Blocking is a strategy for dealing with nuisance variables, which are variables that aren't of interest but have an impact on the variables that are. Because each farm may have varying levels of moisture, fertility, and other factors, the Farm is such a nuisance element for the design given in CRD & RCDB.
- Used when subjects are of different types (male/female).
- Analogous to breaking groups into strata in a survey.
- “Stratifying” is used in a survey (observational study).
- “Blocking” is used in an experiment.
Definition of a Block
A set of experimental units or patients that are similar in ways that are predicted to impact the response to treatments is referred to as a block.
The random assignment of units to treatments is done independently inside each block in a block design.
Blocking occurs prior to group assignment at random. A block design is similar to doing numerous tests at once.
The Benefits of a Block Design
Blocking decreases variance in your results by controlling the influence of some hidden factors that may obfuscate results.
Another type of control is blocks. They can regulate the impact of some external factors by introducing them into the experiment as blocks.
Each block may be used to draw its own conclusions, resulting in increasingly precise results. Blocking is frequently referred to as the fourth statistical design principle.
Disadvantages of using a Block Design
While blocking reduces sample sizes, more participants are required.
Design of Matched Pairs
- When only two treatments are being compared.
- Subjects are compared to one another or...
- Use only one subject and give him or her both treatments. The subject acts as his or her own control in this scenario.
- The order of the treatments is important and should be randomized.
When should a random block design be used?
- The outcome is influenced by an unwelcome/uninteresting variable.
- This variable is measurable.
- Your sample size is insufficient to establish equal groups using basic randomization (see Randomized Block Design vs Completely Randomized Design).
The randomized block design statistics limitations
- We can't have too many variables blocked.
The number of blocks formed grows as the number of blocking factors grows, nearing the sample size — i.e., the number of participants in each block would be quite small, posing a difficulty for the randomized block design.
- Difficulty deciding on the number of blocks to use
Because the number of blocks is equal to the number of categories in the blocking variable, selecting a blocking variable with an appropriate number of categories is critical because:
- If you utilized fewer blocks than you needed, preserving homogeneity within each block may be difficult.
- If you utilized more blocks than your sample size allowed, you'll need to do the following: It's possible that each block will only have a few individuals, making it difficult to appropriately randomize treatment alternatives.
- If you're unsure, go with a number of blocks based on past research.
3. Difficulty recognizing and quantifying the blocking variable
- So when the blocking variable is recognized and controlled, the following steps can be taken: Use a randomized block design as a solution.
- When the blocking variable is known but not within your control: Make an adjustment in the statistical analysis to account for it.
- When you don't know what the blocking variable is: Use basic randomization to ensure that research groups are equal and similar.
If developing trials like this is part of your research interests, you should consider taking an experimental design course.