Applied Statistics Lesson of the Day – Choosing the Number of Levels for Factors in Experimental Design

The experimenter needs to decide the number of levels for each factor in an experiment.

  • For a qualitative (categorical) factor, the number of levels may simply be the number of categories for that factor.  However, because of cost constraints, an experimenter may choose to drop a certain category.  Based on the experimenter’s prior knowledge or hypothesis, the category with the least potential for showing a cause-and-effect relationship between the factor and the response should be dropped.
  • For a quantitative (numeric) factor, the number of levels should reflect the cause-and-effect relationship between the factor and the response.  Again, the experimenter’s prior knowledge or hypothesis is valuable in making this decision.
    • If the relationship in the chosen range of the factor is hypothesized to be roughly linear, then 2 levels (perhaps the minimum and the maximum) should be sufficient.
    • If the relationship in the chosen range of the factor is hypothesized to be roughly quadratic, then 3 levels would be useful.  Often, 3 levels are enough.
    • If the relationship in the chosen range of the factor is hypothesized to be more complicated than a quadratic relationship, consider using 4 or more levels.

2 Responses to Applied Statistics Lesson of the Day – Choosing the Number of Levels for Factors in Experimental Design

  1. Reblogged this on Psychology & Statistics Tutor:Mentor and commented:
    Really great resource for learning more about the Why of stat processes.

Your thoughtful comments are much appreciated!

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: