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ABSTRACT: In experimental research, it is not uncommon to assign clusters to conditions. When analysing the data of such cluster-randomized trials, a multilevel analysis should be applied in order to take into account the dependency of first-level units (i.e., subjects) within a second-level unit (i.e., a cluster). Moreover, the multilevel analysis can handle covariates on both levels. If a first-level covariate is involved, usually the within-cluster effect of this covariate will be estimated, implicitly assuming the contextual effect to be equal. However, this assumption may be violated. The focus of the present simulation study is the effects of ignoring the inequality of the within-cluster and contextual covariate effects on parameter and standard error estimates of the treatment effect, which is the parameter of main interest in experimental research. We found that ignoring the inequality of the within-cluster and contextual effects does not affect the estimation of the treatment effect or its standard errors. However, estimates of the variance components, as well as standard errors of the constant, were found to be biased.
Behavior Research Methods 04/2011; 43(4):1003-13. · 2.12 Impact Factor
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ABSTRACT: This systematic review and meta-analysis examined whether implementation intentions are an effective tool to help people put their intentions to eat a healthy diet into practice. Additionally, it was investigated whether the quality of the outcome measures and the quality of the control conditions that are used in these studies influence implementation intentions' effectiveness.
Twenty three empirical studies investigating the effect of implementation intentions on eating behavior were included. In assessing the empirical evidence, a distinction was made between studies that aim to increase healthy eating (i.e., eating more fruits) and studies that aim to diminish unhealthy eating (i.e., eating fewer unhealthy snacks).
Implementation intentions are an effective tool for promoting the inclusion of healthy food items in one's diet (Cohen's d=.51), but results for diminishing unhealthy eating patterns are less strong (Cohen's d=.29). For studies aiming to increase healthy eating, it was found that higher quality outcome measures and lower quality control conditions tended to yield stronger effects.
Implementation intentions are somewhat more effective in promoting healthy eating than in diminishing unhealthy eating, although for some studies promoting healthy eating effect sizes may have been inflated due to less than optimal control conditions.
Appetite 11/2010; 56(1):183-93. · 2.59 Impact Factor
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ABSTRACT: Multilevel structural equation modeling (multilevel SEM) has become an established method to analyze multilevel multivariate data. The first useful estimation method was the pseudobalanced method. This method is approximate because it assumes that all groups have the same size, and ignores unbalance when it exists. In addition, full information maximum likelihood (ML) estimation is now available, which is often combined with robust chi-squares and standard errors to accommodate unmodeled heterogeneity (MLR). In addition, diagonally weighted least squares (DWLS) methods have become available as estimation methods. This article compares the pseudobalanced estimation method, ML(R), and two DWLS methods by simulating a multilevel factor model with unbalanced data. The simulations included different sample sizes at the individual and group levels and different intraclass correlation (ICC). The within-group part of the model posed no problems. In the between part of the model, the different ICC sizes had no effect. There is a clear interaction effect between number of groups and estimation method. ML reaches unbiasedness fastest, then the two DWLS methods, then MLR, and then the pseudobalanced method (which needs more than 200 groups). We conclude that both ML(R) and DWLS are genuine improvements on the pseudobalanced approximation. With small sample sizes, the robust methods are not recommended.
Statistica Neerlandica 04/2010; 64(2):157 - 170. · 0.50 Impact Factor
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ABSTRACT: A daily diary method was used to examine the daily dynamics of adolescent conflict and perceived relationship satisfaction with mothers, fathers, and best friends among a sample of 72 Dutch adolescents (M = 15.59 years). Multilevel analyses revealed that perceived relationship satisfaction with mothers, fathers, and best friends was lower on days on which conflict occurred with mothers, fathers, and best friends than on days on which no conflict occurred. More specifically, perceived relationship satisfaction was highest in a particular relationship on days when no conflict occurred, second highest on days on which constructive conflict occurred, and lowest on days on which unconstructive conflict occurred. Whereas in adolescents' relationships with their parents, conflict and perceived relationship satisfaction were not found to be related to each other one day later, conflict with their best friends-and especially unconstructive conflict-was found to be related to higher perceived relationship satisfaction one day later.
Journal of Youth and Adolescence 08/2009; 38(6):790-803. · 2.72 Impact Factor
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ABSTRACT: This ex post facto prospective study investigated whether symptoms of posttraumatic stress disorder (PTSD) are associated with increased physical health problems, while controlling for base-rates of symptoms and individual differences in neuroticism. Dutch army soldiers completed standardized questionnaires before theywere deployed to Iraq (n = 479), and about 5 months (n = 382; 80%) and 15 months (n = 331; 69%) after their return home. PTSD-symptoms were evaluated by questionnaires and clinical interviews. The results showed that, on average, participants with PTSD at 5 months re ported increased physical problems after deployment. PTSD-symptoms at 5 months predicted coexisting physical symptoms, after controlling for demographic variables, military factors, injury sustained on deployment, war-zone exposure on deployment, baseline physical problems, baseline PTSD-symptoms, and neuroticism. PTSD-symptoms at 5 months also predicted physical problems at 15 months, while controlling for physical symptoms at 5 months. The results suggest that posttraumatic stress contributes to physical symptoms. Clinicians are advised to be attentive to PTSD when individuals present with physical symptoms, and to pay attention to physical symptoms in patients diagnosed with PTSD.
International Journal of Clinical and Health Psychology. 01/2009;
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ABSTRACT: This meta-analysis examined differences in health-related quality of life (HRQoL) between seekers of surgical and non-surgical treatment, and non-treatment seekers, over and above differences that are explained by weight, age, and gender.
Our literature search focused on the 'Impact of Weight on Quality of Life-Lite' (IWQOL-Lite) and the 'Short Form-36' (SF-36) questionnaires. Included were studies published between 1980 and April 2006 providing pre-treatment descriptive statistics of adult overweight, obese or morbidly obese persons. Excluded were elderly and ill patient groups.
54 articles, with a total number of nearly 100,000 participants, met the inclusion criteria. Persons seeking surgical treatment demonstrated the most severely reduced HRQoL. IWQOL-Lite scores showed larger differences between populations than SF-36 scores. After adjustment for weight, the population differences on the IWQOL disappeared. In contrast, the differences on the SF-36 between the surgical treatment seeking population and the other populations were maintained after adjustment for weight.
The IWQOL-Lite questionnaire predominantly reflects weight-related HRQoL, whereas the SF-36 mostly reflects generic HRQoL that is determined by both weight and other factors. Our meta-analysis provides reference values that are useful when explaining or evaluating obesity-specific (IWQOL-Lite) or generic (SF-36) HRQoL, weight, and demographic characteristics of obese persons seeking or not seeking surgical or non-surgical treatment.
Obesity Surgery 11/2007; 17(10):1357-66. · 3.29 Impact Factor
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ABSTRACT: In this quasi-experimental study among staff of 29 oncology wards, the authors evaluated the effects of a team-based burnout intervention program combining a staff support group with a participatory action research approach. Nine wards were randomly selected to participate in the program. Before the program started (Time 1), directly after the program ended (Time 2), and 6 months later (Time 3), study participants filled out a questionnaire on their work situation and well-being. Results of multilevel analyses showed that staff in the experimental wards experienced significantly less emotional exhaustion at both Time 2 and Time 3 and less depersonalization at Time 2, compared with the control wards. Moreover, changes in burnout levels were significantly related to changes in the perception of job characteristics over time.
Journal of Applied Psychology 02/2007; 92(1):213-27. · 4.31 Impact Factor
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ABSTRACT: Negative interactions with intimate partners may have adverse consequences for well-being, especially for individuals dealing with chronic illness. However, it is not clear whether negative interactions affect both dimensions of positive and negative well-being and factors that may moderate this effect have not been well-described. The aim of the present study was to examine the association between daily received negative responses from the partner and end-of-day positive and negative mood in patients with multiple sclerosis (MS) and their intimate partners. Further, the moderating role of receiving emotional support from the partner on the same day was examined. Sixty-one MS patients and their intimate partners were approached via one MS centre and the neurology department of one hospital in the Netherlands and completed computerized diaries for 14 days. Both partners filled out diaries at the end of each day, recording received negative responses, emotional support and end-of-day positive and negative mood. In line with a domain specific model, patients or partners who reported receiving negative responses on a day had higher end-of-day negative mood, whereas received negative responses were unrelated to end-of-day positive mood. Further, for both patients and partners, the adverse effect of received negative responses on end-of day mood was moderated by receiving emotional support on the same day.
Social Science [?] Medicine 02/2007; 64(2):389-400. · 2.70 Impact Factor
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ABSTRACT: This study examined the effects of daily support exchanges in couples facing multiple sclerosis (MS). Two issues were examined: the imbalance between received and provided support, and the extent to which reciprocal exchanges of received and provided support are associated with end-of-day well-being (positive and negative mood and self-esteem). Guided by equity theory, we expected that one-sided support provision or receipt would be harmful for well-being for both patients and partners. We argued that these negative outcomes could be offset by reciprocating support, that is, when both partners receive and provide support. Sixty-one patients and their partners filled out questionnaires on demographics and disease-related characteristics and subsequently completed computerized daily diaries for 14 days. At the end of each day, both partners completed diaries on end-of-day mood, self-esteem, received and provided emotional and instrumental support, and several control variables (daily hassles and MS-related symptoms for patients). Reciprocity in instrumental support transactions was associated with higher levels of self-esteem among both patients and partners. However, the other results all showed independent effects of support received and provided. Patients’ well-being was related to providing emotional support and instrumental support, whereas partners’ well-being was related to receiving emotional support from patients.
Personal Relationships 11/2006; 13(4):485 - 501. · 0.74 Impact Factor
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ABSTRACT: The current study sought to find indications for the appropriateness of a model in which eating patterns and exercise beliefs influence binge eating and physical exercise, respectively, that, in turn, influence outcome after gastric banding for severe obesity.
Participants were 157 patients (144 females, 13 males) who completed questionnaires approximately 34 months (range = 8-68 months) after laparoscopic adjustable gastric banding (LAGB).
Our data showed a well-fitting model in which external and emotional eating were associated with outcome through binge eating. Several exercise beliefs were associated with physical exercise, but physical exercise was not associated with weight loss or physical health.
Binge eating was related more strongly to the outcome after gastric banding than physical exercise. Future research should examine whether a strong focus on the management of binge eating and external and emotional eating could improve the outcome of morbidly obese patients with unsuccessful weight outcome after obesity surgery.
International Journal of Eating Disorders 08/2006; 39(5):369-75. · 2.95 Impact Factor
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ABSTRACT: Patients with rheumatoid arthritis face the challenge of adjusting to adverse health consequences and accompanying emotions. Styles of emotion regulation may affect health. Purpose: The objective is to examine associations between styles of emotion regulation and perceived health, consisting of psychological well-being, social functioning, physical functioning, and disease activity.
Principal component analysis was used to summarize styles of emotion regulation of 335 patients with rheumatoid arthritis. Relationships between emotion regulation and perceived health were examined with structural equation modeling.
Four styles of emotion regulation were identified: ambiguity, control, orientation, and expression. Ambiguity and control were mutually correlated, as were orientation and expression. Styles of emotion regulation were not uniquely related to perceived physical functioning and disease activity. Emotional ambiguity and orientation were related to poorer, whereas expression and control were related to more favorable psychological well-being and social functioning.
Our cross-sectional study suggests that emotion regulation is not of direct importance for perceived somatic health of patients with rheumatoid arthritis, but it may be of importance for psychological well-being and social functioning, and perhaps through this route for somatic health. The more conscious and controlled aspects of control and expression are positively related to psychosocial health, and the more unconscious automatic aspects of ambiguity and orientation are negatively related. Changing emotion regulation will potentially affect psychosocial health. It would be worthwhile to verify this possibility in prospective research.
Annals of Behavioral Medicine 09/2005; 30(1):44-53. · 4.20 Impact Factor
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ABSTRACT: Emotion regulation has been associated with perceived health in rheumatoid arthritis, which is diagnosed three times more often in women than men. Our aim was to examine gender differences in styles of emotion regulation (ambiguity, control, orientation, and expression) and gender-specificity of the associations between emotion regulation and perceived health (psychological well-being, social functioning, physical functioning, and disease activity) in 244 female and 91 male patients with rheumatoid arthritis. Women reported more emotional orientation than men, but did not differ from men with regard to ambiguity, control, and expression. Structural equation modelling showed that relationships between emotion regulation and perceived health were more frequent and stronger for women than men. This held especially for the affective dimension of health, while associations were similar for both women and men with regard to social and physical functioning. Only for women, the association between ambiguity and disease activity was significant, which appeared to be mediated by affective functioning. The observations that women are more emotionally oriented than men and that emotion regulation is more interwoven with psychological health in women than men, support the usefulness of a gender-sensitive approach in research and health care of patients with rheumatoid arthritis.
Women & Health 02/2005; 42(1):75-97. · 1.00 Impact Factor
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ABSTRACT: An important problem in multilevel modeling is what constitutes a sufficient sample size for accurate estimation. In multilevel analysis, the major restriction is often the higher-level sample size. In this paper, a simulation study is used to determine the influence of different sample sizes at the group level on the accuracy of the estimates (regression coefficients and variances) and their standard errors. In addition, the influence of other factors, such as the lowest-level sample size and different variance distributions between the levels (different intraclass correlations), is examined. The results show that only a small sample size at level two (meaning a sample of 50 or less) leads to biased estimates of the second-level standard errors. In all of the other simulated conditions the estimates of the regression coefficients, the variance components, and the standard errors are unbiased and accurate.
Methodology European Journal of Research Methods for the Behavioral and Social Sciences 01/2005; 1(3):86-92. · 0.33 Impact Factor
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Statistica Neerlandica 02/2004; 58(2):127-137. · 0.50 Impact Factor
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ABSTRACT: To evaluate the reliability of the Dutch version of the Pediatric Evaluation of Disability Inventory (PEDI), an instrument for measuring functional status (capability and performance in self-care, mobility and social function) of young children using parent interviews.
Inter-interviewer reliability was studied after scoring audiotaped interviews by a second researcher. For test-retest reliability the same parent was interviewed twice within three weeks; in inter-respondent reliability both parents of a child were interviewed independently within a few days. On item level, percentage identical scores were computed, and on scale level intraclass correlation coefficients (ICC) and Cronbach's alphas were calculated.
Parents of 63 nondisabled and 53 disabled (various diagnosis) children aged between 7 and 88 months were interviewed.
On scale level, all ICCs were above 0.90 and Cronbach's alpha was 0.89 for the self-care domain, 0.74 for the mobility domain and 0.87 for the social function domain. On item level for the Functional Skills Scale, the mean percentage identical scores varied from 89 to 99, and for the Caregiver Assistance Scale from 54 to 90. Different scores between interviewers resulted partially from ambiguous interpretation of the item and/or the explanation.
Although small adaptations have to be made, the psychometric properties of the Dutch PEDI are found to be good.
Clinical Rehabilitation 08/2003; 17(4):457-62. · 2.12 Impact Factor
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ABSTRACT: Hierarchical structured data cause problems in analysis, because the usual assumptions of independently and identically distributed variables are violated. Muth+®n (1989) described an estimation method for multilevel factor and path analysis with hierarchical data. This article assesses the robustness of the method with unequal groups, small sample sizes at both the individual and the group level, in the presence of a low or a high intraclass correlation (ICC). The within-groups part of the model poses no problems. The most important problem in the between-groups part of the model is the occurrence of inadmissible estimates, especially when group level sample size is small (50) while the intracluster correlation is low. This is partly compensated by using large group sizes. When an admissible solution is reached, the factor loadings are generally accurate. However, the residual variances are underestimated, and the standard errors are generally too small. Having more or larger groups or a higher ICC does not effectively compensate for this. Therefore, although the nominal alpha level is 5%, the operating alpha level is about 8% in all simulated conditions with unbalanced groups. The strongest factor is an inadequate sample size at the group level. Imbalance is only a problem for the overall fit test. For balanced data, the chi-square fit test is accurate. The size of the biases is comparable to the effect of moderate nonnormality in ordinary modeling, and in our view, the approximate solution remains a useful analysis tool, provided the group level sample size is at least 100
Structural Equation Modeling: A Multidisciplinary Journal. 01/2001; 8.
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ABSTRACT: In social sciences, research questions often refer to hierarchically structured data. For instance, the achievements of pupils in classes (in schools) are studied, or the work satisfaction of employees in organizations. The main problem of studying such hierarchical systems, which have more than one level, is the dependence of the observations at the lower levels. Multilevel analyzing programs account for this dependence and in recent years these programs are widely accepted. In this paper we will discuss the influence of different circumstances on the robustness of the parameter estimates (regressioncoefficients and the variance) in two level situations. A simulation study is used to determine the influence of small sample sizes at both level one and level two, and different variance distributions between the levels (the so called 'intraclass correlation').
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International journal of clinical and health psychology, ISSN 1697-2600, Vol. 9, Nº. 3, 2009, pags. 365-372.
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ABSTRACT: A crucial problem in the statistical analysis of hierarchically structured data is the dependence of the observations at the lower levels. Multilevel modeling programs account for this dependence and in recent years these programs have been widely accepted. One of the major assumptions of the tests of significance used in the multilevel programs is normality of the error distributions involved. Simulations were used to assess how important this assumption is for the accuracy of multilevel parameter estimates and their standard errors. Simulations varied the number of groups, the group size, and the intraclass correlation, with the second level residual errors following one of three non-normal distributions. In addition asymptotic maximum likelihood standard errors are compared to robust (Huber/White) standard errors.The results show that non-normal residuals at the second level of the model have little or no effect on the parameter estimates. For the fixed parameters, both the maximum likelihood-based standard errors and the robust standard errors are accurate. For the parameters in the random part of the model, the maximum likelihood-based standard errors at the lowest level are accurate, while the robust standard errors are often overcorrected. The standard errors of the variances of the level-two random effects are highly inaccurate, although the robust errors do perform better than the maximum likelihood errors. For good accuracy, robust standard errors need at least 100 groups. Thus, using robust standard errors as a diagnostic tool seems to be preferable to simply relying on them to solve the problem.
Computational Statistics & Data Analysis.
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ABSTRACT: Objective: This systematic review and meta-analysis examined whether implementation intentions are an effective tool to help people put their intentions to eat a healthy diet into practice. Additionally, it was investigated whether the quality of the outcome measures and the quality of the control conditions that are used in these studies influence implementation intentions’ effectiveness. Methods: Twenty three empirical studies investigating the effect of implementation intentions on eating behavior were included. In assessing the empirical evidence, a distinction was made between studies that aim to increase healthy eating (i.e., eating more fruits) and studies that aim to diminish unhealthy eating (i.e., eating fewer unhealthy snacks). Results: Implementation intentions are an effective tool for promoting the inclusion of healthy food items in one's diet (Cohen's d = .51), but results for diminishing unhealthy eating patterns are less strong (Cohen's d = .29). For studies aiming to increase healthy eating, it was found that higher quality outcome measures and lower quality control conditions tended to yield stronger effects. Conclusion: Implementation intentions are somewhat more effective in promoting healthy eating than in diminishing unhealthy eating, although for some studies promoting healthy eating effect sizes may have been inflated due to less than optimal control conditions.
Appetite.
