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S D Walter

Publications and source records attributed to S D Walter.

At least 109 records · Page 6Linked to original sources
PubMed1988

A reappraisal of the kappa coefficient.

Kappa is frequently used in epidemiology as an index of the quality of measurement for binary characteristics. The authors discuss the strong dependence of kappa on true prevalence, and they examine the relationship of the value of kappa to the degree of attenuation of the odds ratio that results from non-differential misclassification. It is concluded that under certain circumstances kappa can be interpreted as an indicator of validity, i.e. unbiasedness of the odds ratio, rather than simply as one of reliability. Cautions are stressed regarding (1) possible variation in the quality of measurement and (2) possible lack of independence of errors for the paired measurements from which kappa is calculated. An important implication for the design of reliability studies is that they should be conducted in populations where the distribution of the factor of interest is similar to that for the settings in which the measurement technique will ultimately be applied.

Data Interpretation, Statistical↗
PubMed1988

Estimation of test error rates, disease prevalence and relative risk from misclassified data: a review.

We review methods for the analysis of categorical clinical and epidemiological data, in which the observations are subject to misclassification. Under certain conditions, it is possible to estimate error parameters such as sensitivity, specificity, relative risk, or predictive value, even though no definitive classification (gold standard) is available. The parameter estimates are obtained by modelling the data, using maximum likelihood, with or without some constraints. The models recognize that the true classification of an individual is unknown, and so are sometimes referred to as "latent class" models. The latent class approach provides a unified framework for various methods found in a dispersed literature, characterising each by the number of populations or subgroups in the data, and the number of observations made on each individual; the statistical degrees of freedom are implied by the sampling design. Data sets with less than three replicate observations per individual necessarily require constraints for parameter estimation to be possible. Data sets with three or more replicates lead directly to estimates of the misclassification rates, subject to some simple assumptions. Some more complex problems are also discussed, including data where the response variable has more than two levels, sequential and irregular designs and the effects of assumption violations.

Classification↗
PubMed1988

A comparison of physician and patient reports of Pap smear histories.

We describe the level of agreement between patients and their physicians with respect to their use of the Pap smear, the symptoms present, and the smear results. The data derived from a case-control interview study of 181 newly diagnosed invasive squamous cell cervical cancer cases and 905 age-matched controls, a second case-control interview study of 250 cases of cervical dysplasia and 500 age-matched controls, and the medical records of attending physicians for all patients. Cases and controls in both studies reported more smears over the previous 5 years than their medical records indicated; also patients reported their last smear as more recent than noted in physician files. Cancer cases were somewhat more accurate on frequency and timing of smears than the other patient groups. Patients tend to report more symptoms at interview than recorded in their files. Disagreements on smear results were probably because of differences in terminology, especially in distinguishing invasive from pre-cancerous conditions; cancer cases tended to report some dysplasias as cancer, but the controls in both studies under-reported dysplasia. If high physician response rates can be obtained, we would prefer to use physician records as the basis of evaluation of screening programs, especially when accurate dating of screening events is required. However it is less clear whether physician records are to be preferred, if the evaluation is to take symptoms into account.

Adult↗
PubMed1988

Effect of contact letter on control response rates in cancer studies.

Our analysis of response rates for community controls approached in a retrospective study of melanoma indicates that there was an 11 per cent improvement in response rate when the contact letter was changed from inviting participation in a "healthy study" with no mention of cancer to a format which explicitly explained that the study concerned cancer. This improvement was observed for all age-sex groups except younger males. The result suggest that higher response rates may be achieved in studies of cancer if fuller disclosure of the study purposes is made.

Adult↗
PubMed1988

Operation Everest II: oxygen transport during exercise at extreme simulated altitude.

A decrease in maximal O2 uptake has been demonstrated with increasing altitude. However, direct measurements of individual links in the O2 transport chain at extreme altitude have not been obtained previously. In this study we examined eight healthy males, aged 21-31 yr, at rest and during steady-state exercise at sea level and the following inspired O2 pressures (PIO2): 80, 63, 49, and 43 Torr, during a 40-day simulated ascent of Mt. Everest. The subjects exercised on a cycle ergometer, and heart rate was recorded by an electrocardiograph; ventilation, O2 uptake, and CO2 output were measured by open circuit. Arterial and mixed venous blood samples were collected from indwelling radial or brachial and pulmonary arterial catheters for analysis of blood gases, O2 saturation and content, and lactate. As PIO2 decreased, maximal O2 uptake decreased from 3.98 +/- 0.20 l/min at sea level to 1.17 +/- 0.08 l/min at PIO2 43 Torr. This was associated with profound hypoxemia and hypocapnia; at 60 W of exercise at PIO2 43 Torr, arterial PO2 = 28 +/- 1 Torr and PCO2 = 11 +/- 1 Torr, with a marked reduction in mixed venous PO2 [14.8 +/- 1 (SE) Torr]. Considering the major factors responsible for transfer of O2 from the atmosphere to the tissues, the most important adaptations occurred in ventilation where a fourfold increase in alveolar ventilation was observed. Diffusion from alveolus to end-capillary blood was unchanged with altitude. The mass circulatory transport of O2 to the tissue capillaries was also unaffected by altitude except at PIO2 43 Torr where cardiac output was increased for a given O2 uptake. Diffusion from the capillary to the tissue mitochondria, reflected by mixed venous PO2, was also increased with altitude. With increasing altitude, blood lactate was progressively reduced at maximal exercise, whereas at any absolute and relative submaximal work load, blood lactate was higher. These findings suggest that although glycogenolysis may be accentuated at low work loads, it may not be maximally activated at exhaustion.

2,3-Diphosphoglycerate↗
PubMed1987

Evaluating the survival of cancer cases detected by screening.

Early detection of cancer by screening advances the date of diagnosis, but may or may not affect survival. To assess the survival benefit associated with early detection, one must estimate the distribution of time survived post lead-time, that is, after the unknown date when clinical diagnosis would have occurred in the absence of screening. One can then compare the adjusted survival of screen-detected cancer cases to other groups of cases not diagnosed by screening. This paper describes a model for the survival of screen-detected cases, with a hazard function that depends on an individual's lead time, the duration of preclinical disease, and the time since diagnosis. The model is fitted to the ten year survival data from the 132 screen-detected cases of breast cancer in the well-known HIP (Health Insurance Plan of Greater New York) study. Comparison with the survival of several groups of cancer cases not detected by screening (interval cases arising clinically in persons previously screened, cases among persons who refuse screening, and cases among randomized controls not offered screening) yields various estimates of benefit. Use of the interval cases for comparison gives an estimate of about 21 breast cancer deaths prevented among 20,166 women screened in the HIP study; use of the data from the randomized controls gives an estimate of about 25 prevented deaths. The former estimate derives from the screened group of women only, and so the same method of evaluation may also be applied to community screening programmes and other situations that do not entail randomization.

Actuarial Analysis↗
PubMed1987

Coding ordinal independent variables in multiple regression analyses.

The authors present a coding scheme for ordinal independent variables which may be used in various forms of regression analysis. The scheme is useful in dose-response analyses, when the objective is to identify contrasts in the dependent (or response) variable between successive levels of the independent variable, or to identify critical threshold values of the independent variables at which significant changes occur in the response. An example is given of evaluating the survival of lung cancer patients according to their stage of symptomatology. The authors discuss the interpretation of the regression coefficients when this coding scheme is used with linear regression, logistic regression, or in the proportional hazards regression model.

Humans↗
PubMed1987

Observer perception of skin color in a study of malignant melanoma.

Observer perceptions of skin color with a 15-step skin tone panel were evaluated during an as yet unpublished case-control study of malignant melanoma. Skin color is a risk factor for melanoma, and the skin tone panel was introduced in an effort to reduce its misclassification. Reflectances of the 15 artificial skin tones were measured at four wavelengths with a reflectance spectrophotometer. Six observers each evaluated the skin color of eight study subjects twice under three lighting conditions, and the results were transformed to reflectance values. Components of variance analysis demonstrated that between-subject variability contributed 63% or more of the variance at wave-lengths of 400-600 nm, while observers, light source, observation time, and error contributed 30% or less. At 700 nm, only 25.5% of the variance was due to subjects, indicating lower levels of reliability. Similarly, the correlation of visual and spectrophotometric assessment of skin reflectance was higher at 400-600 nm (r = 0.63-0.71) than at 700 nm (r = 0.41). Thus, the value of the skin tone panel-based assessments depends upon knowledge of which wavelengths most closely relate to the physiologic risk factor. For instance, reflectance at 650-700 nm is a better measure of skin melanin content than reflectance at lower wavelengths. Since the role of melanin as a risk factor remains in doubt, the utility of this technique has yet to be demonstrated. However, data from the case-control study and from this validity and reliability study will allow us to develop an analytic approach that minimizes misclassification of skin color as a confounder.

Analysis of Variance↗
PubMed1987

Evaluation of public health preschool child developmental screening: the process and outcomes of a community program.

We conducted a randomized controlled trial of a public health and education screening program aimed at all 4,797 four to five year old children registering for kindergarten in three school districts of southern Ontario, Canada. Children received either the Denver Developmental Screening Test (DDST) with a community health intervention program for positive screeness; the DDST with no intervention for positive screenees; or no screening test. The intervention program consisted of referral to the child's physician for assessment; a review conference between the child's teacher and the school health nurse; parent counseling; and monitoring of the child in school by the school health nurse. At the end of the third school year, no differences were found between positive screenees in the community health intervention group and the "no intervention" groups using individual academic achievement, cognitive, and developmental tests. Parents' reports revealed no differences between groups in children's mental, social, and behavioral well-being. However, parents of intervention program children had more worry about their child's school progress suggesting a potentially harmful labeling effect. In comparison to a random sample of children with normal DDST results, or a random sample of children who had randomly not been screened, the children with positive preschool DDSTs had substantially more school problems three years after screening.

Child Development↗
PubMed1987

Personal and job-related factors as determinants of incidence of back injuries among nursing personnel.

The purpose of this multicenter prospective cohort study was to assess personal and job-related factors as determinants of incidence of reported back injuries among nurses. The study population included 5,649 nurses who were surveyed by questionnaire and then observed for a 12-month study period. An annual injury rate of 4.9% was observed. Four factors were found to be significant (P less than .01) predictors of back injury. All four factors--service area, lifting, job category, and previously reported back injury--maintain significance when a forward stepping model of logistic regression is applied. The adjusted odds ratios observed are 4.26 for service areas where lifting occurs most as compared with areas where lifting occurs least; 2.19 for daily lifters v light, occasional, and nonlifters; 1.77 for nursing aides v registered nurses and supervisory personnel; and 1.73 for individuals who have previously reported back injury v those who have not reported previous injury. These findings strongly suggest that job-related rather than personal characteristics are the major predictors of back injury in nurses.

Accidents, Occupational↗
PubMed1986

Case-control studies for the evaluation of screening.

The case-control approach has been proposed for the evaluation of the efficacy of screening for cancer. It provides information at the level of the individual subject and allows direct estimation of the relative risk of not being screened. The criteria for the choice of the case and control series are examined, with reference to the type of cancer considered and the precise goal assigned to the screening programme. The definition of the exposure is discussed, as well as criteria for the avoidance of commonly occurring biases. Typical situations where case-control studies of screening efficacy appear to be the method of choice are presented.

Adult↗
PubMed1986

An analysis of Berkson's bias in case-control studies.

The bias described by Berkson arises as a mathematical phenomenon, caused by the probabilistic union of different rates of hospitalization for people with different medical phenomena. When the concept is extended to case-control studies, these rates will occur as hd for people with the target disease, he for people with the control condition, and hc for the separate effect of exposure to the suspected etiologic agent. An algebraic analysis of patterns of hospitalization and case-control selection demonstrates that Berkson's bias will be avoided if both cases and controls are chosen from the community or if he = 0. When the cases are chosen from hospitalized patients, the odds ratio will be biased if, as in the usual clinical situation, he not equal to 0. The odds ratio will be falsely elevated if the control groups are chosen from a community population rather than from hospitalized patients, and falsely lowered if the controls are hospitalized patients who do not have the target disease. If the control groups are chosen from patients hospitalized with specific comparison conditions, the odds ratio will be falsely elevated or lowered, depending on the relative magnitudes of hd and hc. In Berkson's mathematical model, the probabilistic calculations depend on the assumption that each of the exposed or diseased clinical conditions has an independent additive effect on hospitalization rates. In reality, however, the concurrence of two or more conditions of disease and exposure may synergistically affect the examining physician's nosocomial decisions and may thereby substantially change the hospitalization rates from what is expected mathematically. In creating hospitalization bias in case-control studies, these selective clinical decisions about referral to hospital may be more cogent than the probabilistic distinctions described by Berkson.

Biometry↗
PubMed1986

The relationship of cancer incidence to terrestrial radiation and population density in Connecticut, 1935-1974.

The relationship of cancer incidence to terrestrial radiation and population density was investigated. Cancer incidence was obtained using 40 years of age-standardized data from the Connecticut Tumor Registry, and environmental radiation was estimated using data from an airborne gamma radiation survey of the entire state. These variables were examined ecologically, using the 169 towns of the state as the analytic units in a weighted regression analysis. The study design involves a large population base in a state having relatively high terrestrial radiation exposure levels overall and reasonable variation in exposure between towns. For all cancer combined, only one of the eight sex-specific analyses by decade yielded a significant radiation regression coefficient, and this was negative. In the sex- and site-specific analyses, almost all the coefficients for radiation were not significantly different from zero. In contrast, significant positive relationships of cancer incidence with population density were found for all cancer, for cancer of the lung for both sexes, for stomach, colonic, and prostatic cancer for males, and for lymphomas, thyroid, breast, and ovarian cancer for females. Both the radiation and population density relationships were adjusted for socioeconomic status. Socioeconomic status was significantly negatively associated with stomach and lung cancer in males and with cervical cancer in females; it was also positively associated with lymphomas and breast cancer in females. A power calculation revealed that, despite the relatively large size of this study, there was only a small probability of detecting a radiation effect of the strength anticipated from previous estimates.

Air Pollution, Radioactive↗
PubMed1986

Occupational risk factors for laryngeal carcinoma: Connecticut, 1975-1980.

A case-control study of possible occupational risk factors for laryngeal carcinoma in white males in the New Haven, Connecticut, area included 92 cases diagnosed between 1975 and 1980 and 181 hospital controls individually matched to the cases on age, year and hospital of admission, county of residence, smoking status (current vs. ex-smoker), and type of tobacco used at the time of admission. Only cases and controls who were alive at the time of the study were included. With the effects of tobacco and alcohol controlled in a conditional linear logistic model, elevated odds ratios were found for men who had ever worked in rubber products manufacturing or transportation equipment manufacturing other than shipbuilding, and for men who had ever been machinists, bartenders, farmers, masons, or metal grinders. However, only one occupation, machinists, had a statistically significant odds ratio (2.5, 95% confidence interval = 1.2-5.2) in these multivariate analyses. Asbestos and nickel were not found to be risk factors for laryngeal carcinoma. Amount of tobacco smoked and alcohol consumed were positively associated with risk for laryngeal cancer.

Aged↗
PubMed1985

Correlations of radiographic analysis of healing fractures with strength: a statistical analysis of experimental osteotomies.

The assessment of fracture healing strength using routine roentgenograms is difficult and controversial. There are few experimental data that correlate radiographic appearance with the actual quantitative strength of healing fractures. However, this method is widely used in clinical practice. A study is presented in which transversely osteotomized rabbit tibiae were allowed to heal for 3 to 8 weeks. A pair of orthogonal roentgenograms was taken of each bone and the bones were tested for strength in a dynamic torsion testing machine. Statistical analyses were done to study the correlations between the roentgenographic and strength parameters. Cortical continuity was found to be the best single predictor of strength of a healing fracture (correlation coefficient r = 0.80). The least important predictor was the callus area (r = 0.17). Fracture displacement, callus thickness, and callus diameter had negative correlations. From these experimental findings in an animal model, we conclude that even under laboratory conditions the information gained from plain radiographs is not sufficient to accurately predict the strength of a healing fracture.

Animals↗
PubMed1985

An epidemiologic study of the relationship between postural asymmetry in the teen years and subsequent back and neck pain.

This epidemiologic study examined the relationship between postural discrepancies in the posterior plane and the subsequent development of back and neck pain. The study population comprised women who were members of the graduating classes of 1957, 1958, and 1959 of an eastern U.S. women's college. Information on postural asymmetry was gathered from two sources: (1) measurements made from posture pictures taken early in the freshman year of college and obtained from each woman in the study, and (2) subjective evaluations made by the physical education department faculty at the time of examination. Information on the development of back and neck disorders and associated risk factors during the subsequent 25 years was obtained by a postal questionnaire. Three parameters of postural asymmetry were examined from the posture picture measurements: elevation of one shoulder, elevation of one hip, and deviation of the spine from the midline of the body. None of these parameters nor the physical education department evaluations was associated with a subsequent report of low-back pain, mid-back pain, or neck pain.

Adolescent↗
PubMed1984

Thoracic spine centers of rotation in the sagittal plane.

The purpose of this study was to determine in vitro the centers of rotation of thoracic functional spinal units in the sagittal plane. The center of rotation is a convenient concept and part of a precise method of documenting the kinematics of a joint moving in a plane. Fresh cadaver functional spinal units from the thoracic region were utilized. Six load types were used that produced motions only in the sagittal plane, namely anterior and posterior shear forces, flexion and extension moments, and compression and distraction forces. The resulting motion with three degrees of freedom was measured with dial gauges. Statistical methods were used to analyze data from the viewpoint of vertebral level, load magnitude, and load type. Only the load type was found to be significantly related to the location of the centers of rotation. Although there was significant variability in the centers of rotation, there were definite locations related to each load type. The average center of rotation was 15-45 mm directly below the geometric center of the moving vertebra. The results of the present study may be helpful in the clinical interpretation of spinal kinematic studies.

Adult↗