---
title: "AP Biology Science Practice 3: Questions and Methods Guide"
description: "Learn AP Biology Science Practice 3 - Questions and Methods: testable questions, null hypotheses, variables, controls, and proposing new investigations."
canonical: "https://fiveable.me/ap-bio/science-practices/science-practice-3-questions-and-methods/study-guide/D8KHGYfMQExiFd8lCJn4"
type: "study-guide"
subject: "AP Biology"
unit: "Science Practices"
lastUpdated: "2026-06-17"
---

# AP Biology Science Practice 3: Questions and Methods Guide

## Summary

Learn AP Biology Science Practice 3 - Questions and Methods: testable questions, null hypotheses, variables, controls, and proposing new investigations.

## Guide

## Overview

[AP Biology](/ap-bio "fv-autolink") Science Practice 3 - Questions and Methods is the skill set you use to design and evaluate experiments. With this practice you pose a testable question, state a [null hypothesis](/ap-bio/key-terms/null-hypothesis "fv-autolink") or predict results, identify the procedures and controls that match the question, and propose a new investigation when the design or evidence falls short.

This practice shows up across all four big ideas and on both the multiple-choice and free-response sections. It is the backbone of FRQ 3 (Scientific Investigation), and pieces of it appear in any question that asks you to think like a researcher.

Quick subskill map:
- 3.A: Identify or pose a testable question
- 3.B: State the null hypothesis or predict results
- 3.C: Identify procedures that align with the question, including variables and controls
- 3.D: Propose a new investigation based on an evaluation of design or evidence

## What Science Practice 3 - Questions and Methods Means

This practice is about the front end and the redesign end of experimentation. You are not just reading results. You are deciding what counts as a good question, what a fair test looks like, and what should happen next.

Think of it in four moves:
- Ask a question that data can actually answer.
- Predict what you would see, or state what no effect would look like.
- Match a method to the question, with variables and controls set up correctly.
- Use what the experiment showed (or failed to show) to plan the next study.

These moves connect directly to the laboratory part of the course, where you design plans, make predictions, and decide which controls keep your test honest.

## What This Practice Requires

Here is what each subskill asks you to do.

**3.A: Pose or identify a testable question**
- A testable question can be answered by collecting data, not by opinion.
- The best questions point to a direct measurement. In a study of two hearing-loss genes, the question "Do [mutations](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink") in the POU4F3 gene affect MYO6 [mRNA](/ap-bio/key-terms/mrna "fv-autolink") levels in cells?" is testable and direct. "Have mutations in other genes also been associated with hearing loss?" is background, not a direct test.

**3.B: State the null hypothesis or predict results**
- The null hypothesis says there is no effect or no difference between groups.
- A prediction states the expected outcome if your idea is correct, often in if-then form.
- You may be asked for either one, so know the difference.

**3.C: Identify procedures that align with the question**
- i. [Independent variable](/ap-bio/key-terms/independent-variable "fv-autolink") is what you change. Dependent variable is what you measure.
- ii. A [control group](/ap-bio/key-terms/control-group "fv-autolink") is treated the same except for the one variable you are testing.
- iii. You also need to justify why that control is appropriate, meaning explain what it lets you rule out.

**3.D: Propose a new investigation**
- After evaluating a design or a result, suggest a logical follow-up.
- A strong proposal changes one clear variable or fixes a weakness, so the new data would add something.

## Skills You Need for This Practice

- Tell the difference between a testable question and a background or opinion question.
- Write a null hypothesis as a statement of no difference or no effect.
- Write a prediction tied to a specific outcome.
- Label independent and dependent variables in any setup.
- Identify the correct control and explain what it isolates.
- Spot a flaw in a design, such as a missing control or a confounding variable.
- Suggest a focused next experiment that changes one variable at a time.

## How It Shows Up on the AP Exam

All six [science practices](/ap-bio/science-practices "fv-autolink") appear on every exam in both sections, so you will see Practice 3 in both multiple-choice and free-response form.

**Multiple-choice**
- Questions that ask which question would best test a proposal (3.A).
- Questions that ask which treatment would be the best control (3.C).
- A lake nutrient study, for example, asks which treatment is the best control, and the answer is an untreated section of the lake because it isolates the effect of the added nutrients.

**Free-response**
- FRQ 3 is Scientific Investigation and is built around this practice. You may be asked to design part of an experiment, name variables, choose and justify a control, or predict results.
- Pieces of 3.B and 3.D can also appear in the longer experimental FRQs (Questions 1 and 2).

Practical tip: when an FRQ asks for a control, name it and then add one sentence explaining what it lets you rule out. That extra sentence often covers the justification part of 3.C.

## Examples Across the Course

This practice is not tied to one unit. Here is how it looks in different parts of the course.

- **[Unit 3](/ap-bio/unit-3 "fv-autolink"), Cellular Energetics:** Question, does [temperature](/ap-bio/unit-3/enzyme-catalysis/study-guide/Jg1jljQ8ZHUvcaKprPGy "fv-autolink") affect enzyme reaction rate? Independent variable is temperature, dependent variable is rate of product formation. A control held at a known optimal temperature lets you compare. Null hypothesis: temperature has no effect on reaction rate.
- **[Unit 6](/ap-bio/unit-6 "fv-autolink"), Gene Expression and Regulation:** In a study of two hearing-loss genes, the direct testable question is whether mutations in one gene change the mRNA levels of the other. Predict that if the proposed regulation is real, mutant cells will show altered MYO6 mRNA levels.

- **Unit 8, Ecology:** Researchers split a lake and treat one side with sucrose and the other with sucrose plus [phosphate](/ap-bio/unit-1/nucleic-acids/study-guide/RKOM4rhL6iJsAMdbDOWU "fv-autolink") to find the limiting nutrient. The independent variable is the added nutrient, the dependent variable is phytoplankton [biomass](/ap-bio/key-terms/biomass "fv-autolink"), and the best control is an untreated section of the same lake.
- **[Unit 5](/ap-bio/unit-5 "fv-autolink"), Heredity:** For a genetic cross, the null hypothesis is that observed [offspring](/ap-bio/unit-7/intro-natural-selection/study-guide/v9Lf9qQpmpSXvd2ZUOqH "fv-autolink") ratios do not differ from the expected Mendelian ratios. That null sets up later statistical testing.

- **[Unit 2](/ap-bio/unit-2 "fv-autolink"), Cells:** Watching red onion cells in salt solutions, you could propose a new investigation that varies NaCl concentration in measured steps to find the point where cells start to shrink. That is a 3.D follow-up built on an earlier observation.

## How to Practice Science Practice 3 - Questions and Methods

- Take any lab you did and write its testable question, null hypothesis, independent variable, dependent variable, and control in five short lines.
- Practice rewriting weak questions. Turn "Is this gene important?" into a measurable version like "Does removing this gene change [protein](/ap-bio/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG "fv-autolink") levels?"
- For each experiment you read, ask "What is the control, and what does it rule out?" Say the answer in one sentence.
- After every result, write one new investigation that changes a single variable.
- Drill the null hypothesis format until it always reads as no difference or no effect.

## Common Mistakes

- Confusing the null hypothesis with the prediction. The null states no effect. The prediction states the expected outcome.
- Swapping independent and dependent variables. You change the independent variable and measure the dependent one.
- Choosing a control that changes more than one thing. A good control differs from the test group by only the variable being studied.
- Naming a control but not justifying it. On the FRQ, add the sentence that explains what the control isolates.
- Picking a background question over a direct test. Background questions gather context but do not directly test the proposal.
- Proposing a vague next experiment. A strong follow-up changes one clear variable so the data mean something.

## Quick Review

- 3.A: Write questions that data can answer, and favor the one that gives a direct test.
- 3.B: Null hypothesis equals no effect. Prediction equals expected result, often if-then.
- 3.C: Independent variable is changed, dependent variable is measured, control isolates one variable, and you should justify it.
- 3.D: Use the design or evidence to plan a focused next study that changes one variable.
- This practice anchors FRQ 3 and appears in multiple-choice and other experimental FRQs across all four big ideas.