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Results for “instrumental variable assumptions”

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PubMed1999

A method for proficiency testing of small peer groups in the College of American Pathologists Coagulation Surveys.

OBJECTIVE: To develop a grading scheme for the proficiency testing of small peer groups of fewer than 10 members for the prothrombin time (PT) and activated partial thromboplastin time (APTT). METHODS: A modified target value for small peer groups was derived based on the assumption that measurement variability in the PT and APTT is more greatly influenced by variations in reagents than in instruments. Criteria for grading were established by statistical simulation to achieve misclassification errors of less than 5% for both incorrectly passing and failing participants. College of American Pathologists Coagulation Survey data were analyzed to determine the number of additional laboratories graded using the proposed scheme, as well as the failure rates among participants in the small peer groups. RESULTS: The modified target value for small peer groups is a weighted average between the mean of the peer group and the mean of all participants using the same reagent (reagent group). Peer groups with as few as 4 members can be graded provided that specific criteria are satisfied: there must be at least 5 peer groups for the same reagent, at least 3 of these 5 peer groups must have more than 3 members, and the coefficient of variation for the reagent group must be less than 10%. This proposed grading scheme decreased the number of ungraded laboratories by 44% to 46% for the PT and 42% to 55% for the APTT. The percentage of failing grades among participants in the small peer groups ranged from 1.3% to 4.1% for the PT and 1.4% to 7.2% for the APTT. These failure rates were 2.8- to 13.0-fold higher than the failure rates in large peer groups (P < or = .05). CONCLUSIONS: The proposed small peer group grading scheme can improve the effectiveness of College of American Pathologists proficiency testing for the PT and APTT and may also be generally applicable to other test methods and analytes.

Anticoagulants↗

PubMed2005

Quality effects of operative delay on mortality in hip fracture treatment.

BACKGROUND: Most hip fracture patients undergo surgery, but there is conflicting evidence on the relation between the timing of surgery and the outcome of treatment. There is considerable variation in the length of surgical delays between hospitals, possibly reflecting the quality of care. AIM: To examine the associations between in-hospital surgical delay and the mortality of hip fracture patients from a practical quality assessment perspective. METHODS: The effects of operative delay on mortality were estimated using various statistical methods applied to observational data from 16 881 first time hip fracture patients aged 65 or older from 47 hospitals (providers) in Finland in 1998-2001. RESULTS: A prolonged in-hospital operative delay was associated with a higher mortality of hip fracture patients in individual level analyses, but the instrumental variable approach indicated that the individual level effect was not caused by the operative delay but by inappropriate methodological assumptions. There was extensive variation between providers in the proportion of late surgery patients. Provider level analyses showed that the effects of the provider of operative delay on mortality are quite small, but there is a clear association between the proportion of late surgery patients and non-optimal treatment. CONCLUSIONS: If provider level heterogeneity is not explicitly taken into account, studies of the effects of surgical delay on outcomes are prone to serious bias. The proportion of patients with prolonged waiting time for surgery at the provider level seems to work as an effective evidence-based quality indicator. Providers should reduce unnecessary delays to surgery and identify more carefully patients not suitable for early surgery.

Age Factors↗

PubMed2001

Transcultural psychiatry: some social and epidemiological research issues.

BACKGROUND: Mental Health Research across cultural groups is often criticised for using imprecise measures of cultural group and for using outcome measures as if they have universal validity. AIMS: 1. To Investigate the effect of using different cultural group variables on the findings of a survey of prevalence of Common Mental Disorders. 2. To demonstrate that assumptions of validity for outcomes measures can affect the interpretation of data from prevalence surveys. METHODS: We recruited Punjabi and English subjects to a phase prevalence survey that included the Amritsar Depression Inventory and the General Health Questionnaire as screening instruments. The Clinical Interview Schedule was the outcome measure. This paper reports on a secondary analysis of the data. We used ethnic group, place of birth, religion, first language and language spoken at interview as possible cultural group variables and compared the prevalence estimates. We then considered the limitations of conventional methods to assess prevalence, by looking at mean scores on each of the three instruments in both cultural groups. RESULTS: Cultural group variables did not influence the prevalence estimates for Common Mental Disorder. Although conventional scoring methods showed no difference in prevalence across cultures, the mean scores on each instrument, when compared across cultural groups, differed for the Amritsar Depression Inventory. This instrument showed a higher mean score for the Punjabis suggesting a higher prevalence. The findings are discussed in the context of value laden 'assumptions' about validity. CONCLUSIONS: The findings of prevalence surveys depend on assumptions of validity. The 'culture' of psychiatry is a closed system in which validation studies support its basic assumptions.

Cross-Cultural Comparison↗

PubMed1989

Three example applications of optimization techniques to Department of Energy contractor radiation protection programs.

Six numerical examples of optimization of radiation protection are provided in the appendices of International Commission on Radiological Protection (ICRP) Publication No. 37 (1983). In each case, the calculations were based on well-defined parameters and assumptions. In this paper, we examined three different numerical examples that were based on empirical data and less-certain assumptions. In the first example, the optimum sampling frequency for a typical 3H bioassay program was found to be once every 2 mo. However, this result depended on assumed values for several variables that were difficult to evaluate. The second example showed that the optimum frequency for recalibrating a group of "cutie pie" (CP) ionization chamber survey instruments was once every 85 d. This result depended largely on the assumption that an improperly operating CP instrument could lead to a serious overexposure. In the third example, one continuous air monitor (CAM) was determined to be the optimum number in a workplace at a Department of Energy (DOE) Pu facility. The optimum location of the CAM was determined from past glove-box release studies. These examples demonstrated that cost-benefit analysis of individual elements of radiation protection programs can be useful even if limited data are available.

Air Pollution, Radioactive↗

PubMed1997

Assessing inner-city patients' hospital experiences. A controlled trial of telephone interviews versus mailed surveys.

OBJECTIVES: Obtaining accurate and representative patient-centered data may be difficult among poor, inner-city patients because of changing addresses, variable access to telephones, and a higher prevalence of illiteracy than in the populations in which many survey instruments were developed and tested. Assumptions about the usefulness of mailed surveys versus telephone interviews may not hold for the urban poor. Therefore, identifying the most efficient mode of survey administration in this population becomes an important methodological question. METHODS: We conducted a randomized trial of patients discharged from the inpatient medicine service of an urban teaching hospital to compare telephone interview with mailed self-administration of a detailed instrument for measuring patients' experiences with hospital care. Our primary outcomes were response rate, missing data, and data collection costs. Patients were excluded if they were not discharged to home or were mentally or physically unable to complete mailed or telephone interviews. The research assistant contacted eligible patients while hospitalized, informed them of the postdischarge survey, and obtained current phone numbers and addresses. Patients then were randomized to receive a 116-item satisfaction survey via one of two survey methods: mail-first (mailed surveys with follow-up on nonrespondents by telephone) or telephone-first (telephone interviews with follow-up of nonrespondents by mail). RESULTS: Of the 252 patients enrolled, 130 were randomized to the mail-first and 122 to the telephone-first method. Response rates were higher with the telephone-first (73%) compared with the mail-first method (50%; P < 0.0001). Surveys obtained by the telephone-first method had fewer missing data (0.7 +/- 2.39) for those items not involved in skip patterns compared with the mail-first method (7.1 +/- 12.3; P < 0.001) and were 42% less expensive per completed survey ($26.32 versus $37.35; P < 0.0001). CONCLUSIONS: In this survey of patients served by an urban teaching hospital, a strategy of telephone interviews with mail follow-up proved less expensive and yielded a higher response rate with more complete data than using a method where mailed surveys were followed by back-up telephone interviews. In addition, we believe that the improved response rate for telephone interviews compared with those reported in the literature for similar populations is the result of informing inpatients of the survey and obtaining telephone numbers and addresses in the hospital.

Female↗

PubMed1975

Variation of energy density in argon laser photocoagulation.

The power density at the retina provided by presently available laser photocoagulators is subject to variation. Three factors contribute to making the retinal image of the laser beam substantially different from the spot size indicated on the selector of the photocoagulator. These are the operator's variable accommodation, the astigmatism introduced by the contact lens, and the depth of focus of the observation system. The assumption that the same setting on the instrument regularly produces the same power density at the retina will lead to errors in comparing lesions. The safest procedure is to start from the threshold in every location.

Argon↗

PubMed2000

Expectations among the elderly about nursing home entry.

OBJECTIVE: To assess whether the covariates that explain expectations of nursing home entry are consistent with the characteristics of those who enter nursing homes. DATA SOURCES: Waves 1 and 2 of the Assets and Health Dynamics Among the Oldest Old (AHEAD) survey. STUDY DESIGN: We model expectations about nursing home entry as a function of expectations about leaving a bequest, living at least ten years, health condition, and other observed characteristics. We use an instrumental variables and generalized least squares (IV-GLS) method based on Hausman and Taylor (1981) to obtain more efficient estimates than fixed effects, without the restrictive assumptions of random effects. PRINCIPAL FINDINGS: Expectations about nursing home entry are reasonably close to the actual probability of nursing home entry. Most of the variables that affect actual entry also have significant effects on expectations about entry. Medicaid subsidies for nursing home care may have little effect on expectations about nursing home entry; individuals in the lowest asset quartile, who are most likely to receive these subsidies, report probabilities not significantly different from those in other quartiles. Application of the IV-GLS approach is supported by a series of specification tests. CONCLUSIONS: We find that expectations about future nursing home entry are consistent with the characteristics of actual entrants. Underestimation of risk of nursing home entry as a reason for low levels of long-term care insurance is not supported by this analysis.

Activities of Daily Living↗

PubMed2006

Developing and testing an instrument for identifying performance incentives in the Greek health care sector.

BACKGROUND: In the era of cost containment, managers are constantly pursuing increased organizational performance and productivity by aiming at the obvious target, i.e. the workforce. The health care sector, in which production processes are more complicated compared to other industries, is not an exception. In light of recent legislation in Greece in which efficiency improvement and achievement of specific performance targets are identified as undisputable health system goals, the purpose of this study was to develop a reliable and valid instrument for investigating the attitudes of Greek physicians, nurses and administrative personnel towards job-related aspects, and the extent to which these motivate them to improve performance and increase productivity. METHODS: A methodological exploratory design was employed in three phases: a) content development and assessment, which resulted in a 28-item instrument, b) pilot testing (N = 74) and c) field testing (N = 353). Internal consistency reliability was tested via Cronbach's alpha coefficient and factor analysis was used to identify the underlying constructs. Tests of scaling assumptions, according to the Multitrait-Multimethod Matrix, were used to confirm the hypothesized component structure. RESULTS: Four components, referring to intrinsic individual needs and external job-related aspects, were revealed and explain 59.61% of the variability. They were subsequently labeled: job attributes, remuneration, co-workers and achievement. Nine items not meeting item-scale criteria were removed, resulting in a 19-item instrument. Scale reliability ranged from 0.782 to 0.901 and internal item consistency and discriminant validity criteria were satisfied. CONCLUSION: Overall, the instrument appears to be a promising tool for hospital administrations in their attempt to identify job-related factors, which motivate their employees. The psychometric properties were good and warrant administration to a larger sample of employees in the Greek healthcare system.

Adult↗

PubMed2005

Correlations between estimated and true dietary intakes: using two instrumental variables.

PURPOSE: We describe a new application of the method of triads that allows an estimate of the correlation between a dietary questionnaire measure (Q) and true intake (T). METHODS: Three surrogate variables Q, M, and P are observed where M and P are both instrumental (often biological) variables. A reference dietary method (R) is not required. The variables M and P may be concentration rather than recovery biomarkers. Estimating equations produce Corr(Q,T), Corr(M,T), Corr(P,T), conditional on assumptions about error correlations. Correlations between errors in both Q and a reference dietary measure can also be estimated if R is available. A small validation study of California Seventh-day Adventists provided food frequency, repeated 24-hour dietary recalls (R), and biological data (blood, overnight urines, and subcutaneous fat). RESULTS: Values of Corr(Q,T) ranged between 0.40 and 0.66. Values of Corr(R,T) were higher, between 0.48 and 0.83. Estimated correlations between errors in R and Q were all positive. CONCLUSIONS: When carefully chosen, M and P, rather than M and R, should better satisfy assumptions about error correlations. Food frequency data and repeated 24-hour recalls both provide estimates of T, but the latter has greater validity. Standard errors suggest that for good precision Corr(Q,T) requires large validation studies (2000-3000 subjects).

Adult↗

PubMed2006

Assessing motivation for change in subjects with alcohol problems: the MAC2-A questionnaire.

AIMS: This article presents the validation study of a questionnaire (MAC2-A), created to assess motivation for change in Italian adult subjects with alcohol related problems who require or are referred for assessment and treatment. The questionnaire evaluates readiness to change (RTC), self-efficacy and discrepancy. METHODS: The questionnaire was validated on 419 subjects referred to 23 health agencies in Italy. RESULTS: The questionnaire showed good internal consistency and temporal stability. Exploratory and confirmatory factor analysis confirms the consistency with the theoretical assumptions. Test-retest showed high reliability on all scales except one. Visual analogue control scales used for reliability and concurrent validity yielded strong correlations with the corresponding variables. CONCLUSION: The study confirms the validity of the instrument, and its consistency with the model. The MAC2-A questionnaire is the first psychometric instrument integrating three factors (discrepancy, self-efficacy, RTC) into a 3D model of motivation for change.

Adult↗

PubMed2007

Instrumental variables and interactions in the causal analysis of a complex clinical trial.

We consider the application of instrumental variable techniques in a longitudinal clinical trial in paediatric HIV/AIDS, with a substantial degree of non-compliance to randomized treatment (Nelfinavir versus placebo) and with left censoring of the outcome variable (HIV RNA concentration). We consider in detail the assumptions and implications behind the inclusion and exclusion of interactions between randomized arm and baseline covariates in modelling actual treatment received, and between treatment and baseline covariates in modelling outcome. Estimated treatment effects were sensitive to inclusion of interactions, and we show how such sensitivity can be explored and explained.

Anti-HIV Agents↗

PubMed1977

Critical evaluation of the Obscure Figures Test as an instrument for measuring "cognitive innovation".

The Obscure Figure Test has been conceptualized by Acker and McReynolds (1965) as a measure of Cognitive Innovation. As a general concept Cognitive Innovation refers to an integration of different kinds of behavioral systems, e.g., creativity and exploratory behavior. Considering recent research about the relationship between curiosity and creativity as well as basic assumptions underlying this test, it is hypothesized that this is an instrument for measuring creativity rather than curiosity. 41 boys and 41 girls ranging from 7 to 10 yr. of age were given a battery of tests of curiosity and creativity. There were significant correlations between Obscure Figures Test and creativity measures. No such relationships were found between obscure figures and curiosity measures. A factor analysis yielded two factors which could be interpreted as "visual exploration" and "creativity", the obscure figures being a marker variable on the latter factor. Results are discussed in terms of the theoretical assumptions underlying the concepts of curiosity, creativity, and Cognitive Innovation. Further implications with respect to the requirements for an operationalization of Cognitive Innovations are mentioned.

Child↗

PubMed2006

Random effects logistic models for analysing efficacy of a longitudinal randomized treatment with non-adherence.

We present a random effects logistic approach for estimating the efficacy of treatment for compliers in a randomized trial with treatment non-adherence and longitudinal binary outcomes. We use our approach to analyse a primary care depression intervention trial. The use of a random effects model to estimate efficacy supplements intent-to-treat longitudinal analyses based on random effects logistic models that are commonly used in primary care depression research. Our estimation approach is an extension of Nagelkerke et al.'s instrumental variables approximation for cross-sectional binary outcomes. Our approach is easily implementable with standard random effects logistic regression software. We show through a simulation study that our approach provides reasonably accurate inferences for the setting of the depression trial under model assumptions. We also evaluate the sensitivity of our approach to model assumptions for the depression trial.

Humans↗

PubMed1995

Body composition by DXA: tried and true?

Dual-energy x-ray absorptiometry (DXA) is unquestionably the method of choice for the assessment of bone mass. Recent generations of DXA instruments also have the capability of assessing soft tissue mass, thereby providing a three-compartment model of body composition, i.e., bone mineral, fat, and bone-free lean masses. Although hydrodensitometry has long been considered the reference method of assessing body composition, assumptions regarding the constancy of the constituents of fat-free mass may compromise its validity in some populations. Because DXA appears to be less dependent on assumptions regarding biological consistency, it is possible that it has the potential to provide a more accurate assessment of body composition across populations than does hydrodensitometry, and that it should be considered the reference method. DXA has been shown to provide precise measures of body composition, but it remains questionable as to whether those measures are accurate. Contributing to the uncertainty regarding validity is the variability among manufacturers of DXA instruments in the methods of calibration, data acquisition, and data analysis. Although DXA holds great promise in becoming the criterion method of assessing body composition, and has been promoted as such by some investigators, available data indicate that this endorsement is premature.

Absorptiometry, Photon↗

PubMedDate not supplied

Estimating a preference-based single index for the Impact of Weight on Quality of Life-Lite (IWQOL-Lite) instrument from the SF-6D.

OBJECTIVE: To facilitate economic evaluations of interventions for treating obesity, we estimated a preference-based single index for the Impact of Weight on Quality of Life-Lite (IWQOL-lite) instrument by mapping it onto the SF-6D preference-based index. METHODS: A heterogeneous sample of 1972 individuals, composed of community volunteers and participants in weight loss programs, clinical trials, and gastric bypass studies, completed the IWQOL-lite, an obesity-specific measure of health-related quality of life (HRQOL), and the SF-36, a generic measure of HRQOL converted into the preference-based SF-6D. Models of the relationship between the IWQOL-lite and SF-6D of increasing complexity were estimated by regression analyses. RESULTS: The best fitting model for predicting SF-6D index scores entered levels for each item as independent variables (R2 =0.530 in the cross-validation sample, with a mean absolute error of 0.0976). This model (1) makes fewer assumptions than those using total score, dimension scores or item scores as dependent variables and (2) provides a robust unbiased estimate of a preference-based index from IWQOL-Lite data where a preference-based measure was not used. The addition of age and body mass index (BMI) led to a slight improvement in the model. CONCLUSIONS: It is possible to facilitate economic evaluations using results obtained from disease-specific instruments using this approach. A weakness of this approach is that there may be aspects of the condition that have not been properly reflected in the SF-6D index. It is, however, useful when a preference-based measure has not been administered and when it would be impractical to conduct a full valuation survey.

Adolescent↗

PubMed1999

A comparison of computerized and standard versions of the Wisconsin Card Sorting Test.

The manual version of the Wisconsin Card Sorting Test (WCST) is the most frequently used neuropsychological instrument for differentiating normal from populations with frontal-lobe impairments. A number of computerized versions have become available, and their ability to accurately detect frontal-lobe dysfunction is based on the assumption of equivalence with the standard manual version. Accordingly, comparisons of the distribution properties of central tendency, variability, and shape between the manual version with four computerized versions of the WCST were performed (n = 22 per condition). None of the computerized versions were found to be equivalent to the manual version on all assessment measures. Given the discrepancies between the manual and computerized versions, it is concluded that the norms provided for the standard manual presentation method should not be used for the computerized versions, and for continued use of the computerized versions, new norms for computer versions need to be established. Thus, clinicians and experimenters must use caution when basing conclusions on scores from computer versions of the WCST.

Adult↗

PubMed2000

Cost-of-illness of epilepsy in Italy. Data from a multicentre observational study (Episcreen).

OBJECTIVE: To investigate the impact of epilepsy in Italy on healthcare resources, producing an average cost per patient per year of follow-up. DESIGN AND SETTING: The Episcreen Project is a multicentre longitudinal Italian observational study; its methodology, organisational network and case report form have been reported in detail elsewhere. Using a subset of patients with epilepsy from this project, we conducted a retrospective cost-of-illness analysis based on clinical records. The analysis was performed from the societal (community) perspective, including both direct and indirect costs. Hospital admissions, day-hospital visits, specialist visits, instrumental examinations, drugs and productivity losses because of visits and hospitalisation were analysed. Each cost variable was valued in 1996 Italian liras (L) using published national tariffs (except for drugs for which published prices were used). A sensitivity analysis was conducted on indirect costs to test the robustness of the assumption that 1 working day lost for each day hospital visit would produce a change of 0.3% in the weight of indirect costs. PATIENTS AND PARTICIPANTS: Patients analysed in this study were registered in the Episcreen database as at 21 November 1996. They were diagnosed with epilepsy at the last visit, had at least 1 follow-up visit (i.e. at least 1 visit after the enrolment visit), and had at least 12 months of follow-up. RESULTS: The average cost per patient per year was L2,726,116 ($US1767). The average cost per patient was higher for children than for adults [L3,629,997 ($US2353) and L2,362,134 ($US1531), respectively), and for newly diagnosed patients for whom the first diagnosis of epilepsy was addressed at the first Episcreen visit [adults: old referrals L1,304,353 ($US845), new referrals L6,901,374 ($US4473); children: old referrals L2,810,504 ($US1822), new referrals L7,814,400 ($US5065)]. Direct costs represented 87.6% of total costs. The major cost driver was hospitalisation (63.7%), followed by drugs (10.5%), day-hospital visits (4.1%), out-patient visits (3.85%), other tests (3.1%) and electroencephalographs (2.3%). Indirect costs (lost productivity) represented 12.4% of total costs. Sensitivity analysis showed that the results are sensitive to the value attributed to lost productivity. CONCLUSIONS: The cost of managing a patient with epilepsy in Italy is influenced by age, syndrome and modality of referral to the centre for epilepsy.

Adolescent↗

PubMed1998

Who is the marginal patient? Understanding instrumental variables estimates of treatment effects.

OBJECTIVE: To clarify the issues of generalizability arising from the use of instrumental variable (IV) methods to estimate treatment effects in nonexperimental medical outcome studies. DATA SOURCE: We generate Monte Carlo data designed to resemble typical data sets where detailed health status information is unavailable and the treatment assignment process is unobserved. The model used to generate our data makes the realistic assumption that unobservable health status characteristics of patients influence the treatment assignment process and the effectiveness of treatment. STUDY DESIGN: We use Monte Carlo data to illustrate the circumstances where IV estimates generalize to an unobservable patient subpopulation and those where IV estimates generalize to the entire patient population represented by the sample used in the analysis. We also simulate the effect of two policy changes that affect practice patterns. Further, we show that IV estimates are useful for predicting the effect of these changes on treatment effectiveness when the subpopulation to which the IV estimate refers is the same or very similar to the population whose treatment status is affected by the policy change. CONCLUSIONS: Health services researchers cannot take for granted that IV estimates generalize to the same population represented by the sample used for analysis. Instead, researchers must rely on their knowledge of clinical practice and theory regarding the treatment assignment process in interpreting their results and in predicting the effect of changes in practice patterns.

Analysis of Variance↗