Overview Section
In Lesson 4 we discussed blocking as a method for removing extraneous sources of variation. In this lesson, we consider blocking in the context of
Objectives
- Concept of Confounding
- Blocking of replicated
factorial designs - Confounding high order interaction effects of the
factorial design in blocks - How to choose the effects to be confounded with blocks
- That a
design with a confounded main effect is actually a Split Plot design - The concept of Partial Confounding and its importance for retrieving information on every interaction effect
Blocking in Replicated Designs Section
In
There is almost always an advantage to blocking when we replicate the treatments. This is true even if we only block using time due to the order of the replicates. However, there are often many other factors that we have available as potential sources of variation that we can include as a block factor, such as batches of material, technician, day of the week, or time of day, or other environmental factors. Thus if we can afford to replicate the design then it is almost always useful to block.
To give a simple example, if we have four factors, the
The more interesting case that we will consider next is when we have an unreplicated design. If we are only planning to do one replicate, can we still benefit from the advantage ascribed to blocking our experiment?