[Show abstract][Hide abstract] ABSTRACT: Current pooling rules for multiply imputed data assume infinite populations.
In some situations this assumption is not feasible as every unit in the
population has been observed, potentially leading to over-covered population
estimates. We simplify the existing pooling rules for situations where the
sampling variance is not of interest. We compare these rules to the
conventional pooling rules and demonstrate their use in a situation where there
is no sampling variance. Using the standard pooling rules in situations where
sampling variance should not be considered, leads to overestimation of the
variance of the estimates of interest, especially when the amount of
missingness is not very large. As a result, populations estimates are
over-covered, which may lead to a loss of statistical power. We conclude that
the theory of multiple imputation can be extended to the situation where the
sample happens to be the population. The simplified pooling rules can be easily
implemented to obtain valid inference in cases where we have observed
essentially all units and in simulation studies addressing the missingness
[Show abstract][Hide abstract] ABSTRACT: Although children both at the upper and lower tail of the body mass index (BMI) distribution are at greater health risk, relatively little is known about the development of thinness prevalence rates in developed countries over time. We studied trends in childhood thinness and assessed changes in the BMI distribution since the onset of the obesity epidemic.
[Show abstract][Hide abstract] ABSTRACT: Abstract Background: People from Asian populations are generally shorter than other ethnic groups. It is unknown if current universal height references are suitable for affluent South Asian children in the Netherlands. Aims: To develop height-for-age charts for contemporary South Asian children aged 0-20 years living in the Netherlands, to evaluate secular trends, and to compare the charts with current Asian Indian, Dutch and WHO references. Subjects and methods: A population-based study measured 3315 South Asian children aged 0-20 years between 2007-2010. Among this cohort, 6876 measurements were taken. Another 7388 measurements were taken of a historical cohort of 1078 children born between 1974-1976 (aged 0-18 years). Results: An upward trend in height was observed for South Asian children living in the Netherlands between 1992-2010. The height-for-age charts of the South Asian historical cohort were similar to current Asian Indian charts. South Asian children in the Netherlands were shorter than their Dutch contemporaries at every age; and these differences increased further during adolescence. Compared to the WHO height-for-age references, there were considerable discrepancies in height, with curves intersecting twice. Conclusion: The discrepancies between the South Asian and Dutch and WHO height-for-age references indicate differences in growth patterns between the source populations.
Annals of Human Biology 06/2014; · 1.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The association between poverty, malnutrition, illness and poor socioeconomic conditions on the one side, and poor growth and short adult stature on the other side, is well recognized. Yet, the simple assumption by implication that poor growth and short stature result from poor living conditions, should be questioned. Recent evidence on the impact of the social network on adolescent growth and adult height further challenges the traditional concept of growth being a mirror of health. Twenty-nine scientists met at Glücksburg castle, Northern Germany, November 15th - 17th 2013, to discuss genetic, endocrine, mathematical and psychological aspects and related issues, of child and adolescent growth and final height.
Pediatric endocrinology reviews: PER 03/2014; 11(3):341-53.
[Show abstract][Hide abstract] ABSTRACT: Multiple imputation methods properly account for the uncertainty of missing data. One of those methods for creating multiple imputations is predictive mean matching (PMM), a general purpose method. Little is known about the performance of PMM in imputing non-normal semicontinuous data (skewed data with a point mass at a certain value and otherwise continuously distributed). We investigate the performance of PMM as well as dedicated methods for imputing semicontinuous data by performing simulation studies under univariate and multivariate missingness mechanisms. We also investigate the performance on real-life datasets. We conclude that PMM performance is at least as good as the investigated dedicated methods for imputing semicontinuous data and, in contrast to other methods, is the only method that yields plausible imputations and preserves the original data distributions.
[Show abstract][Hide abstract] ABSTRACT: It is known that height and body mass index (BMI) are correlated in childhood. However, its impact on the (trend of) national prevalence rates of overweight and obesity has never been investigated. The aim of our study is to investigate the relation between height and national prevalence rates of overweight and obesity in childhood between 1980, 1997, and 2009, and to calculate which fixed value of p (2.0,2.1, …,3.0) in kg/m(p) during childhood is most accurate in predicting adult overweight.
Cross-sectional growth data of children from three Dutch nationwide surveys in 1980, 1997, and 2009, and longitudinal data from the Terneuzen Birth Cohort and the Harpenden Growth Study were used. Relative risks (RR) and 95% confidence intervals (CI) were calculated. Our study showed that tall (>1 standard deviation (SD)) girls aged 5.0-13.9 y were more often overweight (RR = 3.5,95%CI:2.8-4.4) and obese (RR = 3.9,95%CI:2.1-7.4) than short girls (<-1 SD). Similar results were found in boys aged 5.0-14.9 y (RR = 4.4,95%CI:3.4-5.7 and RR = 5.3,95%CI:2.6-11.0). No large differences were found in the other age groups and in comparison with children with an average stature. Tall boys aged 2.0-4.9 y had a significantly higher positive trend in overweight between 1980 and 1997 compared to short boys (RR = 4.0,95%CI:1.38-11.9). For other age groups and in girls, no significant trends were found. The optimal Area Under the Curve (AUC) to predict adult overweight was found for p = 2.0.
Tall girls aged 5.0-13.9y and tall boys aged 5.0-14.9y have much higher prevalence rates of overweight and obesity than their shorter peers. We suggest taking into account the impact of height when evaluating trends and variations of BMI distributions in childhood, and to use BMI to predict adult overweight.
PLoS ONE 01/2014; 9(1):e85769. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Morbid obesity can be a life threatening condition. The aim of our study is to assess the trend in morbid obesity in The Netherlands among children of Dutch origin since 1980, and among children of Turkish and Moroccan origin since 1997.
Cross-sectional height and weight data of children of Dutch, Turkish and Moroccan origin aged 2-18 years were selected from three national Dutch Growth Studies performed in 1980, 1997 and 2009 (n = 54,814). Extended international (IOTF) cut-offs in childhood were used to define morbid obesity (obesity class II and III combined). The morbidity index for overweight was calculated as the prevalence of morbid obesity divided by the prevalence of overweight. Our study showed that the prevalence of morbid obesity in children of Dutch origin was 0.59% in boys and 0.53% in girls in 2009. Significant upward trends occurred since 1980 and 1997. The prevalence was three to four fold higher in Turkish children compared to Dutch children. The Turkish children also had an upward trend since 1997, but this was only statistically significant in boys. The prevalence of morbid obesity in Moroccan children was two to three fold higher than in Dutch children, but it remained almost stable between 1997 and 2009. The Dutch and Turkish children showed an upward trend in morbidity index for overweight since respectively 1980 and 1997, while the Moroccan children showed a downward trend since 1997. In 2009, children of low educated parents had the highest prevalence rates of morbid obesity; 1.06% in Dutch, 2.11% in Turkish and 1.41% in Moroccan children.
An upward trend of morbid obesity in Dutch and Turkish children in The Netherlands occurred. Monitoring and reducing the prevalence of childhood morbid obesity is of high importance for these children, health care and the community.
PLoS ONE 01/2014; 9(4):e94299. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Standard approaches to implement multiple imputation do not automatically incorporate nonlinear relations like interaction effects. This leads to biased parameter estimates when interactions are present in a dataset. With the aim of providing an imputation method which preserves interactions in the data automatically, the use of recursive partitioning as imputation method is examined. Three recursive partitioning techniques are implemented in the multiple imputation by chained equations framework. It is investigated, using simulated data, whether recursive partitioning creates appropriate variability between imputations and unbiased parameter estimates with appropriate confidence intervals. It is concluded that, when interaction effects are present in a dataset, substantial gains are possible by using recursive partitioning for imputation compared to standard applications. In addition, it is shown that the potential of recursive partitioning imputation approaches depends on the relevance of a possible interaction effect, the correlation structure of the data, and the type of possible interaction effect present in the data.
[Show abstract][Hide abstract] ABSTRACT: Recently, the Individual Work Performance Questionnaire (IWPQ) version 0.2 was developed using Rasch analysis. The goal of the current study was to improve targeting of the IWPQ scales by including additional items. The IWPQ 0.2 (original) and 0.3 (including additional items) were examined using Rasch analysis. Additional items that showed misfit or did not improve targeting were removed from the IWPQ 0.3, resulting in a final IWPQ 1.0. Subsequently, the scales showed good model fit and reliability, and were examined for key measurement requirements (e.g., category ordening, unidimensionality, and differential item functioning). Finally, calculation and interpretability of scores were addressed. Compared to its previous version, the final IWPQ 1.0 showed improved targeting for two out of three scales. As a result, it can more reliably measure workers at all levels of ability, discriminate between workers at a wider range on each scale, and detect changes in individual work performance.
Journal of applied measurement 01/2014; 15(2):160-75.
[Show abstract][Hide abstract] ABSTRACT: Objective: Many health-promoting interventions combine multiple behavior change techniques (BCTs) to maximize effectiveness. Although, in theory, BCTs can amplify each other, the available meta-analyses have not been able to identify specific combinations of techniques that provide synergistic effects. This study overcomes some of the shortcomings in the current methodology by applying classification and regression trees (CART) to meta-analytic data in a special way, referred to as Meta-CART. The aim was to identify particular combinations of BCTs that explain intervention success. Method: A reanalysis of data from Michie, Abraham, Whittington, McAteer, and Gupta (2009) was performed. These data included effect sizes from 122 interventions targeted at physical activity and healthy eating, and the coding of the interventions into 26 BCTs. A CART analysis was performed using the BCTs as predictors and treatment success (i.e., effect size) as outcome. A subgroup meta-analysis using a mixed effects model was performed to compare the treatment effect in the subgroups found by CART. Results: Meta-CART identified the following most effective combinations: Provide information about behavior-health link with Prompt intention formation (mean effect size ḡ = 0.46), and Provide information about behavior-health link with Provide information on consequences and Use of follow-up prompts (ḡ = 0.44). Least effective interventions were those using Provide feedback on performance without using Provide instruction (ḡ = 0.05). Conclusions: Specific combinations of BCTs increase the likelihood of achieving change in health behavior, whereas other combinations decrease this likelihood. Meta-CART successfully identified these combinations and thus provides a viable methodology in the context of meta-analysis. (PsycINFO Database Record (c) 2013 APA, all rights reserved).
[Show abstract][Hide abstract] ABSTRACT: South Asian babies born in developed countries are generally lighter than babies from other ethnic groups born in the same country. While the mean birth weight of Caucasian babies in the Netherlands has increased the past decades, it is unknown if the mean birth weight of South Asian babies born in the Netherlands has increased or if the prevalence of low birth weight (LBW) or small-for-gestational-age (SGA) has decreased.The aims of this study are: 1. to investigate secular changes in mean birth weight and the prevalence of LBW and SGA in Surinamese South Asian babies, and 2. to assess differences between Surinamese South Asian and Dutch Caucasian neonates born 2006--2009.
A population based study for which neonatal characteristics of 2014 Surinamese South Asian babies, born between 1974 and 2009 in the Netherlands, and 3104 Dutch Caucasian babies born 2006--2009 were obtained from well-baby clinic records. LBW was defined as a birth weight <2500g. SGA was based on a universal population standard (the Netherlands) and three ethnic specific standards (the Netherlands, UK, Canada).
In Surinamese South Asian babies from 1974 to 2009 no secular trend in mean birth weight and prevalence of LBW was found, whereas SGA prevalence decreased significantly.Surinamese South Asian babies born in 2006--2009 (2993g; 95%CI 2959-3029g) were 450g lighter than Dutch Caucasian babies (3448g; 95%CI 3429-3468g), while LBW and SGA prevalences, based on universal standards, were three times higher. Application of ethnic specific standards from the Netherlands and the UK yielded SGA rates in Surinamese South Asian babies that were similar to Dutch. There were considerable differences between the standards used.
Since 1974, although the mean birth weight of Surinamese South Asian babies remained unchanged, they gained a healthier weight for their gestational age.
BMC Public Health 10/2013; 13(1):931. · 2.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To develop a reference chart to monitor inspiratory muscle strength during pre-operative inspiratory muscle training for patients at high risk of developing postoperative pulmonary complications awaiting coronary artery bypass graft (CABG) surgery.
Secondary data analysis using patients from the intervention arm of a randomised clinical trial.
University medical centre.
Patients at high risk of developing postoperative pulmonary complications awaiting CABG surgery.
Patients performed inspiratory muscle training seven times per week for at least 2 weeks before surgery.
Maximal inspiratory muscle strength.
A new reference chart was produced using a non-linear time trend model with a normal error structure.
The chart is a novel tool for monitoring the progress of inspiratory muscle training for physiotherapy practice. Wider use of this chart is recommended.
[Show abstract][Hide abstract] ABSTRACT: Missing values are a practical issue in the analysis of longitudinal data. Multiple imputation (MI) is a well-known likelihood-based method that has optimal properties in terms of efficiency and consistency if the imputation model is correctly specified. Doubly robust (DR) weighing-based methods protect against misspecification bias if one of the models, but not necessarily both, for the data or the mechanism leading to missing data is correct. We propose a new imputation method that captures the simplicity of MI and protection from the DR method. This method integrates MI and DR to protect against misspecification of the imputation model under a missing at random assumption. Our method avoids analytical complications of missing data particularly in multivariate settings, and is easy to implement in standard statistical packages. Moreover, the proposed method works very well with an intermittent pattern of missingness when other DR methods can not be used. Simulation experiments show that the proposed approach achieves improved performance when one of the models is correct. The method is applied to data from the fireworks disaster study, a randomized clinical trial comparing therapies in disaster-exposed children. We conclude that the new method increases the robustness of imputations.
British Journal of Mathematical and Statistical Psychology 08/2013; · 1.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: BACKGROUND: The steep ramp test (SRT) is a feasible, reliable, and valid exercise test on a cycle ergometer that may be more appealing for use in children in daily clinical practice than the traditional cardiopulmonary exercise test, because of its short duration, its resemblance to children's daily activity pattern and the fact that it does not require respiratory gas analysis. OBJECTIVE: The aim of the current study was to provide sex- and age-related norm values for SRT performance in healthy Dutch Caucasian children and adolescents between the ages of 8 and 19 years. DESIGN: This was a cross-sectional, observational study. METHODS: Two hundred and fifty-two Dutch Caucasian children and adolescents, 118 boys (mean age 13.4 (3.0) years) and 134 girls (mean age 13.4 (2.9) years), performed a SRT (work rate increments of 10, 15, or 20 W·10 s(-1), depending on body height) to voluntary exhaustion to assess peak work rate (WRpeak). Norm values are presented as reference centiles developed using generalized additive models for location, scale, and shape (GAMLSS). RESULTS: WRpeak correlated highly with age (r=0.915 and r=0.811), body mass (r=0.870 and r=0.850), body height (r=0.922 and r=0.896), body surface area (r=0.906 and r=0.885), and fat free mass (r=0.930 and r=0.902), for boys and girls respectively (P<0.001 for all coefficients). The reference curves demonstrated an almost linear increase with age in WRpeak in boys, even when normalized for body mass. In contrast, absolute WRpeak in girls increased constantly until the age of approximately 13 years, where after WRpeak started to level off. WRpeak normalized for body mass showed only a slight increase with age in girls, with a slight decrease in relative WRpeak as of the age of 14 years. LIMITATIONS: The sample may not be entirely representative of the Dutch population. CONCLUSIONS: The current study provides sex- and age-related norm values for SRT performance for both absolute and relative WRpeak thereby facilitating the interpretation of SRT results for clinicians and researchers.
[Show abstract][Hide abstract] ABSTRACT: The goal of growth hormone (GH) treatment in a short child is to attain a fast catch-up growth toward the target height (TH) standard deviation score (SDS), followed by a maintenance phase, a proper pubertal height gain, and an adult height close to TH. The short-term response variable of GH treatment, first-year height velocity (HV) (cm/year or change in height SDS), can either be compared with GH response charts for diagnosis, age and gender, or with predicted HV based on prediction models. Three types of prediction models have been described: the Kabi International Growth Hormone Study models, the Gothenburg models and the Cologne model. With these models, 50-80% of the variance could be explained. When used prospectively, individualized dosing reduces the variation in growth response in comparison with a fixed dose per body weight. Insulin-like growth factor-I-based dose titration also led to a decrease in the variation. It is uncertain whether adding biochemical, genetic or proteomic markers may improve the accuracy of the prediction. Prediction models may lead to a more evidence-based approach to determine the GH dose regimen and may reduce the drug costs for GH treatment. There is a need for user-friendly software programs to make prediction models easily available in the clinic.
Hormone Research in Paediatrics 05/2013; · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In het Terneuzen Onderzoek naar Preventie zijn overgewicht en cardiometabole risicofactoren op jongvolwassen leeftijd bestudeerd in relatie tot de veranderingen in body mass index (BMI) tussen de geboorte en 18-jarige leeftijd. Deze studie is gebaseerd op het Terneuzen Geboorte Cohort, waarbij prospectief verkregen gegevens zijn verkregen via de jeugdgezondheidszorg (JGZ) van GGD Zeeland. Bij de analyses is gebruik gemaakt van de brokenstickmethode en van lineaire en logistische regressieanalyses. Het leeftijdsinterval 2-6 jaar was het meest voorspellend voor overgewicht en de meeste cardiometabole uitkomsten op jongvolwassen leeftijd. Deze resultaten vragen speciale aandacht van de JGZ voor een stijgende BMI-standaarddeviatiescore (SDS) gedurende het leeftijdsinterval 2-6 jaar, ook als er nog geen sprake is van overgewicht. Hierbij kunnen predictie-instrumenten nuttig zijn. Het monitoren en stabiliseren van de BMI SDS gedurende dit leeftijdsinterval draagt waarschijnlijk niet alleen bij aan de preventie van overgewicht op jongvolwassen leeftijd, maar ook aan een goede cardiometabole gezondheidstoestand op latere leeftijd.
JGZ Tijdschrift voor jeugdgezondheidszorg. 04/2013; 45(2).
[Show abstract][Hide abstract] ABSTRACT: This article reviews and compares two types of growth charts for tracking human development over age. Both charts assume the existence of a continuous latent variable, but relate to the observed data in different ways. The D-score diagram summarizes developmental indicators into a single aggregate score measuring global development. The relations between the indicators should be consistent with the Rasch model. If true, the D-score is a measure with interval scale properties, and allows for the calculation of meaningful differences both within and across age. The stage line diagram describes the natural development of ordinal indicators. The method models the transition probabilities between successive stages of the indicator as smoothly varying functions of age. The location of each stage is quantified by the mid-P-value. Both types of diagrams assist in identifying early and delayed development, as well as finding differences in tempo. The relevant techniques are illustrated to track global development during infancy and early childhood (0-2 years) and Tanner pubertal stages (8-21 years). New reference values for both applications are provided.
Statistical Methods in Medical Research 03/2013; · 2.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An important goal of growth monitoring is to identify genetic disorders, diseases or other conditions that manifest themselves through an abnormal growth. The two main conditions that can be detected by height monitoring are Turner's syndrome and growth hormone deficiency. Conditions or risk factors that can be detected by monitoring weight or body mass index include hypernatremic dehydration, celiac disease, cystic fibrosis and obesity. Monitoring infant head growth can be used to detect macrocephaly, developmental disorder and ill health in childhood. This paper describes statistical methods to obtain evidence-based referral criteria in growth monitoring. The referral criteria that we discuss are based on either anthropometric measurement(s) at a fixed age using (1) a Centile or a Standard Deviation Score, (2) a Standard Deviation corrected for parental height, (3) a Likelihood Ratio Statistic and (4) an ellipse, or on multiple measurements over time using (5) a growth rate and (6) a growth curve model. We review the potential uses of these methods, and outline their strengths and limitations.
Statistical Methods in Medical Research 02/2013; · 2.36 Impact Factor