Identifying Controls and Variables: Biology Practice with Explanations

Biology Foundations and Lab Skills Teaching Resources · High school biology; AP extensions where noted.

Use these original practice situations to distinguish a treatment, a response, controlled conditions, and a comparison group. They are paper exercises, not instructions to carry out experiments.

Learning goals and materials

Students will identify variables by their role in a stated design and recognize when a comparison cannot support a causal claim. Prerequisite: scientific inquiry notes. Materials: paper and the situations below.

Terms to use

Independent variable: the factor deliberately varied in the described experiment.
Dependent variable: the measured response.
Controlled conditions: relevant factors kept comparable across treatments.
Control or comparison group: the condition against which another is interpreted. It is not the same as a controlled variable and need not always mean “nothing happens.”

Situation 1: water and germination

A teacher randomly assigns 12 identical containers, each with ten seeds from one seed lot, to three water volumes: 5, 10, or 15 mL initially. There are four containers per volume. All are observed for seven days under the same planned light and temperature conditions. The measured response is the proportion of seeds germinated in each container.

Identify the independent and dependent variables.
Name two conditions to keep comparable.
If 10 mL is the usual classroom condition, what can it provide?
What is the independently treated experimental unit?

Situation 2: two changes at once

One set of seedlings receives a new nutrient solution beside a sunny window. Another receives plain water in a dim cupboard. The sunny set becomes taller.

Can this comparison establish that the nutrient solution caused the difference? Propose one change to improve the design.

Situation 3: observational data

A class measures leaf length and width on leaves already collected by a teacher. Nothing is assigned or changed. Longer leaves tend to be wider.

Identify the two measured variables. Does the class have a manipulated independent variable or an experimental control group? Can the association establish causation?

Answer explanations

Situation 1: water volume is varied; germination proportion per container is measured. Seed source, container type, observation period, and planned environmental conditions should be comparable. The usual 10 mL condition supplies a reference. Each container is the independently treated unit; the seeds within it share conditions. Four containers are not forty independent treatment replicates.
Situation 2: light and nutrient conditions differ together. Either could contribute, so the design confounds them. Keep light comparable while assigning nutrient treatments across multiple independent containers, or use a design that examines both factors separately.
Situation 3: length and width are measured, with no manipulated treatment and no experimental control group. One may be placed on the horizontal axis for analysis, but that does not make it an experimentally manipulated cause. The pattern alone does not establish causation.

Teaching note: Ask for the measurement definition, not just the noun “growth.” For AP extension, distinguish repeated measurements, subsamples, and independent treatment replicates. The older cartoon-based exercise is replaced here; its scenarios and answers are not reused.