Search PubMed⌕ Search

Biomedical subjects

D L DeMets

Publications and source records attributed to D L DeMets.

72 records · Page 4Linked to original sources
PubMed1982

Early stopping in the two-sample problem for bounded random variables.

In a fixed sample size clinical trial, the question often asked is whether the final outcome has been determined before the data has been completely collected. A particular situation occurs when the intake of subjects is slow relative to the time required for the outcome variable to be realized. We assume that the range of values the outcome variable may take can be specified in advance. We also assume that simple randomization has been used and thus, under the null hypothesis, Student's t test is a good approximation to the exact test involving the permutational distribution. In order to determine with partial data whether the outcome of the final test of hypothesis is already certain, certain nonlinear extremal problems with constraints must be solved. Analytic solutions may be expressed in the form of noniterative algorithms. Simulation studies suggest an approximation to the analytic solution. The results also provide a basis for assessing at the end of the trial, whether missing values are of consequence in the test of significance. An application based on an actual clinical trial is presented.

Clinical Trials as Topic↗
PubMed1982

A case report of data monitoring experience: the nocturnal oxygen therapy trial.

The monitoring of accumulating data in a clinical trial is a challenging endeavor, yet one that must be undertaken to fulfill the ethical responsibility to the participating subject. This paper, using the Nocturnal Oxygen Therapy Trial as an example, discusses three issues often faced by data monitoring committees. These are (1) the multiplicity of outcomes, (2) decisions about extension of the patient recruitment period in order to achieve specified sample size goals, and (3) problems in monitoring survival data with a lag in the reporting of events. The decision process can be quite complex and existing statistical methodology can at best serve as a guide in this decision process.

Clinical Trials as Topic↗
PubMed1981

History and physical examination in acute pulmonary embolism in patients without preexisting cardiac or pulmonary disease.

The history and physical examination were assessed in 215 patients with acute pulmonary embolism uncomplicated by preexisting cardiac or pulmonary disease. The patients had been included in the Urokinase Pulmonary Embolism Trial or the Urokinase-Streptokinase Embolism Trial. Presenting syndromes were (1) circulatory collapse with shock (10 percent) or syncope (9 percent); (2) pulmonary infarction with hemoptysis (25 percent) or pleuritic pain and no hemoptysis (41 percent); (3) uncomplicated embolism characterized by dyspnea (12 percent) or nonpleuritic pain usually with tachypnea (3 percent) or deep venous thrombosis with tachypnea (0.5 percent). The most frequent symptoms were dyspnea (84 percent), pleuritic pain (74 percent), apprehension (63 percent) and cough (50 percent). Hemoptysis occurred in only 28 percent. Dyspnea, hemoptysis or pleuritic pain occurred separately or in combination in 94 percent. All three occurred in only 22 percent. The most frequent signs were tachypnea (respiration ate 20/min or more) (85 percent), tachycardia (heart rate 100 beats/min or more) (58 percent), accentuated pulmonary component of the second heart sound (57 percent) and rales (56 percent). Signs of deep venous thrombosis were present in only 41 percent and a pleural friction rub was present in only 18 percent. Either dyspnea or tachypnea occurred in 96 percent. Dyspnea, tachypnea or deep venous thrombosis occurred in 99 percent. As a group, the identified clinical manifestations, although nonspecific, are strongly suggestive of acute pulmonary embolism. Conversely, acute pulmonary embolism was rarely identified in the absence of dyspnea, tachypnea or deep venous thrombosis.

Dyspnea↗
PubMed1981

The randomized clinical trial: bias in analysis.

The realization that bias in patient selection may influence the results of clinical studies has helped to establish the randomized controlled clinical trial in medical research. However, bias can be equally important at other stages of a trial, especially at the time of analysis. Withdrawing patients from consideration in the analysis because of ineligibility on account of study entry criteria, lack of compliance to the protocol, or data of poor quality may be a source of systematic error. Examples to illustrate the possible consequences are taken from trials in the cardiovascular field. We recommended that reported study results should include outcome data from all subjects randomized in the group to which they were originally assigned.

Acute Disease↗
PubMed1977

A co-operative study for the detection of the carrier state of classic hemophilia.

To determine the specificity and sensitivity of current technics for detecting carriers of classic hemophilia, three investigators simultaneously tested obligate carriers and noncarriers for immunologic and procoagulant factor VIII activity. Overall correct classification ranged from 72 per cent (36 of 50) to 94 per cent (47 of 50). The maximum accuracy obtained with the same linear-discriminant-function method on all data was 90 per cent (26 of 29) in detecting carriers without misclassifying normal persons as carriers (none of 21). Lowest accuracy by the same technic was 66 per cent (19 of 29) carrier detection while misclassifying 19 per cent (four of 21) normal persons. Precision of testing for both factor VII activity and antigen was high (standard deviations from 0.004 to 0.026 on a log scale). Differences between participants seemed related to laboratory technics rather than to statistical methods. The factor VIII activity/antigen measurement is a valid technic for detecting in the carrier state of hemophilia A.

Antigens↗
PubMed1977

The clinical features of submassive and massive pulmonary emboli.

Clinical findings in 167 patients with angiographically established pulmonary emboli were analyzed in detail. The clinical symptoms and physical findings in this group were compared with the findings in 160 patients (diagnosis established by angiography) from an earlier similar study. The observations from this, the largest known group of patients with documented pulmonary emboli that has been studied and reported on, revealed that many of the "classic signs and symptoms" occurred infrequently. Most patients in this study had prognostic value. The data from this study demonstrate that no clinical findings are specific for the diagnosis of pulmonary emboli, but the absence of isolated frequently occurring signs and symptoms should mitigate against the presence of pulmonary emboli.

Angiography↗
PubMed1977

Urinary excretion of immunoreactive prostaglandin E: a circadian rhythm and the effect of posture.

The excretion of urinary immunoreactive prostaglandin E (iPGE), sodium, potassium, creatinine and volume was studied in 4 hr collections in normal women at normal activity. iPGE exhibited a circadian rhythm with an amplitude of 29% and peak excretion at 4:55 P. M. There were also significant circadian rhythms for sodium, potassium, creatinine, and volume, all peaking in late afternoon. There were no significant changes either in the total excretion or in the circadian rhythms of iPGE, potassium, or creatinine excretion when the subjects remained in bed for an entire day while the circadian rhythms of sodium and volume were significantly modified in amplitude and phase, respectively. Urinary aldosterone excretion decreased significantly when the subjects were at bed rest. iPGE excretion increased 33% when subjects were first recumbent and then erect for consecutive 4 hr period on the same day (but when subjects were erect 1 day for a 4 hr period, iPGE excretion was lower by 32% than for the same 4 hr period the preceding day when they were recumbent). These data indicate that: 1) the sympathetic nervous system and renin-angiotensin-aldosterone system do not affect the circadian rhythms of urinary iPGE, and 2) short-term experiments of prostaglandin E excretion must be designed to avoid misleading results due to the circadian rhythm.

Adult↗
PubMed1976

Randomized clinical trials. Perspectives on some recent ideas.

In spite of the controversy over the role of randomized clinical trials in medical research, the rationale underlying such trials remains persuasive as compared to recent suggestions for alternative non-randomized studies such as those relying on the use of historical controls and adjustment technics. Others have suggested that recent statistical innovations for improving clinical trials, including adaptive allocation of treatment to patients and sequential stopping procedures, are underutilized. These innovations, though theoretically interesting, are not easily adapted to large-scale, complex medical trials in which there may be multiple end points and delayed response times. Ethical considerations suggest that randomized trials are more suitable than uncontrolled experimentation in protecting the interests of patients. Randomized clinical trials remain the most reliable method for evaluating the efficacy of therapies.

Biometry↗
PubMedDate not supplied

Interim analysis: the alpha spending function approach.

Interim analysis of accumulating data in a clinical trial is now an established practice for ethical and scientific reasons. Repeatedly testing interim data can inflate false positive error rates if not handled appropriately. Group sequential methods are a commonly used frequentist approach to control this error rate. Motivated by experience of clinical trials, the alpha spending function is one way to implement group sequential boundaries that control the type I error rate while allowing flexibility in how many interim analyses are to be conducted and at what times. In this paper, we review the alpha spending function approach, and detail its applicability to a variety of commonly used statistical procedures, including survival and longitudinal methods.

Bias↗
PubMedDate not supplied

The Wisconsin Epidemiologic Study of Diabetic Retinopathy: VIII. The incidence of retinal photocoagulation.

The incidence of focal and panretinal photocoagulation and its relationship to demographic and other characteristics were examined in a population-based study of people with diabetes in southern Wisconsin. For participants whose age at diagnosis was less than 30 years, who were taking insulin, and who had not been previously treated with photocoagulation, the 4 year incidence of panretinal photocoagulation (10.8%) was significantly higher (p less than .0001) than the rate of focal and/or grid photocoagulation of the macula (4.3%). For those whose age at diagnosis was 30 years or older and who had not been previously treated with photocoagulation, the incidence rates of panretinal photocoagulation (4.4%) and focal and/or grid photocoagulation of the macula (3.1%) were not significantly different (p = .11). At follow-up examination, 33.8% of the eyes of younger onset persons and 57.7% of the eyes of older onset persons with Diabetic Retinopathy Study high risk characteristics for severe visual loss had never received panretinal photocoagulation. These relatively high frequencies of untreated eyes in need of panretinal photocoagulation remain a concern.

Adult↗
PubMedDate not supplied

Retinopathy in young-onset diabetic patients.

The relative importance of duration of diabetes before and after 13 yr of age as a risk factor for retinopathy was investigated using data from 200 persons who were younger than 26 yr of age. These persons were identified in a population-based study of diabetic retinopathy in southern Wisconsin in 1980-1982. Retinopathy was found in 9% of persons who were younger than 13 yr and in 34% of persons who were 13 yr or older and had been diagnosed at or after 13 yr. Presence of retinopathy was more strongly associated with the duration of diabetes after 13 yr of age than before it.

Adolescent↗
PubMedDate not supplied

Surrogate endpoints.

Explore the source record for details and available documents.

Anti-HIV Agents↗