BackgroundConfounding bias is a common concern in epidemiological research. Its presence is often determined by comparing exposure effects between univariable- and multivariable regression models, using an arbitrary threshold of a 10% difference to indicate confounding bias. However, many clinical researchers are not aware that the use of this change-in-estimate criterion may lead to wrong conclusions when applied to logistic regression coefficients. This is due to a statistical phenomenon called noncollapsibility, which manifests itself in logistic regression models. This paper aims to clarify the role of noncollapsibility in logistic regression and to provide guidance in determining the presence of confounding bias.MethodsA Monte Carlo simulation study was designed to uncover patterns of confounding bias and noncollapsibility effects in logistic regression. An empirical data example was used to illustrate the inability of the change-in-estimate criterion to distinguish confounding bias from noncollapsibility effects.ResultsThe simulation study showed that, depending on the sign and magnitude of the confounding bias and the noncollapsibility effect, the difference between the effect estimates from univariable- and multivariable regression models may underestimate or overestimate the magnitude of the confounding bias. Because of the noncollapsibility effect, multivariable regression analysis and inverse probability weighting provided different but valid estimates of the confounder-adjusted exposure effect. In our data example, confounding bias was underestimated by the change in estimate due to the presence of a noncollapsibility effect.ConclusionIn logistic regression, the difference between the univariable- and multivariable effect estimate might not only reflect confounding bias but also a noncollapsibility effect. Ideally, the set of confounders is determined at the study design phase and based on subject matter knowledge. To quantify confounding bias, one could compare the unadjusted exposure effect estimate and the estimate from an inverse probability weighted model.
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When planning a study measuring the effects of a neurodevelopmental treatment (NDT), we were confronted with the methodological problem that while measuring the effects of NDT, a rival hypothesis is that the decision to implement the NDT might be related to the quality of nursing care. Therefore, we measured the quality of nursing care as a possible confounding variable in relation to this outcome study. The quality of nursing care was measured on 12 wards participating in the experimental and control groups of the outcome study. Data were collected from 125 patients and 71 nurses and patients' records. The findings showed no significant differences in the quality of nursing care between the 2 groups of wards (P = .49). This method may be useful to other researchers conducting outcome research and who are confronted with a similar methodological problem.
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OBJECTIVES: The objective of this observational study was to establish the possible presence of confounders on the association between temporomandibular disorders (TMD) and headaches in a patient population from a TMD and Orofacial Pain Clinic.METHODS: Several subtypes of headaches were diagnosed: self-reported headache, (probable) migraine, (probable) tension-type headache (TTH), and secondary headache attributed to TMD. The presence of TMD was subdivided into two subtypes: painful TMD and function-related TMD. The associations between the subtypes of TMD and headaches were evaluated by single regression models. Subsequently, to study the influence of possible confounding factors on this association, the regression models were extended with age, gender, bruxism, stress, depression, and somatic complaints.RESULTS: Of the included patients (n=203), 67.5% experienced headaches. In the subsample of patients with a painful TMD (n=58), the prevalence of self-reported headaches increased to 82.8%. The associations found between self-reported headache and (1) painful TMD and (2) function-related TMD, were confounded by the presence of somatic complaints. For probable migraine, both somatic complaints and bruxism confounded the initial association found with painful TMD.DISCUSSION: The findings of this study imply there is a central working mechanism overlapping TMD and headache. Healthcare providers should not look at these disorders separately, but rather at the bigger picture to appreciate the complex nature of the diagnostic and therapeutic process.
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