Stratification studies

Sample heterogeneity (population stratification) Study subjects come from diverse genetic backgrounds. The presence of sample heterogeneity can introduce spurious associations between disease and genetic markers. 

A. Eklund, D. Simonsson, R. Kadsson, and E. N. Bark, "Theoretical and Experimental Studies of Eree Convection and Stratification of... 

Determine the components of variance that should be built Into the statistical design Proper stratification of the study area will allow Identification and quantification of several sources of variation. The sources of variation that can be controlled by the sampling are determined by the particular sampling design and by the pattern of sample collection superimposed over the area. An analysis of variance of the data provides estimates of the components of variance ... 

The randomization scheme assigns a therapy randomly across a study population based on various stratification factors such as site, blocking factor, and perhaps subject demographics. There is no actual patient assignment information in this data table. Here is an example of a randomization scheme with a block size of four and a treatment ratio of 2 2 ... 

Other data sets may be found within the IVRS system that prove useful to the statistical programmer as well. Often the IVRS collects several baseline patient characteristics that are used in the stratification of the randomization scheme and subsequent assignment of study therapy. Finally, the preceding examples show in detail what the treatment variable is, in the treatment column. It is more often the case that the treatment variable is coded, such as A or B or C. It is of paramount importance that you know with absolute certainty how the treatment code can be properly interpreted. 

The randomization data are used in both efficacy and safety analyses, as they are typically the key stratification variable for the trial. The randomization data allow you to answer the question of whether patients who are getting the study therapy fare better than the alternative. CDISC places treatment assignment information in the special demographics domain. 

Pritchard JK, Rosenberg NA. Use of unlinked genetic markers to detect population stratification in association studies. Am J Hum Genet 1999 65[l] 220-228. 

Nearly all pharmacogenomic studies conducted to date in asthma have been case control studies. A major criticism of genetic case control studies of human diseases has been potentially undetected population stratification. Population stra-... 

Reich DE, Goldstein DB. Detecting association in a case-control study while correcting for population stratification. Genet Epidemiol 2001 20 4—16. 

In reservoirs with discharge of hypolimnetic water, the effects on the water temperature have been widely studied. If there is stratification, water temperature downstream from the dam is higher in winter, colder in summer, the daily and annual thermal amplitude is reduced and the maximum annual temperature is delayed [5-7]. Recently, it has been observed that hypolimnetic discharges can reduce the variability of the temperature of the water in reduced time scales, in the range of days to weeks [8]. The low temperatures in summer can modify the composition of the fluvial community, but they can also bring the river to a previous stage of the river continuum [9, 10]. 

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