AP Biology Exam Prep: Independent vs. Dependent Variables, Controls + Next Investigations

Summary

Mr. Poser reviews experimental design skills for AP Biology, focusing on identifying variables and controls and proposing a follow-up investigation.

Independent and dependent variables

• Independent variable (IV): The factor researchers deliberately change or test.
• Dependent variable (DV): The outcome researchers measure, which may be affected by the independent variable.
• On a typical graph, the IV is usually on the x-axis, and the DV is usually on the y-axis.
• Example: To test how water affects plant growth, the amount of water is the IV, and a measure of plant growth—such as size, leaf count, or survival—is the DV.
• In an algae example, if researchers test whether water temperature affects unusual algae growth, water temperature is the IV and algal growth is the DV.

Controls and controlled variables

The video distinguishes between two ideas that are easy to confuse:

• Controlled variables are conditions kept the same across experimental groups so the experiment can isolate the effect of the IV. In an antibiotic test, these might include the bacterial strain, starting amount of bacteria, growth medium, Petri dish size, light, and other growing conditions.
• A control group lacks the specific factor being tested and provides a comparison for the experimental group. It should otherwise be as similar as possible to that group.
  • In the antibiotic example, the Petri dish without antibiotic is the control; the dish with antibiotic is the experimental group.
  • A negative control is treated in the same way as the other samples but is not expected to produce the tested effect.
  • A positive control uses a condition or treatment whose result is already known, helping show that the experimental setup can produce an expected effect.

In a catalase-and-hydrogen-peroxide example, the appropriate control is a beaker of water without hydrogen peroxide, which contains none of the tested substrate.

Proposing a next investigation

The video describes proposing a new investigation based on experimental results or an evaluation of the original design. Possible approaches include:

• Use the previous results to ask a new question or investigate why a result occurred.
• Test a different independent variable against the same dependent variable.
• Improve the procedure by controlling variables more carefully.
• Increase the sample size or add more subjects, groups, and data to strengthen comparisons.

For the northern leopard frog example, scientists could divide fertilized eggs into two groups: one exposed to water containing atrazine at the concentration found in the contaminated pond, and one kept in water without atrazine. Atrazine is the IV; frog development and the proportion that metamorphose into adults are possible DVs. The untreated group serves as the control.

The video also notes that collecting observations or data from laboratory setups is a useful course skill, though the instructor says that particular skill is not assessed on the AP exam.

Speakers and sources

• Mr. Poser — AP Biology teacher and presenter.
• AP-style practice-question scenarios — The algae, catalase, and frog examples are discussed as practice questions; no other speakers are featured.