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Biomedical subjects

S D Walter

Publications and source records attributed to S D Walter.

At least 19 recordsLinked to original sources
PubMed2003

Mammographic screening: case-control studies.

BACKGROUND: The case-control design can be used to evaluate the benefit of cancer screening programmes. MATERIALS AND METHODS: This paper outlines the main methodological features of the case-control design in this context, and indicates some potential biases. It also reviews the existing case-control literature on mammographic screening. RESULTS: Case-control studies consistently indicate a reduction of approximately 50% in breast cancer mortality associated with mammography. This result indicates greater benefit than shown in randomised trials; however, one should recognise that trials indicate effectiveness whereas case-control studies indicate efficacy. The two types of evidence are broadly compatible when one allows for screening non-compliance and contamination in the randomised trials. CONCLUSIONS: The case-control evidence supports and is consistent with the findings of randomised trials of mammography. Effectiveness estimates from trials indicate the benefit of screening to the population as a whole, and are pertinent to the public policy debate as to the value of offering screening. In contrast, case-control studies indicate benefit to actual screening participants. As such, case-control estimates of efficacy are appropriate for individual decision-making by women about their use of mammography when it is potentially available to them.

Adult↗
PubMed2002

Properties of the summary receiver operating characteristic (SROC) curve for diagnostic test data.

The summary receiver operating characteristic (SROC) curve has been recommended to represent the performance of a diagnostic test, based on data from a meta-analysis. However, little is known about the basic properties of the SROC curve or its estimate. In this paper, the position of the SROC curve is characterized in terms of the overall diagnostic odds ratio and the magnitude of inter-study heterogeneity in the odds ratio. The area under the curve (AUC) and an index Q(*) are discussed as potentially useful summaries of the curve. It is shown that AUC is maximized when the study odds ratios are homogeneous, and that it is quite robust to heterogeneity. An upper bound is derived for AUC based on an exact analytic expression for the homogeneous situation, and a lower bound based on the limit case Q(*), defined by the point where sensitivity equals specificity: Q(*) is invariant to heterogeneity. The standard error of AUC is derived for homogeneous studies, and shown to be a reasonable approximation with heterogeneous studies. The expressions for AUC and its standard error are easily computed in the homogeneous case, and avoid the need for numerical integration in the more general case. SE(AUC) and SE(Q(*)) are found to be numerically close, with SE(Q(*)) being larger if the odds ratio is very large. The methods are illustrated using data for the Pap smear screening test for cervical cancer, and for three tests for the diagnosis of metastases in cervical cancer patients.

Area Under Curve↗
PubMed2001

Number needed to treat (NNT): estimation of a measure of clinical benefit.

The number needed to treat (NNT) is becoming increasingly popular as an index for reporting the results of randomized trials and other clinical studies. It represents the expected number of patients who must be treated with an experimental therapy in order to prevent one additional adverse outcome event (or, depending on the context, to expect one additional beneficial outcome), compared to the expected event rates under the control therapy. Although NNT is a clinically useful measure, little work has been done on its statistical properties. In this paper, alternative NNT-type measures are defined for use with discrete or continuous data. Estimators and their variances are obtained for these measures in cross-over or parallel group designs. The ideas are illustrated with data on quality of life in asthma patients.

Albuterol↗
PubMed2001

The attributable mortality and length of intensive care unit stay of clinically important gastrointestinal bleeding in critically ill patients.

OBJECTIVE: To estimate the mortality and length of stay in the intensive care unit (ICU) attributable to clinically important gastrointestinal bleeding in mechanically ventilated critically ill patients. DESIGN: Three strategies were used to estimate the mortality attributable to bleeding in two multicentre databases. The first method matched patients who bled with those who did not (matched cohort), using duration of ICU stay prior to the bleed, each of six domains of the Multiple Organ Dysfunction Score (MODS) measured 3 days prior to the bleed, APACHE II score, age, admitting diagnosis, and duration of mechanical ventilation. The second approach employed Cox proportional hazards regression to match bleeding and non-bleeding patients (model-based matched cohort). The third method, instead of matching, derived estimates based on regression modelling using the entire population (regression method). Three parallel analyses were conducted for the length of ICU stay attributable to clinically important bleeding. SETTING: Sixteen Canadian university-affiliated ICUs. PATIENTS: A total of 1666 critically ill patients receiving mechanical ventilation for at least 48 hours. MEASUREMENTS: We prospectively collected data on patient demographics, APACHE II score, admitting diagnosis, daily MODS, clinically important bleeding, length of ICU stay, and mortality. Independent adjudicators determined the occurrence of clinically important gastrointestinal bleeding, defined as overt bleeding in association with haemodynamic compromise or blood transfusion. RESULTS: Of 1666 patients, 59 developed clinically important gastrointestinal bleeding. The mean APACHE II score was 22.9 +/- 8.6 among bleeding patients and 23.3 +/- 7.7 among non-bleeding patients. The risk of death was increased in patients with bleeding using all three analytic approaches (matched cohort method: relative risk [RR]= 2.9, 95% confidence interval (CI)= 1.6-5.5; model-based matched cohort method: RR = 1.8, 95% CI = 1.1-2.9; and the regression method: RR = 4.1, 95% CI = 2.6-6.5). However, this was not significant for the adjusted regression method (RR = 1.0, 95% CI = 0.6-1.7). The median length of ICU stay attributable to clinically important bleeding for these three methods, respectively, was 3.8 days (95% CI = -0.01 to 7.6 days), 6.7 days (95% CI = 2.7-10.7 days), and 7.9 days (95% CI = 1.4-14.4 days). CONCLUSIONS: Clinically important upper gastrointestinal bleeding has an important attributable morbidity and mortality, associated with a RR of death of 1-4 and an excess length of ICU stay of approximately 4-8 days.

APACHE↗
PubMed2001

Estimating the number needed to treat (NNT) index when the data are subject to error.

The number needed to treat (NNT) index has been proposed as a clinically useful measure to assess the results of randomized trials and other clinical studies. In its usual form, NNT indicates the expected number of patients who must be treated with an experimental therapy in order to prevent one adverse event, compared to the expected event rates under the control therapy. It can be formulated as a function of the proportions of patients who respond to treatment by more than a certain amount, the clinically important difference. We may also wish to evaluate two group studies comparing treatment and control responses, and to consider net benefit from treatment (by also allowing for individuals who deteriorate as well as those who respond positively). In this paper, we investigate the effect on NNT caused by measurement errors in continuous outcome measures. Such errors can lead to bias in the estimated proportions of subjects with clinically important responses, and hence bias the associated values of NNT. General expressions for the bias are derived, and enumerated for typical scenarios. For many situations, reliability of 80 per cent or more in the observations is required to restrict the bias to tolerable levels.

Bias↗
PubMed2001

A comparison of methods to detect publication bias in meta-analysis.

Meta-analyses are subject to bias for many of reasons, including publication bias. Asymmetry in a funnel plot of study size against treatment effect is often used to identify such bias. We compare the performance of three simple methods of testing for bias: the rank correlation method; a simple linear regression of the standardized estimate of treatment effect on the precision of the estimate; and a regression of the treatment effect on sample size. The tests are applied to simulated meta-analyses in the presence and absence of publication bias. Both one-sided and two-sided censoring of studies based on statistical significance was used. The results indicate that none of the tests performs consistently well. Test performance varied with the magnitude of the true treatment effect, distribution of study size and whether a one- or two-tailed significance test was employed. Overall, the power of the tests was low when the number of studies per meta-analysis was close to that often observed in practice. Tests that showed the highest power also had type I error rates higher than the nominal level. Based on the empirical type I error rates, a regression of treatment effect on sample size, weighted by the inverse of the variance of the logit of the pooled proportion (using the marginal total) is the preferred method.

Computer Simulation↗
PubMed2001

Sources of heterogeneity in the meta-analysis of observational studies: the example of SIDS and sleeping position.

The study objective was to provide an example of how risk estimates might vary across studies of observational design, even when a causal association is present and to explore the possible sources of such variation. A meta-analysis of studies on the association between prone sleeping position and sudden infant death syndrome (SIDS) is used to illustrate how risk estimates might vary across studies. Data used were reported case-control studies of the association between sleeping position and SIDS that were published between 1970 and 1994. If the pooled odds ratio had been relied on to assess the association between sleeping position and SIDS without an accompanying examination of the reasons for heterogeneity, important insights into the causal significance of the relationship may have been lost. In meta-analyses of observational studies it is important to investigate the reasons for heterogeneity across studies.

Bias↗
PubMed2001

Gross motor function of children with down syndrome: creation of motor growth curves.

OBJECTIVE: To create gross motor function growth curves for children with Down syndrome (DS) and to estimate the probability that motor functions are achieved by different ages. DESIGN: Nonlinear growth curve analysis by using a 2-parameter (rate, upper limit) model. SETTING: Early intervention programs, schools, and children's homes. PARTICIPANTS: One hundred twenty-one children with DS, ages 1 month to 6 years. MAIN OUTCOME MEASURES: Gross Motor Function Measure (GMFM) and severity of motor impairment. RESULTS: The curves for children with mild (n = 51) and moderate/severe (n = 70) impairment were characterized by a greater increase in GMFM scores during infancy and smaller increases as the children approached the predicted maximum score of 85.9 or 87.9. The estimated probability that a child would roll by 6 months was 51%; sit by 12 months, 78%; crawl by 18 months, 34%; walk by 24 months, 40%; and run, walk up stairs, and jump by 5 years, 45% to 52%. CONCLUSIONS: Children with DS require more time to learn movements as movement complexity increases. Impairment severity affected the rate but not the upper limit of motor function. The results have implications for counseling parents, making decisions about motor interventions, and anticipating the time frame for achievement of motor functions.

Child↗
PubMed2001

Relation of distribution- and anchor-based approaches in interpretation of changes in health-related quality of life.

BACKGROUND: Approaches to interpretation of quality of life changes in clinical trials have fallen into two camps: those that rely on the distribution of changes and the Effect Size (ES), and those that use some external anchor, such as patient judgments of change, which is then used to compute a Minimally Important Difference (MID), the proportion benefiting from treatment, p(B), and the Number Needed to Treat (NNT). OBJECTIVE: To examine the relationship between the ES and p(B), and the impact of the MID on this relationship. METHODS: Simulation was used based on a normal distribution to compute the proportion of patients benefiting in both parallel group and crossover designs, for various values of the ES and the MID. The agreement of the simulation with empirical data from four studies of asthma and respiratory disease was assessed. The effect of skewness in the distributions of change scores on the relationship between ES and p(B) was also examined. RESULTS: The simulation showed a near-linear relationship between ES and p(B), which was nearly independent of the value of the MID. Agreement of the simulation with the empirical data were excellent. Although the curves differed for crossover and parallel group designs, the general form was similar. Introducing moderate skew into the distributions had minimal impact on the relationship. CONCLUSIONS: The proportion of patients who will benefit from treatment can be directly estimated from the ES, and is nearly independent of the choice of MID. Effect size and anchor based approaches provide equivalent information in this situation.

Chronic Disease↗
PubMed2001

Development and testing of formal protocols for oxygen prescribing.

The absence of standardized assessment protocols with well- defined measurement properties limits comparison of outcomes among those receiving long-term oxygen therapy (LTOT). We describe simple protocols for a hospital test, a simulated home test, and an actual home test, their reliability and relationship to each other. Stable patients with exercise hypoxemia participated. In 74 patients who completed four exercise tests, correlations between tests ranged from 0.85 to 0.78. Of these 27.0% had the same prescription from all four tests. In 46% prescriptions were within 1 L/ min and in 27% within 2 L/min. During exercise the hospital tests suggested slightly higher oxygen prescriptions than did the simulated home tests (2.5 L/min versus 2.0 L/min, p < 0.001). In 23 patients who participated in actual home assessments, the correlations between the home test, the hospital, and the simulated home tests were 0.22 (95% CI -0.24 to 0.67) and 0.27 (95% CI -0.18 to 0.72). In conclusion, standardizing tests for the assessment of LTOT is important. We describe simple hospital and simulated home tests that are reproducible, easy to carry out, and correlate well with each other.

Aged↗
PubMed2001

Using the gross motor function measure to evaluate motor development in children with Down syndrome.

The purpose of this article is to describe our clinical experiences in using the Gross Motor Function Measure (GMFM) to evaluate motor development in children with Down syndrome and to provide strategies we found helpful in enhancing a child's adherence to standardized testing. The issues discussed are: (1) strategies for test administration; (2) modifications in administration and scoring; (3) reliability of the GMFM using the modified administration and scoring procedures; and (4) applications of the GMFM for clinical practice. The strategies and recommendations address the particular characteristics of children with Down syndrome and allow for their progress to be monitored relative to other children with Down syndrome rather than to children without motor delays. Future studies validating the use of specific goal areas for the administration and scoring of the GMFM for children with Down syndrome are recommended.

Activities of Daily Living↗
PubMed2000

Choice of effect measure for epidemiological data.

The debate concerning the choice of effect measure for epidemiologic data has been renewed in the literature, and it suggests some continuing disagreement between the pertinent clinical and statistical criteria. In this article, some defining characteristics of the main choices of effect measure [risk difference (RD), relative risk (RR), and odds ratio (OR)] for binary data are presented and compared, with consideration of both the clinical and statistical perspectives. Relationships of these measures to the relative risk reduction (RRR) and number needed to treat (NNT) are also discussed. A numerical comparison of models of constant RD, RR, and OR is made to assess when and by how much they might differ in practice. Typically the models show only small numerical differences, unless extreme extrapolation is involved. The RD and RR models can predict impossible event rates, either less than zero or greater than 100%. Each measure has potential theoretical justification. RD and RR may enjoy some advantages for communication of risk, but OR may be preferred for data analysis. A clear distinction should be maintained between the objectives of data analysis and subsequent risk communication, and different effect measures may be needed for each.

Humans↗
PubMed2000

Needs-based primary medical care capitation: development and evaluation of alternative approaches.

OBJECTIVE: To develop and evaluate alternative methods of adjusting primary medical care capitation payments for variations in relative need for health care among enrolled practice populations. METHODS: We developed alternative needs-based capitation formulae and applied them to a sample of capitation-funded primary care practices to assess each formula's performance against a reference standard of capitation payments based on age, sex and self-assessed health status of the enrolled populations. The alternative formulae were based on: (1) age and sex; (2) age, sex and individually-measured socioeconomic characteristics; (3) age, sex and socioeconomic characteristics imputed from census data for enrollees' neighbourhood of residence; (4) age, sex and standardized mortality ratio for enrollees' neighbourhood of residence. RESULTS: Age/sex-adjusted capitation payments for the six practices studied ranged from 10% higher to 18% lower than the reference standard payments. Capitation formulae based on socioeconomic and mortality data did not perform consistently better than the current age/sex-based formula. CONCLUSIONS: Primary medical care capitation payments adjusted only for age and sex do not reflect the relative health care needs of enrolled practice populations. Our alternative formulae based on socioeconomic and mortality data also failed to reflect relative needs. Methods that use other approaches to adjusting for differences in relative need among enrolled populations should be investigated.

Adolescent↗
PubMed2000

Do larger people have more naevi? Naevus frequency versus naevus density.

BACKGROUND: It is unclear which of the number or the density of naevi on the skin is the more appropriate measure of risk of melanoma. Furthermore, the relationship between the number of naevi and their density in an individual has not been explored. Thus, for example, it is unknown if larger people tend to have more naevi by virtue of having a larger skin area, or if the density of naevi is similar in people of different body sizes. In this study, we explored the relationship between the number and the density of naevi in a sample of adolescents. SUBJECTS AND METHODS: A sample survey of naevi in 472 grade 9 secondary school students (aged 14-15 years) was conducted in Tasmania, Australia during 1992, and a subset of these individuals was followed up in 1997. Counts of naevi of various sizes were taken on the arm, leg, and back. Naevus density was estimated by using an algorithm to estimate body surface area from the height and weight of an individual. More general relationships of the naevus counts to height and weight were also explored. Finally, we considered whether the relationship between naevus density and the anthropometric variables could be confounded by exposure to ultraviolet radiation. RESULTS: The mean number of naevi was very similar in the two samples. Naevus density was slightly lower in the 1997 sample, mainly because of increasing body size in the cohort. The numbers of naevi were only weakly related to height and weight in males, and there was essentially no relationship in females. Regression analysis showed significant relationships of weight to the back naevus counts in males in 1992 and 1997, and to the arm naevus count in males in 1997; otherwise, none of the regression coefficients for height and weight were statistically significant. This picture did not change following adjustment for potentially confounding variables indicating time spent outdoors or in the sun. Furthermore, there was no evidence that time spent in the sun was related to the body mass index. CONCLUSIONS: It appears that the number and density of naevi in an individual are unrelated. Accordingly, with the present state of knowledge concerning the risk of melanoma, both the number and density of naevi should be considered as equally valid in future studies as markers of the risk of melanoma, and in studies on the natural history of naevi. If the disease mechanism is systemic, and not related to particular naevi, naevus density might form the better marker of risk. However, if the disease mechanism is related to effects on particular naevi, then the risk would vary in proportion to the number of naevi.

Adolescent↗
PubMed2000

A new visual indicator of chlamydial cervicitis?

OBJECTIVES: To determine the usefulness of endocervical discharge opacity as a risk indicator for chlamydial infection in relation to two acknowledged visual indicators--yellow endocervical discharge and easily induced mucosal bleeding of the cervix. METHODS: Women from two family planning clinics, a therapeutic abortion clinic, and a university student health clinic (n = 1418 total) consented to a pelvic examination and chlamydia testing, and completed a questionnaire on socio-demographics, sexual behaviour, medical history, and symptoms. A case of chlamydia was defined as positive by culture or blocked enzyme immunoassay in an endocervical swab. RESULTS: The prevalence of chlamydial infection in the clinics was 6.3%. All three of the visual indicators--yellow endocervical discharge, easily induced bleeding, and opaque cervical discharge--were statistically significantly and independently associated with chlamydial infection (odds ratios 2.8, 2.3, and 2.9 respectively), independent of clinic type. Adjustment for the other visual indicators made little difference to the odds ratios. CONCLUSION: Opacity of endocervical discharge was at least as important as the other two commonly acknowledged indicators of chlamydial cervicitis--yellow endocervical discharge and easily induced mucosal bleeding of the cervix.

Adolescent↗
PubMed2000

Improved scaling of the gross motor function measure for children with cerebral palsy: evidence of reliability and validity.

BACKGROUND AND PURPOSE: This study examined the reliability, validity, and responsiveness to change of measurements obtained with a 66-item version of the Gross Motor Function Measure (GMFM-66) developed using Rasch analysis. SUBJECTS AND METHODS: The validity of measurements obtained with the GMFM-66 was assessed by examining the hierarchy of items and the GMFM-66 scores for different groups of children from a stratified random community-based sample of 537 children with cerebral palsy (CP). A subset of 228 children who had been reassessed at 12 months was used to test the hypothesis that children who are young (<5 years of age) and have "mild" CP will demonstrate greater change in GMFM-66 scores than children who are older ((5 years of age) and whose CP is more severe. Data from an additional 19 children with CP who were assessed twice, one week apart, were used to examine test-retest reliability. RESULTS: The overall changes in GMFM-66 scores over 12 months and a time ( severity ( age interaction supported our hypotheses. Test-retest reliability was high (intraclass correlation coefficient=.99). CONCLUSION AND DISCUSSION: This study demonstrated that the GMFM-66 has good psychometric properties. By providing a hierarchical structure and interval scaling, the GMFM-66 can provide a better understanding of motor development for children with CP than the 88 item GMFM and can improve the scoring and interpretation of data obtained with the GMFM.

Activities of Daily Living↗