In most experiments, you’ll have a number of factors to deal with. These are elements that affect the outcomes of your experiment. They fall into a few basic categories:

  • Experimental factors are those that you can specify and set yourself. For example, the maximum temperature to which you can heat a solution.
  • Classification factors can’t be specified or set, but they can be recognized and your samples selected accordingly. For example, a person’s age or gender.
  • Treatment factors are those which are of interest to you in your experiment and that you’ll want to manipulate in order to test your hypothesis.
  • Nuisance factors aren’t of interest to you for the experiment but might affect your results regardless.

There are two basic types of treatment factors that you’ll use:

  • Quantitative factors can be set to any specific level required – for example, pH levels.
  • Qualitative factors contain a number of categories– for example, different plant species or a person’s gender.


A popular example in explaining factors is the simple-sounding task of baking cookies. Most people would simply follow a recipe – or, let’s face it, buy the cookie dough pre-made and bake whatever we don’t eat raw. But how did the recipe come to be in the first place? Someone had to experiment with ingredients and baking methods for the right combination.

  • Flour: The ratios of flour to liquid and flour to fat are crucial to the texture of a cookie. Too much flour, and you end up with a dry, crumbly cookie. Too little, and you end up with an overly flat, crispy cookie.
  • Sugar: The type of sugar used can change the way a cookie reacts to the baking process because using granulated (white) sugar usually creates a crisper, flatter cookie. Using brown sugar creates a moister, chewier cookie.
  • Fat: Rubbing the fat into the flour creates a softer cookie. Using butter creates a flatter cookie than using margarine.
  • Eggs: Eggs create a less crumbly, chewier cookie.
  • Baking powder: Using baking powder causes a cookie to rise or spread, creating a ‘cakey’ texture or a more crisp cookie.
  • Temperature: Low-temperature baking gives a cookie more time to spread out while cooking, meaning it’s more likely to be flatter and crisper.

Think of each of these ingredients and the baking temperature as factors in an experiment. You can’t test each factor independently – you need to have all ingredients to produce the cookies. But you can modify the amount, type of ingredient, and temperature at which they’re baked, to find the combination that yields your perfect cookie.


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