A hard-to-change factor is a factor that is difficult to randomize completely because of time or cost constraints. For example, temperature is a common hard-to-change factor because adjusting temperature often requires significant time to stabilize.
Hard-to-change factors are often confused with blocking variables. However, there are several important differences between blocks and hard-to-change factors:
- In a blocked design, the blocks are nuisance factors that are only included in a design to obtain a more precise estimate of the error term. However, you are interested in estimating the effect of hard-to-change factors, such as how temperature affects the moisture of a cake.
- In a blocked experiment, the interaction between the blocking variable and the factors is not of interest. When you have a hard-to-change factor, you might be interested in interactions between the hard-to-change variable and other factors in the experiment.
- Designs with hard-to-change and easy-to-change factors have two different sizes of experimental units. The hard-to-change factors are applied to a large experimental unit. Within this unit, the observational units are small experimental units used to study the easy-to-change factors. With a block design, the experimental units are all the same size.
- Blocks are usually random factors while hard-to-change factors are usually fixed.
- Blocks are a collection of experimental units. Hard-to-change factors are applied to the experimental units.