What is Blocking?
In the statistical theory of the design of experiments, blocking is the arranging of experimental units in groups (blocks) that are similar to one another. Typically, a blocking factor is a source of variability that is not of primary interest to the experimenter. An example of a blocking factor might be the sex of a patient; by blocking on sex, this source of variability is controlled for, thus leading to greater accuracy.
Nuisance Factors
For randomized block designs, there is one factor or variable that is of primary interest. However, there are also several other nuisance factors. Nuisance factors are those that may affect the measured result, but are not of primary interest. For example, in applying a treatment, nuisance factors might be the specific operator who prepared the treatment, the time of day the experiment was run, and the room temperature. All experiments have nuisance factors. The experimenter will typically need to spend some time deciding which nuisance factors are important enough to keep track of or control, if possible, during the experiment.
When we can control nuisance factors, an important technique known as blocking can be used to reduce or eliminate the contribution to experimental error contributed by nuisance factors. The basic concept is to create homogeneous blocks in which the nuisance factors are held constant and the factor of interest is allowed to vary. Within blocks, it is possible to assess the effect of different levels of the factor of interest without having to worry about variations due to changes of the block factors, which are accounted for in the analysis.
The general rule is: "Block what you can; randomize what you cannot. " Blocking is used to remove the effects of a few of the most important nuisance variables. Randomization is then used to reduce the contaminating effects of the remaining nuisance variables.
Example of a Blocked Design
The progress of a particular type of cancer differs in women and men. A clinical experiment to compare two therapies for their cancer therefore treats gender as a blocking variable, as illustrated in . Two separate randomizations are done—one assigning the female subjects to the treatments and one assigning the male subjects. It is important to note that there is no randomization involved in making up the blocks. They are groups of subjects that differ in some way (gender in this case) that is apparent before the experiment begins.
Block Design
An example of a blocked design, where the blocking factor is gender.