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At least 19 recordsLinked to original sources
PubMed1995

Statistical considerations regarding the use of ratios to adjust data.

OBJECTIVE: The use of ratios to adjust data (i.e. 'index' variables) is common in obesity and related research. The rationale for the use of ratios often seems to be the desire to control or eliminate the influence of the variable in the denominator. The purpose of this paper is to gain a greater appreciation of the statistical assumptions underlying ratios and their impact on data interpretation. RESULTS: We demonstrate the limitations of the indiscriminant use of ratios to adjust data. Specifically, we show that: (1) given linearity, a zero intercept between the numerator and denominator are necessary and sufficient conditions for a ratio to remove the confounding effects of the denominator; (2) seemingly minor departures from a zero intercept can have major consequences on the ratio's ability to control for the denominator; (3) the ratio of two normally distributed variables cannot be normally distributed, and this may violate the assumptions of subsequent parametric statistical analyses; (4) the use of ratios affects the error distribution of the data which may also violate the assumptions of subsequent parametric statistical analyses; (5) the use of ratios cannot easily take nonlinear effects between the numerator and denominator into account; (6) the use of ratios can introduce spurious correlations among the ratios and other variables; (7) the use of ratios can create interpretive difficulties. We also clarify that the mean of ratios is not necessarily equivalent to the ratio of the means of the numerator and denominator. Finally, we present and discuss formulae for the reliability of ratios and residuals. CONCLUSION: Because of the above issues, we question the indiscriminant use of ratios and advocate that investigators consider regression-based approaches as alternatives.

Body Constitution↗
PubMedDate not supplied

Application of attribute control charts to risk-adjusted data for monitoring and improving health care performance.

This article proposes a new class of control charts that may be used for monitoring and improving the quality of care. Unlike conventional control charts that rely on observed performance data, these charts use risk-adjusted data in addition to the observed data. The resulting time-ordered charts are capable of reducing time-to-time variation that may stem from uncontrollable changes in patient mix over time. Depending on how observed and risk-adjusted data are combined, proposed charts are categorized under the framework of either additive or multiplicative models. Risk-adjusted rates are obtained using multivariate logistic regression models. It was found that the risk-adjusted control charts could be effective in reducing biases that arise from variation in patient mix. These charts can potentially achieve higher sensitivity and specificity compared with ordinary control charts.

Cesarean Section↗
PubMed1992

Study duration for group sequential clinical trials with censored survival data adjusting for stratification.

The study duration in a clinical trial with censored survival data is the sum of the accrual duration, which determines the sample size in a traditional sense, and the follow-up duration, which more or less controls the number of events to be observed. We propose a design procedure for determining the study duration or for calculating the power in a group sequential clinical trial with censored survival data and possibly unequal patient allocation between treatments, adjusting for stratified randomization. The group sequential method is based on the use function approach. We describe a clinical trial recently activated by the Eastern Cooperative Oncology Group for an illustration of the proposed procedure.

Carcinoma, Non-Small-Cell Lung↗
PubMed1991

Choice and conflict about census data: adjusting the American census count.

In this paper "reasons for inevitable defects in the census count are listed in the first section; the second section reports efforts by the U.S. Census Bureau to identify sources of error in census coverage, and make estimates of the size of the errors.... The paper describes the conflict between statistical practices, laws and public policy about census adjustment in the United States, and concludes by considering the extent to which causes [of error] in America are likely to be found in other countries."

Americas↗
PubMed1991

Changes in rates of unscheduled hospital readmissions and changes in efficiency following the introduction of the Medicare prospective payment system. An analysis using risk-adjusted data.

The purpose of this study was to analyze changes in rates of unscheduled readmissions and changes in technical efficiency following the introduction of the Medicare Prospective Payment System (PPS). We developed the Risk-Adjusted Readmissions Index (RARI), which allowed us to make comparisons in rates of unanticipated readmissions across hospitals and over time. Data envelopment analysis (DEA), a linear programming technique, was used to measure changes in technical efficiency by comparing the inputs used and the outputs produced across a cohort of hospitals, while adjusting for changes over time in case mix and case complexity. Rates of unscheduled readmissions and efficiency scores were computed for a sample of 245 hospitals for each year. Although both readmission rates and efficiency scores increased for most hospitals, there was no evidence that those hospitals that experienced the greatest increases in efficiency had the largest increases in their rates of unscheduled readmissions.

Abstracting and Indexing↗
PubMed1985

A comparison of methods for organ-weight data adjustment in chicks.

An experiment was conducted with 168 Arbor Acre X Peterson unsexed, crossbred broiler chicks to compare methods of expressing organ-weight data and to assess changes in organ weights and physiological parameters as body weight (97 to 791 g) and age (5 to 26 days) increased. Actual wet weight of liver, heart, intestine, spleen, and pancreas and percent bone ash increased (P less than .01) as age and body weight increased. Tibia length-to-width ratio decreased (P less than .01) as age and body weight increased. Blood hemoglobin, hematocrit, and plasma protein were not affected (P greater than .1) by age or by body weight. Liver, heart, and intestinal weight decreased (P less than .01) and spleen weight increased (P less than .01) as body weight and age increased when these tissue weights were expressed as percent of body weight. Liver weight adjusted for body weight by covariance analysis, however, remained constant; adjusted heart and intestinal weights decreased (P less than .01), and adjusted spleen weights increased (P less than .01) with increasing age and body weight. The covariate, body weight, was not significant (P greater than .1) for pancreas weight, tibia length-to-width ratio, and percent bone ash. Except for spleen, adjustment by covariance analysis more effectively reduced variation due to body weight than did expression as percent of body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1996

Using severity-adjusted data to impact clinical pathways.

Feeling the weight of managed care, doctors and nurses at a Midwestern health system have joined forces to reduce costs and average lengths of stay (LOS) in a key practice area--orthopedics. The result has been a 40 percent decrease in average length of stay over a three-year period.

Critical Pathways↗
PubMed1997

Improving the quality of the NCQA (National Committee for Quality Assurance) Annual Member Health Care Survey Version 1.0.

The National Committee for Quality Assurance (NCQA) developed a standardized survey instrument in 1995 designed to measure enrollee satisfaction with the care and services received from health plans across the United States. After the survey was administered for a large number of health plans and thousands of responses were received, some areas for survey improvement have emerged. The objective of this research was to evaluate the NCQA Annual Member Health Care Survey Version 1.0 (the standard form) relative to an alternate survey form created in cooperation with the HMO Group, Maritz Marketing Research, Inc., and Healthcare Research Systems, Ltd. The alternate form of the NCQA survey was constructed to test several theories of measurement improvement via rewording of items, reordering of items, deletion of items, and addition of items. The most important findings of the research project are reported herein. Ten geographically dispersed member health maintenance organizations (HMOs) of The HMO Group took part in the project. A split-half design was used to test the standard and alternate survey forms concurrently. Surveys were administered by using the NCQA-recommended mail methodology of survey and cover letter, reminder card, and second survey. Assuming a 50% response rate, a target of 400 responses (200 for each survey form) per HMO was planned. The window for responding was allowed to remain open 4 weeks beyond the mailing of the second survey to achieve the desired response rate. A total of 4,056 responses were collected (2,022 for the standard form and 2,034 for the alternate form). It was found that the addition of "No Experience" and "No Opinion" response options to the majority of satisfaction items reduced the random error associated with informed responses and produced statistically significant higher correlations with the global satisfaction items relative to the standard form items. Only four of the eight Short Form 12 summary scales, General Health, Reported Health Transition, Mental Health, and Social Functioning, were useful for adjusting data (covariation). The entire set of comorbidities (chronic disease checklist) could be eliminated without losing significant data adjustment capability. The multiple linear regression models generated by using the global satisfaction items on the alternate form had higher adjusted R2 values than the standard-form models. The alternate-form item Overall Value correlated highly with cost items and general satisfaction item, making it a useful global satisfaction variable for predictive modeling.

Health Care Surveys↗
PubMed1991

A suggestion for improving intelligibility in multivariate confounder adjustment using alcohol intake and birth weight as an example. A 'confounder score' approach in analyzing continuous data.

Adjustment for multiple confounding is usually done by applying some kind of regression model. This requires assumptions about the type of relationship between the exposure of interest and outcome. It has been suggested to use confounder scores within strata of which the relationship can be scrutinised without such assumptions. Adjusting continuous outcomes for confounding factors, like birth weight for gestational age and lung function measures for age, is sometimes done by using the ratio of the observed outcome to an outcome expected from a reference series external to the study, with a concomitant risk of introducing new confounding. In some cases, the ideas from both these approaches can, slightly modified, be successfully combined. By using an internal rather than an external reference series, i.e. a group within the data with constant exposure, expected values for outcome can be derived as the fitted values from some model, appropriate for the purpose, depicting outcome conditional on all the potential confounders that are registered and for which control is desired. The relationship between the exposure of interest and the outcome, now represented by the fractional departure for each individual from the value expected conditional on her/his confounder status, can be scrutinised without any assumptions about the form of this relationship but, contrary to the confounder score approach, on the basis of the entire data set. This method also provides a way of presenting the relationship of interest, adjusted for confounding, that is easier to understand than the traditional regression approach. The impact of alcohol intake during pregnancy on birth weight will be given as an example.

Alcohol Drinking↗
PubMed1997

A note on the grouping of surveillance data when adjusting for reporting delays.

Analyses that adjust disease incidence data for reporting delays are often based on grouped data. A way of grouping data based on the quarter in which a case is reported, using all cases reported by a given cutoff date, is compared with the usual method of categorization based on a time-delay between diagnosis and reporting. The two methods of tabulation are illustrated using cases of the acquired immunodeficiency syndrome (AIDS) diagnosed and reported in Australia. A simple simulation study confirms that estimates of adjusted quarterly AIDS counts based on the quarter of report grouping are less variable than those based on the time-delay grouping.

Acquired Immunodeficiency Syndrome↗
PubMed1995

Sex-specific causal factors and effects of common environment for symptoms of anxiety and depression in twins.

Two thousand five hundred seventy intact pairs and 724 single responders from Norwegian twins aged 18-25 years completed questionnaires with information about anxiety and depression and perceived cotwin closeness. The aim of the study was the univariate estimation of sex-specific genetic and environmental effects on an index tapping symptoms of anxiety and depression. An index of social closeness between cotwins was significantly related to the cotwin correlation for anxiety/depression scores. MZ pairs were reported to be closer than DZ pairs, and like-sexed DZ pairs were closer than unlike-sexed pairs. The symptom data were adjusted for this apparent violation of the "equal-environment" assumption in twin studies, but the adjustment did not dramatically affect the parameter estimates of genetic and environmental effects on anxiety/depression. A model specifying male (aM) and female (aF) genetic additive effects, shared environment for males (cM), and individual environmental effects (eM and eF) fitted the adjusted data very well. An alternative model, specifying aM = aF, cM = cF, and eM = eF, and no correlation between those environmental factors shared by brothers and those shared by sisters, fitted equally well. Estimated proportions of total variance from the first model were aM2 = .30, aF2 = .52, and cM2 = .21. The estimates from the second model were aM2 = aF2 = .43 and cM2 = cF2 = .11.

Adolescent↗
PubMed2002

Long-chain n-3-to-n-6 polyunsaturated fatty acid ratios in breast adipose tissue from women with and without breast cancer.

Animal studies suggest that dietary polyunsaturated fatty acids (PUFAs) of the n-6 class, found in corn and safflower oils, may be precursors of intermediates involved in the development of mammary tumors, whereas long-chain (LC) n-3 PUFAs, found in fish oil, can inhibit these effects. This case-control study was designed to examine the relationship between the PUFA composition of breast adipose tissue and the risk of breast cancer. Using fatty acid levels in breast adipose tissue as a biomarker of past qualitative dietary intake of fatty acids, we examined the hypothesis that breast cancer risk is negatively associated with specific LC n-3 PUFAs (eicosapentaenoic acid and docosahexaenoic acid) and positively associated with n-6 PUFAs (linoleic acid and arachidonic acid). Breast adipose tissue was collected from 73 breast cancer patients and 74 controls with macromastia. The fatty acid levels were determined by gas-liquid chromatography. A logistic regression model was used to obtain odds ratio estimates while adjusting for age. The age-adjusted n-6 PUFA (linoleic acid and arachidonic acid) content was significantly higher in cases than in controls (P = 0.02). There was a trend in the age-adjusted data suggesting that, at a given level of n-6 PUFA, LC n-3 PUFAs (eicosapentaenoic acid and docosahexaenoic acid) may have a protective effect (P = 0.06). A similar inverse relationship was observed with LC n-3-to-n-6 ratio when the data were adjusted for age (P = 0.09). We conclude that total n-6 PUFAs may be contributing to the high risk of breast cancer in the United States and that LC n-3 PUFAs, derived from fish oils, may have a protective effect.

Adipose Tissue↗
PubMedDate not supplied

Cohort survival in ageing populations: a model life table approach.

"In populations where higher proportions are surviving longer, cohort life tables are a valuable means of measuring changes and anticipating future developments. Yet cohort life tables are calculated only infrequently from national mortality statistics because of difficulties in adjusting observed data and projecting cohort survival. To facilitate regular updating, an approach to cohort life table construction is needed that circumvents the problems of data adjustment and projection. This paper proposes a method based on model life tables which provides information consistent with official life tables and projections of life expectancy. The paper also compares the results with those obtained by other methods and discusses some implications of trends in cohort survival." (SUMMARY IN ITA AND FRE)

Cohort Studies↗