Definition of an Experimental Unit
An experimental unit is the smallest division of experimental material to which a treatment can be applied independently. It is the individual subject, group, or object that receives a specific experimental treatment or intervention, and on which an observation or measurement is made. Each unit should be independent of other units to ensure valid statistical analysis.
Key Principles and Independence
The critical characteristic of an experimental unit is its independence. This means that applying a treatment to one unit does not affect how treatments are applied to other units, and observations from one unit are not influenced by observations from others. This independence allows for proper randomization and valid conclusions about cause and effect.
Practical Examples
In a medical trial, individual patients receiving different drug doses are experimental units. In an agricultural study, plots of land treated with different fertilizers are experimental units. If a teacher tries two teaching methods, the individual students within each class might be considered experimental units, but often the entire class is the unit if the treatment (teaching method) is applied to the class as a whole.
Importance in Research
Correctly identifying experimental units is fundamental for sound experimental design and accurate statistical inference. Misidentifying experimental units can lead to pseudoreplication, where researchers mistakenly believe they have more independent observations than they actually do, leading to an overestimation of statistical significance and incorrect conclusions.