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R L Prentice

Publications and source records attributed to R L Prentice.

At least 91 records · Page 5Linked to original sources
PubMed1979

A qualitative discrepancy between censored data rank tests.

Two popular censored data rank tests were used to compare cancer incidence rates between dogs receiving bone marrow transplantation and control dogs. The logrank test gave a significance level of 0.01. In contrast, Gehan's generalized Wilcoxon test gave a significance level of 0.76. The statistics are displayed in a manner that shows how such contrasting significance levels can arise. The Gehan statistic is shown to be subject to a serious criticism that does not apply to other Wilcoxon generalizations. Further insight into the data is obtained using a time-dependent logrank test and the proportional hazards regression model.

Animals↗
PubMed1979

Hazard rate models with covariates.

Many problems, particularly in medical research, concern the relationship between certain covariates and the time to occurrence of an event. The hazard or failure rate function provides a conceptually simple representation of time to occurrence data that readily adapts to include such generalizations as competing risks and covariates that vary with time. Two partially parametric models for the hazard function are considered. These are the proportional hazards model of Cox (1972) and the class of log-linear or accelerated failure time models. A synthesis of the literature on estimation from these models under prospective sampling indicates that, although important advances have occurred during the past decade, further effort is warranted on such topics as distribution theory, tests of fit, robustness, and the full utilization of a methodology that permits non-standard features. It is further argued that a good deal of fruitful research could be done on applying the same models under a variety of other sampling schemes. A discussion of estimation from case-control studies illustrates this point.

Morbidity↗
PubMed1978

Estimation of multiple relative risk functions in matched case-control studies.

A linear logistic model used to estimate multiple risk functions in both cohort and case-control studies is adapted for sampling plans wherein each case is matched with R controls. The resulting methodology substantially liberalizes current practice by permitting simultaneous analysis of multiple discrete and continuous risk factors. Interactions among risk factors, and between risk factors and matching variables, may be explored. Data from two studies of oesophageal cancer, one conducted among Singapore Chinese and the other on the Caspian littoral of Iran, illustrate the methods.

Epidemiologic Methods↗
PubMed1978

The analysis of failure times in the presence of competing risks.

Distinct problems in the analysis of failure times with competing causes of failure include the estimation of treatment or exposure effects on specific failure types, the study of interrelations among failure types, and the estimation of failure rates for some causes given the removal of certain other failure types. The usual formation of these problems is in terms of conceptual or latent failure times for each failure type. This approach is criticized on the basis of unwarranted assumptions, lack of physical interpretation and identifiability problems. An alternative approach utilizing cause-specific hazard functions for observable quantities, including time-dependent covariates, is proposed. Cause-specific hazard functions are shown to be the basic estimable quantities in the competing risks framework. A method, involving the estimation of parameters that relate time-dependent risk indicators for some causes to cause-specific hazard functions for other causes, is proposed for the study of interrelations among failure types. Further, it is argued that the problem of estimation of failure rates under the removal of certain causes is not well posed until a mechanism for cause removal is specified. Following such a specification, one will sometimes be in a position to make sensible extrapolations from available data to situations involving cause removal. A clinical program in bone marrow transplantation for leukemia provides a setting for discussion and illustration of each of these ideas. Failure due to censoring in a survivorship study leads to further discussion.

Bone Marrow Transplantation↗
PubMed1978

Regression analysis of grouped survival data with application to breast cancer data.

Use of the proportional hazards regression model (Cox 1972) substantially liberalized the analysis of censored survival data with covariates. Available procedures for estimation of the relative risk parameter, however, do not adequately handle grouped survival data, or large data sets with many tied failure times. The grouped data version of the proportional hazards model is proposed here for such estimation. Asymptotic likelihood results are given, both for the estimation of the regression coefficient and the survivor function. Some special results are given for testing the hypothesis of a zero regression coefficient which leads, for example, to a generalization of the log-rank test for the comparison of several survival curves. Application to breast cancer data, from the National Cancer Institute-sponsored End Results Group, indicates that previously noted race differences in breast cancer survival times are explained to a large extent by differences in disease extent and other demographic characteristics at diagnosis.

Black People↗
PubMed1977

Single dose of two grams of metronidazole for Trichomonas vaginalis infection.

In this study, 243 women with symptomatic trichomonal vaginitis were treated with eight 250 mg. tablets of metronidazole for themselves, and eight tablets were given to each of their male consorts. They were given instructions to ingest "all eight in a row" and "avoid alcohol for 48 hours." Ninety-five per cent of 203 women that returned for re-examination were cured. This single-dose treatment is effective, well tolerated, and less expensive than current recommended dosages.

Administration, Oral↗
PubMed1977

Marrow transplantation for treatment of aplastic anemia. An analysis of factors associated with graft rejection.

Seventy-three consecutive patients with severe aplastic anemia were treated by marrow grafts from normal, HLA-identical siblings, and 68 lived long enough to demonstrate engraftment. In 21 patients the garft was rejected, and 19 of these patients died. This analysis, using a binary logistic regression model, was aimed at identifying factors that predicted marrow-graft rejection. Of the 24 factors entered into the analysis, only two strongly correlated with graft rejection: a positive relative response index in mixed leukocyte culture indicating sensitization of patient against donor (P less than 0.01); and a low number of marrow cells ( less than 3 X 10(8) cells per kilogram) used for transplantation (P less than 0.01). These findings suggest that more powerful immunosuppressive conditioning regimens should be used in patients who are sensitized, and that the greatest possible amount of donor marrow, perhaps supplemented by stem cells derived from the peripheral blood, should be obtained.

Adolescent↗
PubMed1977

Extra-amniotic blood clot simulating placenta previa on ultrasound scan.

The ultrasonic appearance of extra-amniotic blood clot simulating placenta previa is presented. The single case presented is the most illustrative and best documented example of this phenomenon in a series of 10 patients. The ultrasonic characteristics of this phenomenon are discussed.

Adult↗
PubMed1977

Treatment of aplastic anemia by marrow transplantation from HLA identical siblings. Prognostic factors associated with graft versus host disease and survival.

73 consecutive patients with severe aplastic anemia were treated by marrow transplantation from hematologically normal HLA identical siblings. 68 patients lived long enough to document marrow engraftment. 21 rejected the graft and 19 of these died. 47 sustained engraftment and 18 of these died. In 16 patients, death was associated with graft versus host disease. 29 patients with sustained engraftment are alive with complete hematologic restoration between 8 mo and 5 yr. This analysis, by using a proportional hazards regression model, was directed at identifying factors that predicted survival (and absence of graft versus host disease). Of the 24 factors entered into the analysis only two strongly correlated with survival: (a) sex match of donor and recipient (P less than 0.01), and (b) absence of refractoriness to random donor platelets at the time of transplantation (P less than 0.05). Refractoriness adversely influenced the survival of the sex mismatched patients, These data suggest that X and Y-associated transplantation antigen systems are important determinants of the outcome of marrow grafts between HLA identical siblings for the treatment of aplastic anemia. The machanism by which refractoriness to random donor platelets influences survival is currently unclear.

ABO Blood-Group System↗
PubMed1977

The incidence of gestational diabetes.

The incidnece of gestational diabetes has been quoted as being between 1 and 2%. The criteria used to establish this incidence may be in error, leading to identification of fewer patients. Using the criteria established by O'Sullivan et al, we were able to determine the incidence of gestational diabetes to be 6.0% in a private patient population. In addition, in previous pregnancies of these gestational diabetic patients, the incidence of congenital abnormalities was 13.6%, and perinatal mortality was 4.5 per cent.

Adult↗
PubMed1976

Combination chemotherapy of the malignant lymphomas: a controlled clinical trial.

The Eastern Cooperative Oncology Group has studied 113 patients with generalized progressive malignant lymphomas in a randomized clinical trial. Pathologic diagnosis was subclassified by cell type and nodal pattern by The Pathology Panel for Lymphome Clinical Trials. Patients were randomly assigned treatment with either cyclophosphamide (C), vincristine (O), and prednisone (P) (COP) or CO without prednisone. Initial treatment was given for 8 weeks and further randomization of responders to observation or additional chemotherapy was carried out. A significant difference in complete remission rate between treatments was shown: with COP, 43%, and with CO, 17%, indicating an important role for prednisone in inducing CR. COP was also associated with longer remission durations and improved survival. Complete remission following initial chemotherapy is also associated with longer duration of disease-free time and survival. The initial pathologic cell types and nodal pattern also strongly influence survival. Extended "maintainence" CO treatment improved disease-free remission duration, but not survival.

Adolescent↗
PubMed1976

A generalization of the probit and logit methods for dose response curves.

The relationship between response probability and dosage in quantal response bioassay is modelled using a four parameter class. In addition to location and scale quantities the model includes two shape parameters that essentially index skewness and heaviness of tails of the dose-response curve. The class of models includes such special cases as the logistic, normal, extreme minimum value, extreme maximum value, double exponential, exponential and reflected exponential distribution functions. Score tests are derived for logistic and normal hypotheses and certain submodels are discussed for which the model fitting is computationally convenient. The data of Bliss (1935) illustrates the potential improvement over usual methods in the estimation of critical dose levels.

Animals↗