FIXED AND RANDOM FACTORS

When you perform ANOVA with SPSS, you may have noticed that in Generalized Linear Model  => Univariate…  we have put the variable “group”, which represents the dose of exposure, under FIXED FACTOR(s), but there is another option: RANDOM FACTOR(s).

What about  the difference, using a very simplified terminology? FIXED FACTOR represents a factor which is modified following the design of the researcher. In other words, the researcher FIXES a difference between each category of the factor, as for example the concentration of exposure.  On the other hand, RANDOM FACTOR is a grouping variable on which the researcher cannot act, but can randomly influence the results. To do an example: if we perform a multicentric clinical trial, a category which represents each center is a random factor. The same protocol is used in all the centers, but we cannot know a priori whether  there are uncontrollable differences that may influence the results… YES, when we perform N times the same experiment under the same nominal conditions we introduce a RANDOM FACTOR, as we cannot exclude that there is some type of uncontrollable confounder which may influence the experimental trend.

Therefore, if we introduce the variable “experiment” in which the number of the experiment is indicated, we may think to use it as random factor and perform ANOVA with a fixed factor and a random factor.

 

 

Note that for random factors post-hoc tests cannot be performed as irrelevant for the analysis. IT MAY INFLUENCE THE ANOVA RESULTS, BUT IT PROPERLY TAKES INTO ACCOUNT THE FACT THAT SEVERAL EXPERIMENTS ARE PERFORMED DESPITE OF ONE. It is particularly efficient with a relatively low number of experiments (e.g. 3-5). For a higher number of experiments, other methods may be used (see the next discussions).

In the next post, I will show you the Output of such an analysis.