How do you design a controlled biological experiment that produces valid, trustworthy data?
Plan and carry out controlled investigations: ask a testable question, form a hypothesis relating an independent and a dependent variable, identify the variables that must be controlled, and explain the role of the control group (Virginia 2018 Biology SOL BIO.1.a, BIO.1.b).
A SOL-level answer on experimental design for the Virginia Biology EOC: testable questions, hypotheses, independent, dependent, and controlled variables, the control group, and why a valid design isolates one variable at a time.
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What this topic is asking
Virginia Biology SOL standard BIO.1 is the scientific and engineering practices. The first part, BIO.1.a and BIO.1.b, is about designing and carrying out a controlled investigation: turning a question into a testable hypothesis, choosing variables, and building a fair test. The Biology EOC weaves this through the whole exam, so almost any topic, from osmosis to natural selection, can become a question about experimental design. You must be able to identify the independent, dependent, and controlled variables and explain the job of the control group.
A testable question and a hypothesis
A good hypothesis is specific and falsifiable: "If the temperature is increased, then yeast will produce gas faster, up to a point." It names what you change and what you expect to happen, and it can be supported or refuted by the data. The Biology SOL often gives you a scenario and asks you to choose or write a suitable hypothesis, so practice phrasing one that names both variables.
The three kinds of variable
Keeping the controlled variables constant is what makes an experiment a fair test. If you changed the fertilizer amount and also gave some plants more water, you could not tell which factor caused any difference in growth. By holding everything constant except the independent variable, you can attribute any change in the dependent variable to that one cause. This is the logic of cause and effect that the SOL tests directly.
The control group
A control group is a setup that does not receive the experimental treatment, or that receives a standard treatment. It is the baseline you compare the experimental groups against. For example, to test whether a fertilizer increases growth, the control group gets no fertilizer, and the experimental groups get different amounts. If the experimental plants grow taller than the control, the fertilizer plausibly caused the difference. Without a control, you have nothing to compare to, so you cannot say the treatment did anything.
Reliability: sample size and repeats
One plant or one trial can give a misleading result by chance. Using a larger sample size (more plants, more test tubes) and running repeated trials reduces the effect of random variation and lets you calculate an average that better represents the true effect. The SOL expects you to recognize that a larger, repeated investigation gives more reliable data and that conclusions drawn from a single measurement are weak.
Try this
Q1. A study tests whether caffeine increases the heart rate of water fleas. Identify the independent and dependent variables. [2]
- Cue. Independent: the amount (concentration) of caffeine; dependent: the heart rate of the water fleas.
Q2. Explain why an experiment should change only one variable at a time. [2]
- Cue. So that any change in the dependent variable can be attributed to the one independent variable; changing two at once means you cannot tell which caused the effect.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksA student wants to test whether the amount of fertilizer affects how tall tomato plants grow. Which is the independent variable? (A) the height of the plants. (B) the amount of fertilizer. (C) the type of soil. (D) the amount of water given.Show worked answer →
A 1-point multiple-choice item on identifying variables.
The correct answer is B. The independent variable is the one the student deliberately changes, here the amount of fertilizer. A is the dependent variable (the outcome measured). C and D are variables that should be kept the same (controlled) so they do not affect the result.
The test rewards distinguishing the variable that is changed (independent) from the one that is measured (dependent).
VA Biology SOL (2024 released style)2 marksAn investigation tests whether light color affects the rate of photosynthesis in a water plant. (a) State a suitable hypothesis. (b) Identify one variable that must be kept constant and explain why.Show worked answer →
A 2-point technology-enhanced or short-response style item on hypotheses and controlled variables.
(a) 1 point: a testable, if-then hypothesis linking the variables, for example "If the light color is changed, then the rate of photosynthesis will differ, because different wavelengths are absorbed differently." Any answer that predicts an effect of light color (independent) on photosynthesis rate (dependent) earns the point.
(b) 1 point: name one controlled variable (light intensity, distance of the lamp, temperature, type and amount of plant, or carbon dioxide available) and explain that holding it constant ensures any change in photosynthesis is caused by the light color and not by something else.
Markers reward a hypothesis that names both variables and a controlled variable with a reason tied to fair testing.
Related dot points
- Construct and interpret data tables and graphs: organize data, choose an appropriate graph type, read trends and values from a graph, and calculate simple quantities such as means and rates from data (Virginia 2018 Biology SOL BIO.1.c).
A SOL-level answer on organizing and interpreting data for the Virginia Biology EOC: building data tables, choosing line, bar, and scatter graphs, reading trends, and calculating means and rates from data.
- Construct and critique conclusions and explanations: make a claim supported by evidence and reasoning, judge whether the data support the hypothesis, and identify sources of error and uncertainty in an investigation (Virginia 2018 Biology SOL BIO.1.d).
A SOL-level answer on conclusions for the Virginia Biology EOC: claim, evidence and reasoning, deciding whether data support a hypothesis, distinguishing correlation from causation, and identifying sources of error and uncertainty.
- Develop and use models to explain and predict, judging their merits and limitations, and obtain, evaluate, and communicate scientific information from reliable sources (Virginia 2018 Biology SOL BIO.1.e, BIO.1.f).
A SOL-level answer on scientific models and communication for the Virginia Biology EOC: what models are and their limits, the difference between a hypothesis, theory, and law, and how to evaluate and communicate reliable scientific information.
- Explain photosynthesis as the capture, transformation, and storage of energy: light energy and the reactants carbon dioxide and water are converted in chloroplasts into glucose and oxygen (Virginia 2018 Biology SOL BIO.2.e).
A SOL-level answer on photosynthesis for the Virginia Biology EOC: the reactants and products, the role of chlorophyll and chloroplasts, the energy transformation from light to chemical energy, and the factors that limit the rate.
- Explain that enzymes are protein catalysts with specific functions: they lower activation energy, act on specific substrates at an active site, and are affected by temperature, pH, and concentration (Virginia 2018 Biology SOL BIO.2.c, BIO.2.d).
A SOL-level answer on enzymes for the Virginia Biology EOC: enzymes as protein catalysts, activation energy, the active site and specificity, and how temperature, pH, and concentration affect enzyme activity, including denaturation.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)