Design the experiment — variables, controls and hypotheses in AP Biology

The first free-response questions on the AP Biology exam are built around an experiment, and several of their points reward the same moves every year: name the independent and dependent variables, say which group is the control and why, name a variable that has to be held constant, state a null hypothesis, predict what the data would look like if a claim were true, and justify the design. These points are easy to lose by being vague, and easy to secure with practice. This material trains those moves on a series of short invented experiments, each described in full inside the question.

The scenarios cover the whole course: an enzyme tested at five temperatures, a plant hormone applied to pea stems at several concentrations, a pesticide sprayed on aphids, a gene knockout in yeast and its effect on fermentation, light color and oxygen production in floating leaf disks, a fertilizer and algae growth, a drug that blocks a cell receptor, and bacteria growing unevenly inside an incubator. For each one you decide which factor the researcher changes on purpose, which one is measured and in what units, which group is the best negative control (a treatment without the active ingredient, not simply no treatment), what a positive control is for, and which conditions must stay the same so that only one factor differs between groups.

Other questions ask you to write hypotheses the way the exam expects: a null hypothesis that predicts no effect, an alternative hypothesis that predicts one, and a prediction that follows logically from a stated hypothesis. You also practice critical reading of a design: recognizing a confounding variable such as a difference in species or in position inside an incubator, explaining why many replicates give a more reliable estimate of the mean, and choosing the follow-up experiment that tests a specific alternative explanation rather than simply repeating the original.

The material offers a quiz and a flashcard deck. The quiz explains after every answer why the right choice fits the design and why the others do not. The flashcards give precise definitions of the twelve terms the exam uses when it describes an investigation, from independent variable and controlled variable to positive control, replicate and confounding variable. It pairs naturally with the data-analysis material in this category: first design the experiment, then analyze its results.

• Identify the independent, dependent and controlled variables of an experiment
• Choose an appropriate negative control and explain the purpose of a positive control
• State a null hypothesis and an alternative hypothesis for a described investigation
• Predict results that would support a given hypothesis
• Recognize confounding variables and propose a follow-up experiment that tests an alternative explanation

Practice material written by Zestly, based on the College Board AP Biology course framework (science practice 3, questions and methods: skills 3.A–3.D).