Math Candel

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Probabilistic feature models for unfolding

een wetenschappelijke proeve op het gebied van de sociale wetenschappen, in het bijzonder de psyc... more een wetenschappelijke proeve op het gebied van de sociale wetenschappen, in het bijzonder de psychologie Proefschrift ter verkrijging van de graad van doctor aan de Katholieke Universiteit Nijmegen, volgens besluit van het college van decanen in het openbaar te verdedigen op maandag 4 november 1991, des namiddags te 3.30 uur door

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Maximin Efficiencies under Treatment-Dependent Costs and Outcome Variances for Parallel, AA/BB, and AB/BA Designs

Computational and Mathematical Methods in Medicine, 2018

If there are no carryover effects, AB/BA crossover designs are more efficient than parallel (A/B)... more If there are no carryover effects, AB/BA crossover designs are more efficient than parallel (A/B) and extended parallel (AA/BB) group designs. This study extends these results in that (a) optimal instead of equal treatment allocation is examined, (b) allowance for treatment-dependent outcome variances is made, and (c) next to treatment effects, also treatment by period interaction effects are examined. Starting from a linear mixed model analysis, the optimal allocation requires knowledge on intraclass correlations in A and B, which typically is rather vague. To solve this, maximin versions of the designs are derived, which guarantee a power level across plausible ranges of the intraclass correlations at the lowest research costs. For the treatment effect, an extensive numerical evaluation shows that if the treatment costs of A and B are equal, or if the sum of the costs of one treatment and measurement per person is less than the remaining subject-specific costs (e.g., recruitment c...

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How to design and analyse cluster randomized trials with a small number of clusters? Comment on Leyrat et al

International Journal of Epidemiology, 2018

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Price minimizing behaviours by smokers in Europe (2006–20): evidence from the International Tobacco Control Project

European journal of public health, Oct 10, 2022

Background: Effectiveness of tobacco taxation can be undermined through smokers applying price-mi... more Background: Effectiveness of tobacco taxation can be undermined through smokers applying price-minimizing behaviours rather than quitting or reducing consumption. Common price-minimizing strategies are buying cheaper tobacco [discount brands or roll-your-own (RYO) tobacco], bulk buying and cross-border purchasing. This study analyses trends in and factors associated with such behaviours in four European countries from 2006 to 2020. Methods: Data came from adult smokers participating in the International Tobacco Control (ITC) Surveys conducted between 2006 and 2020 in England (9 waves, n ¼ 768-4149), France (4 waves, n ¼ 1415-1735), Germany (5 waves, n ¼ 513-1515) and the Netherlands (10 waves, n ¼ 1191-2177). Country-specific generalized estimating equation regression models were fit to assess trends in smoking RYO tobacco, discount brands, bulk buying and cross-border purchasing within the European Union. Results: Buying discount brands or RYO tobacco was the most common strategy in all countries, except France. Except for buying discount brands, estimates of price-minimizing behaviours were highest in France (2019: RYO ¼ 27.2%, discount brands ¼ 17.3%, bulk buying ¼ 34.1%, cross-border purchasing ¼ 34.2%), and lowest in Germany (2018: RYO ¼ 18.6%, discount brands ¼ 43.7%, bulk buying ¼ 8.0%, cross-border purchasing ¼ 9.8%). Direction and magnitude of trends differed by country, and behaviour. Young smokers were less likely to buy in bulk. Low-income and low-education smokers were more likely to purchase RYO tobacco or discount brands. The association with discount brands was not found for French low-income smokers. Conclusions: Smoking cheaper tobacco is the most prevalent price-minimizing strategy in three countries (England, Germany and Netherlands), and more prevalent among low-income individuals. Harmonizing prices across products and countries would reduce switching to cheaper tobacco.

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Repairing the efficiency loss due to varying cluster sizes in two-level two-armed randomized trials with heterogeneous clustering

Statistics in Medicine, 2016

Repairing the efficiency loss due to varying cluster sizes in two-level two-armed randomized tria... more

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SamP2CeT: an interactive computer program for sample size and power calculation for two-level cost-effectiveness trials

Computational Statistics, 2018

The cost-effectiveness of interventions (e.g. new medical therapies or health care technologies) ... more The cost-effectiveness of interventions (e.g. new medical therapies or health care technologies) is often evaluated in randomized clinical trials, where individuals are nested within clusters, for instance patients within general practices. In such two-level costeffectiveness trials, one can randomly assign treatments to individuals within clusters (multicentre trial) or to entire clusters (cluster randomized trial). Such trials need careful planning to evaluate the cost-effectiveness of interventions within the available research resources. The optimal number of clusters and the optimal number of subjects per cluster for both types of cost-effectiveness trials can be determined by using optimal design theory. However, the construction of the optimal design requires information on model parameters, which may be unknown at the planning stage of a trial. To overcome this problem, a maximin strategy is employed. We have developed a computer program SamP2CeT in R to perform these sample size calculations. SamP2CeT provides a graphical user interface which enables the researchers to optimize the numbers of clusters and subjects per cluster in their cost-effectiveness trial as a function of study costs and outcome variances. In case of insufficient knowledge on model parameters, SamP2CeT also provides safe numbers of clusters and subjects per cluster, based on a maximin strategy. SamP2CeT can be used to calculate the smallest budget needed for a user-specified power level, the largest power attainable with a userspecified budget, and also has the facility to calculate the power for a user-specified design. Recent methodological developments on sample size and power calculation for two-level cost-effectiveness trials have been implemented in SamP2CeT. This program is user-friendly, as illustrated for two published cost-effectiveness trials.

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Smokers’ strategies to reduce tobacco spending: self-reported use and differences across subgroups. Findings from the International Tobacco Control (ITC) Netherlands Survey

BMC Public Health, Apr 21, 2023

Background The cost of tobacco is one of the most reported reasons for smoking cessation. Rather ... more Background The cost of tobacco is one of the most reported reasons for smoking cessation. Rather than quitting, smokers can use also strategies to reduce tobacco expenditure while continuing smoking, such as smoking less or using price-minimising strategies. The Netherlands announced to increase the price of a pack cigarettes from seven (2018) to ten euros (2023), to reduce tobacco prevalence and consumption. This study explores the self-reported strategies to reduce tobacco spending among Dutch smokers, and whether this differed per age, income, and education. Additionally, we analysed among quitters in these subgroups whether price played a role in their decision to quit. Methods Cross-sectional survey data from the International Tobacco Control (ITC) Netherlands Wave 2 (September-November 2020, N = 1915) was used. Strategies to reduce spending among smokers (N = 1790) were: reducing consumption, bulk buying, switching to cheaper products or buying from low-taxed sources. These were collapsed into: reducing consumption (solely or in combination with other behaviours), solely price-minimising behaviours (such as buying cheaper brands), or no strategies to reduce spending. Associations between strategies and characteristics were analysed through multinomial and binary logistic regression models. Second, we explored which subgroups were more likely to report that price played a role in their decision to quit among quitters (N = 125). Results The majority of smokers used strategies to reduce tobacco spending: 35.6% reduced consumption and 19.3% used solely price-minimising strategies. 82.1% of quitters reported that price played a role in their decision to quit. Low-income individuals were more likely to report price as a reason for quitting and reduce consumption, but also to buy cheaper products. Highly nicotine dependent smokers were more likely to use price-minimising behaviours, and less likely to reduce consumption. Conclusions The majority reported using strategies to reduce spending or that price played a role in their decision to quit. Reducing consumption was the most reported strategy. Low-income smokers were more likely to reportedly

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Sample size calculation and optimal design for regression-based norming of tests and questionnaires

Psychological Methods, 2021

To prevent mistakes in psychological assessment, the precision of test norms is important. This c... more To prevent mistakes in psychological assessment, the precision of test norms is important. This can be achieved by drawing a large normative sample and using regression-based norming. Based on that norming method, a procedure for sample size planning to make inference on Z-scores and percentile rank scores is proposed. Sampling variance formulas for these norm statistics are derived and used to obtain the optimal design, that is, the optimal predictor distribution, for the normative sample, thereby maximizing precision of estimation. This is done under five regression models with a quantitative and a categorical predictor, differing in whether they allow for interaction and nonlinearity. Efficient robust designs are given in case of uncertainty about the regression model. Furthermore, formulas are provided to compute the normative sample size such that individuals' positions relative to the derived norms can be assessed with prespecified power and precision. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

An integrated approach towards behavior change: How conscious and unconscious cognitions jointly determine (reduced) red meat intake

Exploring the gateway hypothesis of e-cigarettes and tobacco: a prospective replication study among adolescents in the Netherlands and Flanders

Tobacco Control, 2021

BackgroundStudies demonstrated that adolescent e-cigarette use is associated with subsequent toba... more BackgroundStudies demonstrated that adolescent e-cigarette use is associated with subsequent tobacco smoking, commonly referred to as thegateway effect. However, most studies only investigated gateways from e-cigarettes to tobacco smoking. This study replicates a cornerstone study revealing a positive association between both adolescent e-cigarette use and subsequent tobacco use; and tobacco and subsequent e-cigarette use in the Netherlands and Flanders.DesignThe longitudinal design included baseline (n=2839) and 6-month (n=1276) and 12-month (n=1025) follow-up surveys among a school-based cohort (mean age: 13.62). Ten high schools were recruited as a convenience sample. The analyses involved (1) associations of baseline e-cigarette use and subsequent tobacco smoking among never smokers; (2) associations of e-cigarette use frequency at baseline and tobacco smoking frequency at follow-up; and (3) the association of baseline tobacco smoking and subsequent e-cigarette use among non-use...

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Factors Associated with Primary Health Care Providers’ Alcohol Screening Behavior in Colombia, Mexico and Peru

Substance Abuse, 2021

Background Screening for unhealthy alcohol use in routine consultations can aid primary health ca... more Background Screening for unhealthy alcohol use in routine consultations can aid primary health care (PHC) providers in detecting patients with hazardous or harmful consumption and providing them with appropriate care. As part of larger trial testing strategies to improve implementation of alcohol screening in PHC, this study investigated the motivational (role security, therapeutic commitment, self-efficacy) and organizational context (leadership, work culture, resources, monitoring, community engagement) factors that were associated with the proportion of adult patients screened with AUDIT-C by PHC providers in Colombia, Mexico and Peru. Additionally, the study investigated whether the effect of the factors interacted with implementation strategies and the country. Methods Pen-and-paper questionnaires were completed by 386 providers at the start of their study participation (79% female, Mage = 39.5, 37.6% doctors, 15.0% nurses, 9.6% psychologists, 37.8% other professional roles). T...

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Optimal two-stage sampling for mean estimation in multilevel populations when cluster size is informative

Statistical Methods in Medical Research, 2020

To estimate the mean of a quantitative variable in a hierarchical population, it is logistically ... more To estimate the mean of a quantitative variable in a hierarchical population, it is logistically convenient to sample in two stages (two-stage sampling), i.e. selecting first clusters, and then individuals from the sampled clusters. Allowing cluster size to vary in the population and to be related to the mean of the outcome variable of interest (informative cluster size), the following competing sampling designs are considered: sampling clusters with probability proportional to cluster size, and then the same number of individuals per cluster; drawing clusters with equal probability, and then the same percentage of individuals per cluster; and selecting clusters with equal probability, and then the same number of individuals per cluster. For each design, optimal sample sizes are derived under a budget constraint. The three optimal two-stage sampling designs are compared, in terms of efficiency, with each other and with simple random sampling of individuals. Sampling clusters with pr...

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How Financial Incentives Increase Smoking Cessation: A Two-Level Path Analysis

Nicotine & Tobacco Research, 2020

Introduction Financial incentives effectively increase smoking cessation rates, but it is unclear... more Introduction Financial incentives effectively increase smoking cessation rates, but it is unclear via which psychological mechanisms incentives influence quit behavior. The current study examines how receiving financial incentives for smoking cessation leads to quitting smoking and investigates several mediators and moderators of that relationship. Aims and Methods The study sample consisted of 604 tobacco-smoking employees from 61 companies in the Netherlands who completed a baseline and follow-up questionnaire. The current study is a secondary analysis from a cluster randomized trial where employees received smoking cessation group counseling at the workplace. Participants in the intervention group additionally received financial incentives of €350 in total for 12-month continuous smoking abstinence. We used a two-level path analysis to test a model that assesses the effects of financial incentives through smoking cessation program evaluation, medication use, nicotine replacement ...

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An overview of mixture modelling for latent evolutions in longitudinal data: Modelling approaches, fit statistics and software

Advances in Life Course Research, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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How the New European Union’s (Pictorial) Tobacco Health Warnings Influence Quit Attempts and Smoking Cessation: Findings from the 2016–2017 International Tobacco Control (ITC) Netherlands Surveys

International Journal of Environmental Research and Public Health, 2019

In 2016, the Netherlands was required to introduce new European Union (EU)’s (pictorial) tobacco ... more In 2016, the Netherlands was required to introduce new European Union (EU)’s (pictorial) tobacco health warnings. Our objective was to describe the pathways through which the new EU tobacco health warnings may influence quit attempts and smoking cessation among Dutch smokers. Longitudinal data from 2016 and 2017 from the International Tobacco Control (ITC) Netherlands Survey were used. Smokers who participated in both surveys were included (N = 1017). Structural equation modeling was applied to examine the hypothesized pathways. Health warning salience was positively associated with more health worries (β = 0.301, p < 0.001) and a more positive attitude towards quitting (β = 0.180, p < 0.001), which, in turn, were associated with a stronger quit intention (health worries: β = 0.304, p < 0.001; attitude: β = 0.340, p < 0.001). Quit intention was a strong predictor of quit attempts (β = 0.336, p = 0.001). Health warning salience was also associated with stronger perceived ...

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Efficient design of cluster randomized trials with treatment‐dependent costs and treatment‐dependent unknown variances

Statistics in Medicine, 2018

Cluster randomized trials evaluate the effect of a treatment on persons nested within clusters, w... more Cluster randomized trials evaluate the effect of a treatment on persons nested within clusters, where treatment is randomly assigned to clusters. Current equations for the optimal sample size at the cluster and person level assume that the outcome variances and/or the study costs are known and homogeneous between treatment arms. This paper presents efficient yet robust designs for cluster randomized trials with treatment‐dependent costs and treatment‐dependent unknown variances, and compares these with 2 practical designs. First, the maximin design (MMD) is derived, which maximizes the minimum efficiency (minimizes the maximum sampling variance) of the treatment effect estimator over a range of treatment‐to‐control variance ratios. The MMD is then compared with the optimal design for homogeneous variances and costs (balanced design), and with that for homogeneous variances and treatment‐dependent costs (cost‐considered design). The results show that the balanced design is the MMD if...

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The effectiveness of an interactive organ donation education intervention for Dutch lower-educated students: a cluster randomized controlled trial

Background: Organ donation registration rates remain low, especially among people with lower educ... more Background: Organ donation registration rates remain low, especially among people with lower educational levels. An interactive educational intervention was developed to prepare lower-educated students in the Netherlands for making a well-informed decision about organ donation. This article reports on the effects of this intervention on the intention to register (i.e. the primary outcome in the study at hand) and beliefs regarding organ donation.Methods: The effectiveness was investigated in a post-test only cluster randomized controlled trial, in which the intervention was offered to the experimental group and after measurement also to the control group. Randomization to the experimental and control groups took place at a class level. Teachers of schools for intermediate vocational education who taught a course on Citizenship delivered three intervention elements (i.e. video fragments and discussion, quizzes with tailored feedback and exercise filling out a registration form) to th...

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Relative efficiencies of two‐stage sampling schemes for mean estimation in multilevel populations when cluster size is informative

Statistics in Medicine, 2018

In multilevel populations, there are two types of population means of an outcome variable ie, the... more In multilevel populations, there are two types of population means of an outcome variable ie, the average of all individual outcomes ignoring cluster membership and the average of cluster‐specific means. To estimate the first mean, individuals can be sampled directly with simple random sampling or with two‐stage sampling (TSS), that is, sampling clusters first, and then individuals within the sampled clusters. When cluster size varies in the population, three TSS schemes can be considered, ie, sampling clusters with probability proportional to cluster size and then sampling the same number of individuals per cluster; sampling clusters with equal probability and then sampling the same percentage of individuals per cluster; and sampling clusters with equal probability and then sampling the same number of individuals per cluster. Unbiased estimation of the average of all individual outcomes is discussed under each sampling scheme assuming cluster size to be informative. Furthermore, th...

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Support for a point-of-sale cigarette display ban among smokers: findings from the international tobacco control (ITC) Netherlands survey

BMC Public Health, 2018

Background: Displaying tobacco products at point-of-sale (PoS) has become an important marketing ... more Background: Displaying tobacco products at point-of-sale (PoS) has become an important marketing strategy for the tobacco industry. This study was designed to (1) examine how support for a PoS cigarette display ban changed among Dutch smokers between 2010 and 2015 and (2) identify the variables that predict support among smokers for a PoS cigarette display ban. Methods: Longitudinal data from six annual survey waves (2010-2015) from the International Tobacco Control (ITC) Netherlands Survey were analyzed. The sample consisted of between 1279 and 1800 smokers per year. Smokers were asked whether they supported a complete ban on displays of cigarettes inside shops and stores. Results: Support for a PoS cigarette display ban increased from 28.9% in 2010 to 42.5% in 2015 (OR = 1.40, p < 0.001). A multiple logistic regression analysis revealed that support for a PoS display ban of cigarettes was more likely among smokers who had more knowledge about the health risks of smoking (OR = 3.97, p < 0.001), believed smoking-related health risks to be severe (OR = 1.39, p < 0.001), had a more positive attitude towards quitting smoking (OR = 1.44, p = 0. 006), reported stronger social norms to quit smoking (OR = 1.29, p = 0.035), had a higher self-efficacy for quitting smoking (OR = 1.31, p = 0.001), and had stronger intentions to quit smoking (OR = 1.23, p = 0.006). Conclusions: This paper showed that support for a PoS display ban of cigarettes increased among smokers in the Netherlands over the years. To further increase support, educational campaigns about the dangers of smoking, and campaigns that encourage quitting may be needed.

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Stages of Change Model has Limited Value in Explaining the Change in Use of Cannabis among Adolescent Participants in an Efficacious Motivational Interviewing Intervention

Journal of psychoactive drugs, Jan 26, 2017

Previously, a Dutch randomized controlled trial evaluating an intervention aimed at changing adol... more Previously, a Dutch randomized controlled trial evaluating an intervention aimed at changing adolescents' cannabis use, called Moti-4, has shown its efficacy. A secondary analysis of the Moti-4 data investigated the process of change specified by the Stage of Change (SOC) model in cannabis use during the trial. Seventy-one Moti-4 participants and 60 controls were recruited for the study with a pre-test, post-test (T1), and six-month follow-up (T2). All participants showed signs of problematic cannabis use. No contribution of the Moti-4 intervention to a change in SOC between T1 and T2 was found. Although motivation for treatment and motivation for change can be conceived as independent predictors of treatment outcome, the SOC a person is in does not mediate the effect of the intervention on change in cannabis use. However, a reduction in cannabis use was associated with a positive change in "action willingness," in line with the SOC model. In contrast to model expectat...

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