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Results for “instrumental variable assumptions”

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PubMed1990

Single-breath carbon monoxide diffusing capacity prediction equations from a Mediterranean population.

Because of unanswered questions about prediction equations for the single-breath carbon monoxide diffusing capacity (DLCO) and as part of a larger collaborative project, standardized DLCO measurements were carried out in a selected sample of 361 healthy nonsmoking volunteers (194 men and 167 women) living in the Barcelona metropolitan area (Spain). Except for the test FIO2 (0.18), the study essentially followed the American Thoracic Society (ATS) and European Community for Coal and Steel (ECCS) recommendations for standardizing the methodology of measuring DLCO. Prediction equations for ages 20 through 70 were calculated separately for both sexes. Simple linear equations using age, height, and body weight as independent variables predicted the DLCO indices (DLCO, VA, and DL/VA) as well as more complex equations. In addition, a complete analysis of the residuals (predicted measured values) showed that the assumptions of the multiple regression analysis (independence, homoscedasticity and Gaussian distribution of residuals) were fulfilled using simple linear equations. Correction for the instrumental and anatomic dead spaces decreased the DLCO an average of 4.7%. The standard error of estimates was lower than those reported from other series in the literature. The predicted values from this study were lower than those reported by some investigators and were in reasonable agreement with other studies. A portion but not all of the differences could be explained on the basis of recognized differences in testing methodology. The results of this study may be of value to clinical laboratories seeking predictive equations for DLCO most appropriate for their testing methodology and patient population, and may assist in the resolution of some controversies regarding differences among predictive equations for DLCO.

Adolescent↗

PubMed1997

Econometric approaches to epidemiologic data: relating endogeneity and unobserved heterogeneity to confounding.

The concepts of endogeneity and unobserved heterogeneity are well-known among econometricians. However, these issues are rarely addressed in epidemiologic studies. This paper explores these two concepts, their relationship to each other, and the implications for analysis in epidemiologic studies. An endogenous variable is defined as a predictor variable which is partly determined by factors within the model itself, while unobserved heterogeneity is conceptualized as a vector of missing variables acting through the error term. Under certain assumptions, the simultaneous existence of an endogenous variable and unobserved heterogeneity is shown to act in a manner analogous to confounding. Specifically, this occurs due to an association between the error term in the equation and the endogenous predictor variable. The accepted econometric solution to this problem is to replace the endogenous variable with an 'instrumental variable' which is not correlated with the error term and thus not susceptible to confounding. The validity of these concepts and of the proposed solution are discussed.

Confounding Factors, Epidemiologic↗

PubMed2006

Extracting chemical information from spectral data with multiplicative light scattering effects by optical path-length estimation and correction.

When analyzing complex mixtures that exhibit sample-to-sample variability using spectroscopic instrumentation, the variation in the optical path length, resulting from the physical variations inherent within the individual samples, will result in significant multiplicative light scattering perturbations. Although a number of algorithms have been proposed to address the effect of multiplicative light scattering, each has associated with it a number of underlying assumptions, which necessitates additional information relating to the spectra being attained. This information is difficult to obtain in practice and frequently is not available. Thus, with a view to removing the need for the attainment of additional information, a new algorithm, optical path-length estimation and correction (OPLEC), is proposed. The methodology is applied to two near-infrared transmittance spectral data sets (powder mixture data and wheat kernel data), and the results are compared with the extended multiplicative signal correction (EMSC) and extended inverted signal correction (EISC) algorithms. Within the study, it is concluded that the EMSC algorithm cannot be applied to the wheat kernel data set due to core information for the implementation of the algorithm not being available, while the analysis of the powder mixture data using EISC resulted in incorrect conclusions being drawn and hence a calibration model whose performance was unacceptable. In contrast, OPLEC was observed to effectively mitigate the detrimental effects of physical light scattering and significantly improve the prediction accuracy of the calibration models for the two spectral data sets investigated without any additional information pertaining to the calibration samples being required.

Algorithms↗

PubMed2005

Patient, consumer, client, or customer: what do people want to be called?

OBJECTIVE: To clarify preferred labels for people receiving health care. BACKGROUND: The proper label to describe people receiving care has evoked considerable debate among providers and bio-ethicists, but there is little evidence as to the preferences of the people involved. DESIGN: We analysed dictionary definitions as to the derivation and connotations of such potential labels as: patient, client, customer, consumer, partner and survivor. We then surveyed outpatients from four clinical populations in Ontario, Canada about their feelings about these labels. SETTING AND PARTICIPANTS: People from breast cancer (n = 202), prostate disease (n = 202) and fracture (n = 202) clinics in an urban Canadian teaching hospital (Sharpe study), and people with HIV/AIDS at 10 specialty care clinics and three primary care practices affiliated with the HIV Ontario Observational Database (n = 431). VARIABLES AND OUTCOME MEASURES: The survey instruments included questions about opinion of label, role in treatment decision-making (the Problem Solving Decision Making scale), trust, use of information and health status. RESULTS: Our respondents moderately liked the label 'patient'. The other alternatives evoked moderate to strong dislike. CONCLUSIONS: Many alternatives to 'patient' incorporate assumptions (e.g. a market relationship) which care recipients may also find objectionable. People who are receiving care find the label 'patient' much less objectionable than the alternatives that have been suggested.

Adult↗

PubMed2006

Complex causal process diagrams for analyzing the health impacts of policy interventions.

Causal diagrams are rigorous tools for controlling confounding. They also can be used to describe complex causal systems, which is done routinely in communicable disease epidemiology. The use of change diagrams has advantages over static diagrams, because change diagrams are more tractable, relate better to interventions, and have clearer interpretations. Causal diagrams are a useful basis for modeling. They make assumptions explicit, provide a framework for analysis, generate testable predictions, explore the effects of interventions, and identify data gaps. Causal diagrams can be used to integrate different types of information and to facilitate communication both among public health experts and between public health experts and experts in other fields. Causal diagrams allow the use of instrumental variables, which can help control confounding and reverse causation.

Audiovisual Aids↗

PubMed2006

Differential item functioning (DIF) and the Mini-Mental State Examination (MMSE). Overview, sample, and issues of translation.

BACKGROUND: Various forms of differential item functioning (DIF) in the Mini-Mental State Examination (MMSE) have been identified. Items have been found to perform differently for individuals of different educational levels, racial/ethnic groups, and/or of groups whose first language is not English. The articles in this section illustrate the use of different methods to examine DIF in relation to English and Spanish language administration of the MMSE. OBJECTIVES: The aim of this article is to provide a context for interpretation of the findings contained in the following set of papers examining DIF in the MMSE. METHODS: The performance of the MMSE, when administered in English and Spanish, was reviewed. "Translation" has been discussed in the context of measurement bias, illustrating the variability in Spanish translations. Presented are the readability of the MMSE, description of the translation method, the study design and sample for the data set used, together with treatment of missing data, and model assumptions related to the analyses described in the accompanying set of papers examining DIF. CONCLUSIONS: The examination of item bias in cognitive impairment assessment instruments has practical and theoretical implications in the context of health disparities. Considerable DIF has been identified in the MMSE. A critical factor that may contribute to measurement bias is language translation and conversion. Once DIF has been established consistently in a measure, decisions regarding adjustments proceed. Perhaps the development of guidelines for appropriate adjustments for DIF correction in self-reported measures represents the next challenge in addressing measurement equivalence in crosscultural research.

Aged↗

PubMedDate not supplied

The measurement of normative American Shoulder and Elbow Surgeons scores.

We determined baseline scores on the American Shoulder and Elbow Surgeons (ASES) questionnaire in individuals who had no known shoulder condition and evaluated related factors that could influence the score. A modified version of the questionnaire was completed by 343 patients from an outpatient orthopaedic center being seen for conditions unrelated to the shoulder. A separate group completed the questionnaire at two different times to assess its reliability. The mean ASES score was 92.2 +/- 14.5 points. The instrument was found to be very reliable (intraclass correlation coefficient = 0.96). Only the variables of shoulder status and sports participation entered the regression model to explain the variance in scores. Subjects aged 60 years or older exhibited decreased ability to lift above shoulder level and reach behind the back when compared with younger cohorts. Previous investigators have made the assumption that a normal preinjury ASES score is 100 points. This study raises questions about this assumption. This information can serve as a basis to compare normative scores with those of patients with active shoulder disease.

Adolescent↗

PubMed1971

Blood flow, slip, and viscometry.

The viscosity of blood, measured by the usual viscometers in which slip is not considered, is found to be flow dependent, varying markedly with shear rate, pressure gradient, and vessel diameter in the lower ranges of these factors. The study postulates, on grounds thought reasonable, that slip may be present in blood flow, as a function of the nature of the wall surfaces, shear stress at the wall, and relative cell volume (RCV) adjacent to the wall. It presumes that blood possesses a specific, flow-independent viscosity, and determines theoretically the viscosity indications of viscometers if blood slipped in the instruments. The study shows that if the slip function is of a certain plausible form, these viscosity indications would exhibit a flow dependence of much the same pattern as the actual indications supplied by the usual viscometers. The slip postulate permits, therefore, an interpretation of the "anomalous" flow behavior of blood, dispensing with the prevailing assumption of an ad hoc variability of its viscosity with flow factors. To the extent that viscometric data for blood may be representative of other non-newtonian fluids, the slip postulate may be applicable to these fluids.

Biomechanical Phenomena↗

PubMed2004

Varus/valgus alignment of the femur in total knee arthroplasty. Can accuracy be improved by pre-operative CT scanning?

In a series of 80 consecutive knee replacements, pre-operative scout scans of the femur were taken and the angle between the slope of the worn femur and the mechanical axis was measured. At surgery the angle of the distal femoral cut was adjusted to make it perpendicular to the mechanical axis. Post-operative scans revealed that 97.5% of femoral components were implanted within 2 degrees of the target. The angle between a perfectly positioned virtual intra-medullary rod on the pre-operative films and the mechanical axis showed marked variations between individuals indicating that the use of a fixed angular correction from an intra-medullary rod is unreliable even under perfect circumstances. In practice it is subject to further inaccuracy due to variability in the anatomy of the femur and rod placement. With the advent of robotically assisted surgery, the technical limits of conventional instruments need to be explored in terms of improving accuracy for each individual patient. In terms of the coronal alignment of the femur, this involves some degree of preplanning rather than relying on arbitrary targets based on incorrect assumptions.

Arthroplasty, Replacement, Knee↗

PubMed1988

A new method of measuring the cross-sectional area of connective tissue structures.

There appears to be no generally accepted method of measuring in-situ the cross-sectional area of connective tissues, particularly small ones, before mechanical testing. An instrument has therefore been devised to measure the cross-sectional area of one such tissue, the rabbit medial collateral ligament, directly and nondestructively. However, the methodology is general and could be applied to other tissues with appropriate changes in detail. The concept employed in the instrument is to measure the thickness of the tissue as a function of position along the width of the tissue. The plot obtained of thickness versus width position is integrated to provide the cross-sectional area. This area is accurate to within 5 percent, depending mainly on alignment of the instrument and pre-load of the ligament. Results on the mid-substance of the rabbit medial collateral ligaments are repeatable and reproducible. Values of maximum width and thickness are less variable than those obtained with a vernier caliper. The measured area is considerably less than that estimated assuming rectangular cross-section and slightly less than that estimated on the assumption of elliptical cross-section.

Animals↗

PubMedDate not supplied

Quality time: how parents' schooling affects child health through its interaction with childcare time in Bangladesh.

A child health production function is presented with the key feature being an interaction term between a caregiver's schooling and their exposure time to the child. The production function is estimated using a 2SLS fixed effects model with lagged childcare time, resource allocation and child health as instruments for the first differences of these same endogenous variables. The 1978 Intrafamily Food Distribution and Feeding Practices Survey dataset from Bangladesh is used together with census data. The production function estimates indicate that part of the salutary effects of parental education on child health require that the child actually be exposed to the educated parent. Given the demographic makeup of the study sample and the assumption that age education and gender completely account for productivity, teenage brothers and fathers would have the highest marginal productivity for child health and mothers and grandmothers the least. If economic opportunity draws mothers away from childcare, the presence of other household members with higher schooling levels offers the potential for an improvement in the overall quality of childcare time. In the present study the households failed to set the marginal labour product of child health for each of the caregivers equal. Thus, the quality of childcare may not be the household's sole concern in determining time allocation.

Adolescent↗

PubMed1997

Proposed model for estimating dose to inhabitants of 60Co contaminated buildings.

A model to predict the time weighted exposures to gamma radiation was developed for buildings constructed with structural steel having some contamination from 60Co. Several buildings throughout sixteen city blocks in downtown Taipei were built about ten years ago with this material. These buildings were used for residential, business, and educational purposes with radiation levels ranging from background to five hundred times background. A comprehensive epidemiologic study by the National Yang Ming University Medical School in Taipei is underway to study the effects of this exposure to the building occupants. An evaluation of external radiation exposure was performed using survey instruments and thermoluminescent dosimeters. Exposure data from the survey instruments were used in a computer model developed to calculate cumulative radiation exposure estimates for the epidemiologic research. While the survey instrument data provided radiation levels at a point in time, the thermoluminescent dosimeters were placed in fixed locations and on several volunteers for a period of one month to verify the modeling results. The model itself is a mathematical algorithm that provides estimates with minimum and maximum range values by taking into account differences in the survey data between adults and children, variable occupancy patterns, background radiation, and radioactive decay. Several assumptions (background rates, height adjustment values, and occupancy factors) are easily adjusted to improve the estimated radiation exposures. The model predicted the exposures as measured by the thermoluminescent dosimeters with greater reliability for adults than for children. The differences between the two methods were about 10-15% for the adults and about 60% for the child. This strategy, its advantages, limitations, and its performance against actual thermoluminescent dosimeter measurements are presented.

Activities of Daily Living↗

PubMed1996

Comparisons between fat measurements by dual-energy X-ray absorptiometry, underwater weighing and magnetic resonance imaging in healthy women.

OBJECTIVE: To compare the results of measurements of total and regional fat by dual-energy X-ray absorptiometry (DXA), underwater weighing (UWW) and magnetic resonance imaging (MRI) in women. DESIGN: 13 volunteer pre-menopausal women measured with a Norland XR26 HS DXA instrument, hydrodensitometry equipment and a 0.08 Tesla MRI scanner. SETTING: Aberdeen Royal Infirmary. RESULTS: There were high correlations between measurements of total-body fat by the different techniques, but poor agreement. The mean values of fat as a percentage of the total body mass were UWW = 28.6%, DXA = 40.0%, MRI = 23.0%, although the MRI excluded the head, forearms and feet, estimated from the DXA measurements to contain 8% of the body fat. Although the correlations were apparently linear in the ranges examined, those including the Norland DXA had high intercepts. Bland and Altman plots showed no dependence of the differences on the degree of fatness. Evidence was obtained from in vivo comparisons and phantom measurements that the fat calibration of the Norland DXA had changed during a three year period, suggesting that fat calibration errors contributed to the MRI/DXA differences. Previous comparisons between the Norland DXA and other brands indicate that Hologic and Lunar instruments would show a closer agreement with MRI and UWW. Profiles of fat distribution along the body showed variations in the DXA/MRI ratio, particularly in the chest, with the DXA pattern thought to be the less accurate. The DXA measurements of bone mineral, fat and lean tissue were used to demonstrate the variable proportion of bone in fat-free tissue, leading to errors in UWW. CONCLUSIONS: Each method has its assumptions, which are often not sufficiently appreciated. Each method may be useful for testing and improving the assumptions in the other methods. No method can yet be regarded as a satisfactory reference technique.

Absorptiometry, Photon↗

PubMed1985

The effect of the transcutaneous electrode on the variability of dermal oxygen tension changes.

Transcutaneous oxygen measurements (TCPO2) are being used increasingly for the assessment of tissue viability in the ischaemic limb. The major objective with such a technique is to determine the critical level of TCPO2 which defines the boundary between skin viability and non-viability. To be able to do this, an assumption is made that measurements from different centres using various TCPO2 sensors are comparable. This study shows that static and dynamic changes of TCPO2 (air to oxygen breathing) made with two commercially available instruments are not directly comparable. We suggest that the physical characteristics of present TCPO2 electrodes are not optimal for measurements on adult skin. TCPO2 measurements made in conditions of skin ischaemia should be interpreted with caution because the ratio of local oxygen demand to blood flow is such that the measurement may be underestimated by TCPO2 electrodes with a high oxygen consumption.

Adult↗

PubMed2001

Readiness to adopt adequate postural habits: an application of the Transtheoretical Model in the context of back pain prevention.

Based on a biomechanical model, an adequate body posture can contribute to the prevention of back pain and back pain chronicity. This study examines the explanatory value of the Transtheoretical Model (TTM) for the adoption of adequate postural habits in a cross-sectional sample of 149 employees of a German administration unit (mean age 40.2 years, 50% female). Using newly developed instruments with satisfactory psychometric properties, basic assumptions of the TTM could be confirmed: self-efficacy and the perceived pros for maintaining a good body posture increased significantly across the stages, while the perceived cons decreased. Additionally, the use of preventive strategies for back pain prevention increased linearly and significantly across the stages of change. The study supports the applicability of the TTM for postural behavior. Considering stages of change as an intervening variable may contribute to clarifying the relationship between participation in low back schools and prevention of back pain chronicity. Longitudinal and intervention study data are needed to support these assumptions.

Adult↗

PubMed2004

Avoiding pitfalls of correlation coefficients in the assessment of measurement instruments in rehabilitation research.

OBJECTIVE: To provide a practical guide on how to avoid the pitfalls of correlated correlation coefficients when comparing multiple instruments in rehabilitation research. DESIGN: An observational study comparing a number of instruments measuring quality of life (QoL) compared with an external criterion. SUBJECTS: Sixty-eight patients admitted to a rheumatology ward for intensive treatment of rheumatoid arthritis. METHODS: Patients completed three new (QoL) instruments and an established instrument before and after intensive treatment for rheumatoid arthritis. MAIN OUTCOME MEASURES: Correlation coefficients together with their confidence intervals and a test for the difference between a set of correlated correlation coefficients for the change in the EuroQoL Quality of Life scale (EuroQoL), the World Health Organization Quality Of Life-Abbreviated version (WHOQoL-BREF) and the Quality of Life Profile (QLP) against the Stanford Health Assessment Questionnaire (HAQ). RESULTS: Although the range of correlation between the new instruments and the external criterion was between -0.37 and -0.59 and suggested that one new instrument was far more responsive than the others,; an omnibus test for an overall difference could find no difference in responsiveness. CONCLUSIONS: It is conceptually simple to use correlation coefficients to assess the properties of multiple instruments measured on the same subjects to find a 'best' instrument. However, proper interpretation of results when correlated correlation coefficients are calculated is complex. We recommend analysis includes: (a) that simple plots of the pairs of analysed variables are shown, (b) that simple linear model-fitting statistics, e.g., the R-squared statistic, accompany the plots, (c) that confidence intervals are presented for correlation coefficients, (d) that an omnibus statistical test for the difference between correlated correlation coefficients is presented, and (e) that normal model assumptions are tested.

Arthritis, Rheumatoid↗

PubMed2005

Estimating treatment effects from randomized clinical trials with noncompliance and loss to follow-up: the role of instrumental variable methods.

Perfectly implemented randomized clinical trials, particularly of complex interventions, are extremely rare. Almost always they are characterized by imperfect adherence to the randomly allocated treatment and variable amounts of missing outcome data. Here we start by describing a wide variety of examples and then introduce instrumental variable methods for the analysis of such trials. We concentrate mainly on situations in which compliance is all or nothing (either the patient receives the allocated treatment or they do not--in the latter case they may receive no treatment or a treatment other than the one allocated). The main purpose of the review is to illustrate the use of latent class (finite mixture) models, using maximum likelihood, for complier-average causal effect estimation under varying assumptions concerning the mechanism of the missing outcome data.

Depression↗

PubMed1991

Quantal analysis and synaptic efficacy in the CNS.

Quantal analysis of synaptic transmission at connections between neurons in the CNS has provided insights concerning the structural constraints on transmitter release and postsynaptic responsiveness. However, it has proven difficult in many cases to resolve the size and variability of a single quantum or to distinguish clear peaks in amplitude histograms of evoked responses, due in part to the superposition of background instrumental and biological noise. These limitations raise questions about recent attempts to use direct or indirect methods of quantal analysis in order to distinguish between pre- and postsynaptic loci of the modifications underlying long-term potentiation, particularly since the interpretations are model-dependent and the statistical treatments and experimental techniques employed incorporate simplifying assumptions not yet proven.

Animals↗