EXPERIMENTAL UNITS, FACTOR AND LEVELS OF FACTOR
EXPERIMENTAL UNITS, FACTOR AND LEVELS OF FACTOR
EXPERIMENTAL UNITS
Ø For showing and conducting an experiment, the experimental material is divided into smaller parts and each part is referred to as an experimental unit. The experimental unit is randomly assigned to treatment.
Ø Experimental unit — person or object upon which the treatment is applied.
Ø An experimental unit is the smallest unit of experimental material to which a treatment can be assigned.
Ø Experimental units should be representative of the population about which a conclusion is going to be drawn.
EXAMPLE OF EXPERIMENTAL UNITS
Ø The cage of animals
If animals are group housed in a cage and all animals within that cage receive the same treatment, for example in the drinking water or diet, then the experimental unit is the cage of animals. If animals are group housed but can each receive a different treatment, for example by injection, then the experimental unit would be the individual animal.
Ø The individual animal
The two variables of interest which define the four groups are gender and exercise. Animals are either male or female independently of other animals, and they are also allocated to different activity levels independently of other animals. Thus the experimental unit is the individual mouse.
FACTOR
Ø A variable under the control of the experimenter. A factor of an experiment is a controlled independent variable; This is a variable whose levels are set by the experimenter.
Ø Factors are instructive variables to be studied in an investigation
EXAMPLE OF FACTOR
1. In a study of the effects of colors and prices on sales of bike, the factors being studied are color (qualitative variable) and price (quantitative variable).
2. In an investigation of the effects of education on income, the factor being studied is education level (qualitative but ordinal).
CLASSIFICATION OF FACTOR
Ø Factors can be classified as either controllable or uncontrollable variables.
Ø The controllable variables will be denoted to throughout the material as factors.
Ø People are usually considered a Noise Factor — an uncontrollable factor that causes variability under normal operating situations, but we can control it during the experiment by blocking and randomization.
TYPES OF FACTOR
Ø Experimental factors: levels of the factor are assigned at random to the experimental units.
Ø Observational factors: levels of the factor are characteristic of the experimental units and is not under the control of the investigators.
Ø There could be observational factors in an experimental study.
EXAMPLE
In the “ drug study” , if we are also interested in the effects of age and gender on the recovery rate, then these observational factors; while the treatment (new drug or old drug) is an experimental factor.
FACTOR LEVAL
Ø Factor levels are the “values” of that factor in an experiment.
Ø For example, in the study involving color of cars, the factor car color could have four levels: red, black, blue and grey.
In a design involving vaccination, the treatment could have two levels: vaccine and placebo
