz exposure after single and continuous dosing as well as the effects of concomitant rifampicin and further to evaluate the weight based dosage recommendations used to counteract the rifampicin-efavirenz interaction. Methods: Efavirenz pharmacokinetics were simulated using a physiologically-based pharmacokinetic model implemented in the Simcyp population-based simulator. Physicochemical and metabolism data obtained from the literature were used as input for prediction of pharmacokinetic parameters. The model was used to simulate the effects of rifampicin on efavirenz pharmacokinetics in 400 virtual patients, taking into account bodyweight and CYP2B6 phenotype. Results: Apart from the absorption phase, the simulation model predicted efavirenz concentration-time profiles reasonably well with close agreement with clinical data. The simulated effects of rifampicin co-administration on efavirenz treatment showed only a minor decrease of 16% (95%CI 13; 19) in efavirenz area under the concentration-time curve (AUC), in magnitude with what has been clinically observed (22%). Efavirenz exposure depended on CYP2B6 phenotype and bodyweight. Increasing the efavirenz dose during concomitant rifampicin was predicted to be most successful in patients over 50 kg regardless of CYP2B6 status. Conclusion: Our findings, although based on a simulation approach using limited in vitro data, support the current recommendations for using a 50 kg bodyweight cut-off for efavirenz dose increment when co-treating with rifampicin.
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| 5. |
- Sjöqvist, Hugo, 1992-, et al.
(författare)
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An Analysis of Fast Learning Methods for Classifying Forest Cover Types
- 2020
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Ingår i: Applied Artificial Intelligence. - : Taylor & Francis Group. - 0883-9514 .- 1087-6545. ; 34:10, s. 691-709
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Tidskriftsartikel (refereegranskat)abstract
- Proper mapping and classification of Forest cover types are integral in understanding the processes governing the interaction mechanism of the surface with the atmosphere. In the presence of massive satellite and aerial measurements, a proper manual categorization has become a tedious job. In this study, we implement three different modest machine learning classifiers along with three statistical feature selectors to classify different cover types from cartographic variables. Our results showed that, among the chosen classifiers, the standard Random Forest Classifier together with Principal Components performs exceptionally well, not only in overall assessment but across all seven categories. Our results are found to be significantly better than existing studies involving more complex Deep Learning models.
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| 6. |
- Iftimi, Adina, et al.
(författare)
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Second-order analysis of marked inhomogeneous spatio-temporal point processes : applications to earthquake data
- 2019
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Ingår i: Scandinavian Journal of Statistics. - : Wiley. - 0303-6898 .- 1467-9469. ; 46:3, s. 661-685
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Tidskriftsartikel (refereegranskat)abstract
- To analyse interactions in marked spatio-temporal point processes (MSTPPs), we introduce marked second-order reduced moment measures and K-functions for inhomogeneous second-order intensity reweighted stationary MSTPPs. These summary statistics, which allow us to quantify dependence between different mark-based classifications of the points, are depending on the specific mark space and mark reference measure chosen. Unbiased and consistent minus-sampling estimators are derived for all statistics considered and a test for random labelling is indicated. In addition, we treat Voronoi intensity estimators for MSTPPs. These new statistics are finally employed to analyse an Andaman sea earthquake data set.
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| 7. |
- Abrahamsson, L, et al.
(författare)
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Continuous tumour growth models, lead time estimation and length bias in breast cancer screening studies
- 2020
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Ingår i: Statistical methods in medical research. - : SAGE Publications. - 1477-0334 .- 0962-2802. ; 29:2, s. 374-395
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Tidskriftsartikel (refereegranskat)abstract
- Comparisons of survival times between screen-detected and symptomatically detected breast cancer cases are subject to lead time and length biases. Whilst the existence of these biases is well known, correction procedures for these are not always clear, as are not the interpretation of these biases. In this paper we derive, based on a recently developed continuous tumour growth model, conditional lead time distributions, using information on each individual's tumour size, screening history and percent mammographic density. We show how these distributions can be used to obtain an individual-based (conditional) procedure for correcting survival comparisons. In stratified analyses, our correction procedure works markedly better than a previously used unconditional lead time correction, based on multi-state Markov modelling. In a study of postmenopausal invasive breast cancer patients, we estimate that, in large (>12 mm) tumours, the multi-state Markov model correction over-corrects five-year survival by 2–3 percentage points. The traditional view of length bias is that tumours being present in a woman's breast for a long time, due to being slow-growing, have a greater chance of being screen-detected. This gives a survival advantage for screening cases which is not due to the earlier detection by screening. We use simulated data to share the new insight that, not only the tumour growth rate but also the symptomatic tumour size will affect the sampling procedure, and thus be a part of the length bias through any link between tumour size and survival. We explain how this has a bearing on how observable breast cancer-specific survival curves should be interpreted. We also propose an approach for correcting survival comparisons for the length bias.
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| 8. |
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| 9. |
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| 10. |
- Ahmed, Syed Masud, et al.
(författare)
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Targeted intervention for the ultra poor in rural Bangladesh: Does it make any difference in their health-seeking behaviour?
- 2006
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Ingår i: Social Science & Medicine. - : Elsevier BV. - 0277-9536. ; 63:11, s. 2899-2911
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Tidskriftsartikel (refereegranskat)abstract
- It is now well recognised that regular microcredit intervention is not enough to effectively reach the ultra poor in rural Bangladesh, in fact it actively excludes them for structural reasons. A grants-based integrated intervention was developed (with health inputs to mitigate the income-erosion effect of illness) to examine whether such a targeted intervention could change the health-seeking behaviour of the ultra-poor towards greater use of health services and "formal allopathic" providers during illness, besides improving their poverty status and capacity for health expenditure. The study was carried out in three northern districts of Bangladesh with high density of ultra poor households, using a pre-test/post-test control group design. A pre-intervention baseline (2189 interventions and 2134 controls) survey was undertaken in 2002 followed by an intervention (of 18 months duration) and a post-intervention follow-up survey of the same households in 2004. Structured interviews were conducted to elicit information on health-seeking behaviour of household members. Findings reveal an overall change in health-seeking behaviour in the study population, but the intervention reduced self-care by 7 percentage units and increased formal allopathic care by 9 percentage units. The intervention increased the proportion of non-deficit households by 43 percentage units, as well as the capacity to spend more than Tk. 25 for treatment of illness during the reference period by 11 percentage units. Higher health expenditure and time (pre- to -post-intervention period) was associated with increased use of health care from formal allopathic providers. However, gender differences in health-seeking and health-expenditure disfavouring women were also noted. The programmatic implications of these findings are discussed in the context of improving the ability of health systems to reach the ultra poor.
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