Baseline factors

If the outcome variables were binary, categorical or ordinal then the CMH test would be used with age and sex as above defining the four strata. The emphasis here, is still of course, to compare the treatments, but adjustment for these baseline factors has improved the efficiency of the analysis by comparing the treatments within each stratum (like with like) and then averaging those effects over the strata, as in the multi-centre setting. Again we will come out of this analysis with, for example in the binary case, an adjusted OR and a corresponding confidence interval. 

As the number of baseline factors increases, however, this approach becomes a little unwieldy. The method of analysis of covariance is a more general methodology that can deal with this increase in complexity and which can also give improved ways of exploring interactions. We will develop this methodology later in the chapter. 

The proportional hazards model, as the name suggests, assumes that the hazard ratio is a constant. As such it provides a direct extension of the logrank test, which is a simple two treatment group comparison. Indeed if the proportional hazards model is fitted to data without the inclusion of baseline factors then the p-value for the test Hg c = 0 will be essentially the same as the p-value arising out of the logrank test. 

As we have already seen, there will be settings where the pattern of differences between treatment groups does not conform to proportional hazards, where the hazard ratio is not a constant, single value. Such situations are best handled by using an alternative model to incorporate baseline factors. The accelerated failure time model is an analysis of variance technique which models the survival time itself, but on the log scale ... 

Adjusting the analysis for baseline factors - with change from baseline as the outcome variable, include baseline as a covariate to avoid regression... 

Gordon MO, Beiser JA, Brandt JD, et al.The Ocular Hypertension Treatment Study baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol 2002 120 714-720. 

When developing strategies to change or improve a situation, it s important to understand the current or baseline factors relating to that situation. This chapter offers a brief overview of the dynamics of the present civil engineering workforce as they pertain to diversity, including an overview of the traditional workforce and the challenges faced by those professionals who do not necessarily fit the traditional mold. 

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