Statistics allocation bias

We recorded the water content of individual vials in the order of their moving out from the freeze-dryer and constructed the water content distribution over the vials in the freeze-drayer. Based on those data we obtained information about the operation state, i.e., the bias of heat flux in the shelf or the residence of water vapor in the chamber. The information was very helpful in the maintenance of the machine to achieve uniform conditions in the chamber. Furthermore, we determined the uniformity of the vials in each batch or the consistency among batches based on the information of the water content distribution by statistical analysis. An example of allocation of trays in a freeze-dryer and three-dimensional distribution of moisture levels in individual vials is shown in Figure 24. Frequency distributions of vial moisture and their normal distribution curves in different batches made with different freeze-dryers are shown in Figure 25. 

The calculation of a statistical test and the obtaining of a value for P addresses only chance as the possible explanation for the difference. Being statistically significant is therefore not a sufficient basis for a conclusion that the difference is real - allocation and assessment bias must also be considered. 

By definition, the experimental unit is the smallest unit randomly allocated to a distinct level of a treatment factor. Note that if there is no randomization, there is no experimental unit and (in nearly all cases) no experiment. Although it is possible to perform experiments without randomization, it is difficult to do well, and risky unless the experimental system is very well understood (7). Randomization is important for several reasons. Randomization changes the sources of bias into sources of variation in general, a noisy assay is better than a biased assay. Further, randomization allows estimates of variation to represent variation in the population this in turn justifies statistical inference (standard errors, confidence intervals, etc.). A common practice in cell-culture bioassay is to rotate among a small collection of layouts rather than use random allocation. Whereas rotation among a collection of layouts is certainly better than a fixed layout, it is both possible and practical to use carefully structured randomization on a routine basis, particularly when using a robot. 

Bather J (1992) Response adaptive allocation and selection bias. In Flournoy N, Rosenberger WF (eds), Adaptive Designs. Institute of Mathematical Statistics, Hayward, CA. 

Sometimes, experiments are repeated with a particular set of levels for all the factors to check the statistical validation and repeatability by the replicate data. This is called replication. To get rid of any bias, allocation of experimental material and the order of experimental runs are randomly selected. This is called randomisation. To arrange the experimental material into groups, or blocks, that should be more homogeneous than the entire set of material is called blocking. So, when experiments are carried ont these things should be remembered. There are several methodologies for design of experiments. Some DOE methods are presented below. 

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