The practical steps needed for planning and conducting an experiment include: recognizing the goal of the experiment, choice of factors, choice of response, choice of the design, analysis and then drawing conclusions. This pretty much covers the steps involved in the scientific method.
- Recognition and statement of the problem
- Choice of factors, levels, and ranges
- Selection of the response variable(s)
- Choice of design
- Conducting the experiment
- Statistical analysis
- Drawing conclusions, and making recommendations
What this course will deal with primarily is the choice of the design. This focus includes all the related issues about how we handle these factors in conducting our experiments.
We usually talk about "treatment" factors, which are the factors of primary interest to you. In addition to treatment factors, there are nuisance factors which are not your primary focus, but you have to deal with them. Sometimes these are called blocking factors, mainly because we will try to block on these factors to prevent them from influencing the results.
There are other ways that we can categorize factors:
Experimental vs. Classification Factors
- Experimental Factors
- These are factors that you can specify (and set the levels) and then assign at random as the treatment to the experimental units. Examples would be temperature, level of an additive fertilizer amount per acre, etc.SampleText
- Classification Factors
- These can't be changed or assigned, these come as labels on the experimental units. The age and sex of the participants are classification factors which can't be changed or randomly assigned. But you can select individuals from these groups randomly.
Quantitative vs. Qualitative Factors
- Quantitative Factors
- You can assign any specified level of a quantitative factor. Examples: percent or pH level of a chemical.
- Qualitative Factors
- These factors have categories which are different types. Examples might be species of a plant or animal, a brand in the marketing field, gender, - these are not ordered or continuous but are arranged perhaps in sets.
Try It! Section
Think about your own field of study and jot down several of the factors that are pertinent in your own research area? Into what categories do these fall?
Get statistical thinking involved early when you are preparing to design an experiment! Getting well into an experiment before you have considered these implications can be disastrous. Think and experiment sequentially. Experimentation is a process where what you know informs the design of the next experiment, and what you learn from it becomes the knowledge base to design the next.