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Biomedical subjects

S J Senn

Publications and source records attributed to S J Senn.

At least 19 recordsLinked to original sources
PubMed1992

Is the 'simple carry-over' model useful?

It is argued that the models usually employed for carry-over effects in cross-over trials are not reasonable. The example of a non-linear dose response for a dose-finding trial arranged in a Williams square is developed to show that if carry-over is present to any appreciable degree the usual statistical models provide no guaranteed protection against its effects. It is concluded that the most reasonably defended assumption about carry-over effects is that no important carry-over has taken place and that, where this assumption cannot be defended, statistical models provide no satisfactory substitute for it.

Bias↗
PubMed1991

Crossover trials, degrees of freedom, the carryover problem and its dual.

The problem of carryover in crossover trials has received a great deal of attention in the statistical literature. Carryover is just one form of period by treatment interaction; yet a parallel problem of patient by treatment interaction, which may be regarded as dual to that of carryover, has received little attention. We suggest that the phenomenon of patient by treatment interaction requires a repeated measures approach to the analysis of crossover trials. A simple solution using predefined contrasts is presented and illustrated by example.

Analysis of Variance↗
PubMed1991

Falsificationism and clinical trials.

The relevance of the philosophy of Sir Karl Popper to the planning, conduct and analysis of clinical trials is examined. It is shown that blinding and randomization can only be regarded as valuable for the purpose of refuting universal hypotheses. The purpose of inclusion criteria is also examined. It is concluded that a misplaced belief in induction is responsible for many false notions regarding clinical trials.

Clinical Trials as Topic↗
PubMed1990

The graphical representation of clinical trials with particular reference to measurements over time.

Clinical trials are frequently analysed as if they were surveys when they are in fact experiments. In particular the experimental basis of clinical trials is rarely reflected in the graphs which are used to illustrate them. Proposals are made, and illustrated by example, as to how appropriate figures may be devised, both for parallel-group designs and for crossover trials, to illustrate the experimental nature of clinical trials with measurements over time.

Clinical Trials as Topic↗
PubMed1989

Covariate imbalance and random allocation in clinical trials.

A model is developed to estimate the effect of covariate imbalance on the size of a test of treatment efficacy in randomized clinical trials comparing two treatments when dispersion parameters are known. It is concluded that tests of homogeneity on the covariates should not be performed, that covariate imbalance is just as much a problem for large studies as for small ones in terms of effect on size, and that the effect of correlation between covariates and measures of efficacy is more complex than has previously been suggested. The best way to adjust for covariate imbalance is by an analysis of covariance.

Analysis of Variance↗
PubMed1989

The use of baselines in clinical trials of bronchodilators.

Two important qualities of controlled clinical trials are that they reduce dependence on historical standards for evaluating therapy and separate the effect of treatment from the confounding influence of time. Whatever the theory of the clinical trial, however, time has not easily been banished from the analysis of medical experiments in practice, and many doctors still prefer to evaluate treatments by comparing now with then rather than treatment with control. This is especially so in trials of bronchodilators in the treatment of asthma where, it is shown, baselines are used in ways which can increase the variability of measurement and even bias the results.

Asthma↗
PubMed1978

Resource allocation. Some problems in applying the national formula to area and district revenue allocations.

The inadequacy of the current national formula in dealing with flows of patients across administrative boundaries is illustrated. In particular, the problems of dealing with varying admission rates for inpatients and in allocating revenues for outpatient services are discussed. We draw attention to an oversight in the recommendations concerning psychiatric services and criticise the general approach to the allocation of revenue under this heading. It is concluded that the national formula should not be applied for revenue allocation at district level in an unmodified form.

Catchment Area, Health↗