12.1 Variables, Hypotheses, and Experimental Design
Key Takeaways
- GED Science gives you about 34 questions in 90 minutes; passing is 145 on the 100-200 scale, and most items ask you to reason from a passage, graph, or experiment rather than recall facts.
- The independent variable is the one condition the investigator deliberately changes; the dependent variable is the result that is measured in response.
- A fair test changes only one major factor, holds controlled variables constant, includes a control group, and uses repeated trials to reduce random error.
- A strong hypothesis is a testable if-then prediction that names a direction and a measurable result, not a vague opinion.
- When data look unusual, GED items often ask you to name the source of error or the single design change that would most improve reliability.
Know the Test Before You Read the Experiment
The GED Science module gives you roughly 34 questions in 90 minutes (about 2.6 minutes each), and you pass the subject at a scaled score of 145 out of a 100-200 range. Content splits into Life Science (40%), Physical Science (40%), and Earth and Space Science (20%), but the skill being measured is the same across all three: scientific reasoning in realistic situations, with a heavy emphasis on designing and interpreting investigations.
GED Science is not mainly a memorization test, so when an item hands you a procedure, slow down long enough to find four things: the investigation question, the changed condition, the measured result, and the conditions that must stay the same.
A hypothesis is a testable prediction, usually phrased as an if-then statement connecting one variable to another: if the amount of salt in water increases, then bean-seed germination will decrease. A weak hypothesis only says "salt affects seeds." A strong one names a direction, a measurable result, and a comparison group. The GED rewards the precise version because it can actually be checked against data.
Variable Map
| Part of design | GED question wording | Example (salt-and-seeds study) |
|---|---|---|
| Independent variable | What is changed or tested? | Amount of salt added to each cup of water |
| Dependent variable | What is measured? | Number of seeds that sprout |
| Controlled variables | What is kept the same? | Seed type, water volume, light, soil, temperature, time |
| Control group | What gives a baseline? | Seeds watered with no salt added |
| Repeated trials | What reduces random error? | Testing 10 seeds at each salt level, not 1 |
Fair Comparison and Confounding Variables
A fair experiment changes only one major factor at a time. Suppose Group A gets fertilizer, extra sunlight, and a larger pot, while Group B gets none of those. If Group A grows taller, the data do not prove fertilizer caused the growth, because sunlight or pot size could be responsible. Those uncontrolled differences are confounding variables, and recognizing them is one of the most-tested skills on this module.
GED items frequently ask for the best improvement to a flawed design. Strong answer choices add a control group, increase the number of trials, hold conditions consistent, use a more precise measuring tool, or measure the result at regular intervals. Weak choices change several factors at once, substitute opinion for data, or collect numbers that do not match the hypothesis.
Experimental vs. Observational Designs
In an experiment, the investigator deliberately manipulates the independent variable: assigning one batch of identical batteries to a cold chamber and another to room temperature is experimental. In an observational study, the investigator only records what already exists: logging battery failures reported by customers in different climates is observational. Observational designs can reveal patterns but carry more possible outside causes, so the GED often asks whether a conclusion is too strong.
A study showing that people who live near parks report more exercise supports an association only; it does not prove the park caused the exercise unless other factors were controlled. Confusing correlation with causation is a classic trap.
Five-Step Design Checklist
Run this on every GED experiment item:
- Identify the question being tested.
- Name the independent and dependent variables.
- List the controlled variables that make the comparison fair.
- Check for a control group or baseline.
- Decide whether the stated conclusion stays inside what the data actually show.
Sources of Error and Redesign
A source of error is anything that makes a measured result less accurate or less reproducible. A thermometer left in direct sun reads too high; a balance not zeroed makes every mass too large; a sample of one exaggerates random differences. When an item asks how to improve reliability, pick the change that directly attacks the weakness. If different students measured plant height with different rulers, the fix is one standard ruler or method, not "measure more plants." If only one tank was tested per temperature, the fix is several tanks per temperature.
The best redesign makes the data more comparable and repeatable, not merely more elaborate.
Reliability vs. Validity
Two terms quietly drive many GED design items. Reliability is whether repeating the procedure produces consistent results; validity is whether the experiment actually measures what it claims to measure. Repeated trials and standardized tools improve reliability. A control group and tightly held controlled variables improve validity. A study can be reliable but invalid: if every student uses the same mis-zeroed balance, the masses are consistent (reliable) yet wrong (invalid). Watch for answer choices that fix one without the other, and match the fix to the specific flaw the item describes.
Spotting the Trap Answers
- A choice that changes two or more factors at once is almost never the best fair-test improvement.
- A choice that swaps opinion or a survey for measured data weakens, not strengthens, a physical experiment.
- A choice that says the result "proves" a cause from an observational study overclaims; pick the wording that says "suggests" or "is associated with."
- A choice that adds complexity (more equipment, fancier method) without addressing the named weakness is a distractor.
A student tests whether light color affects algae growth. Four jars receive equal pond water and nutrients, sit at the same temperature, and each is placed under a different color of light for 10 days, after which algae mass is measured. What is the dependent variable?
A gardener wants to test whether Fertilizer A makes tomato plants grow taller than no fertilizer. Which design gives the strongest evidence?