Identify independent, dependent, and controlled variables in an ACT Research Summary, spot the control group, and explain why an experimental step was included — all from the passage.
The one factor the experimenter deliberately CHANGES between trials.
The column whose values were set on purpose (e.g., 0, 5, 10, 15 g).
The outcome the experimenter MEASURES and records as a result.
The column of results, hypothesized to respond to the independent variable.
A factor held CONSTANT across all trials to keep the comparison fair.
Same value in every row, so it cannot explain differences in the result.
The trial with the treatment at zero/none — the baseline for comparison.
Answer "which is the control?" with the untreated / baseline trial.
Match the step to isolating a variable, improving reliability, or extending range.
Repeating a trial = reliability; distilled water = isolate a hidden variable.
One ACT Science passage type describes a set of experiments a scientist ran, then asks about how the experiments were designed rather than only what the data shows. These questions reward understanding of experimental logic — which factor was changed, which was measured, which were held fixed, and why a particular step was included. As always, every answer comes from the passage; you supply the reasoning, not the content.
Every experiment is organized around three roles, and the ACT tests all three:
A quick test: the column whose value the experimenter set on purpose is the independent variable; the column they recorded as a result is the dependent variable. Consider a study of how salt affects how quickly ice melts:
| Trial | Salt on ice (g) | Room temperature (°C) | Time to melt (min) |
|---|---|---|---|
| 1 | 0 | 22 | 41 |
| 2 | 5 | 22 | 28 |
| 3 | 10 | 22 | 19 |
| 4 | 15 | 22 | 13 |
The independent variable is the salt (the experimenter changed it: 0, 5, 10, 15 g). The dependent variable is the time to melt (the measured result). The controlled variable shown is room temperature, held at in every trial so any change in melting time can be attributed to the salt and not to a warmer room.
The whole point of controlling variables is to make the comparison fair. If Trial 4 had also been run in a warmer room, you could not tell whether the ice melted faster because of the extra salt or because of the heat. The ACT loves to ask, "Why was the room temperature kept at in all four trials?" The answer is always some version of: so that salt is the only factor differing between trials, allowing its effect to be isolated.
A control group is the trial run with the independent variable at its baseline — often "none" or "zero" — to serve as a comparison point. In the table above, Trial 1 (0 g of salt) is the control: it shows how long the ice takes to melt with no salt, so the other trials can be measured against it. Without a control, you would have nothing to compare the treated trials to. When the ACT asks "Which trial serves as the control?", look for the one with the treatment set to none, zero, or the ordinary/untreated baseline.
A common stem asks the purpose of a specific step: adding a second thermometer, repeating a trial three times, using distilled instead of tap water. The reasoning is almost always one of:
Match the step to the design goal it serves, and the correct choice usually names that goal in plain language.
Research Summaries often run Experiment 1 and Experiment 2 that differ in exactly one setup detail. The ACT then asks what that difference lets the scientist learn. Read the two method paragraphs side by side and find the single thing that changed. If Experiment 2 repeats Experiment 1 but in darkness instead of light, the comparison isolates the effect of light — that one changed factor is the point of running both.
The trap on Understanding Experiments questions is answering from the data table when the question is about the design. "Which variable was controlled?" is not found in the numbers alone — it is found by asking which column stays the same across every trial while another is deliberately varied. Slow down on the method paragraphs; they are where these answers live. When you can name the independent, dependent, and controlled variables and point to the control group, you can answer almost any design question the passage throws at you.
A student tests how the amount of light affects the oxygen a water plant releases. She places identical plants under lamps of different brightness and counts oxygen bubbles per minute.
| Trial | Light brightness (lux) | Water temperature (°C) | Bubbles per minute |
|---|---|---|---|
| 1 | 500 | 20 | 12 |
| 2 | 1,000 | 20 | 21 |
| 3 | 1,500 | 20 | 29 |
What is the independent variable in this experiment?
A researcher adds different masses of a catalyst to identical reactions and records how long each reaction takes to finish.
| Trial | Catalyst added (mg) | Reaction time (s) |
|---|---|---|
| 1 | 0 | 300 |
| 2 | 20 | 180 |
| 3 | 40 | 110 |
What is the dependent variable?
A biologist tests whether a fertilizer speeds seedling growth. Four pots receive different fertilizer doses; one pot receives none. All pots get identical soil, water, and light.
| Pot | Fertilizer (mL) | Height after 2 weeks (cm) |
|---|---|---|
| 1 | 0 | 6 |
| 2 | 5 | 9 |
| 3 | 10 | 13 |
| 4 | 15 | 15 |
Which pot serves as the control group?
In the fertilizer study above, every pot received the same soil, the same amount of water, and the same light. Why did the biologist keep these three factors identical across all four pots?
A chemist measures how fast a tablet dissolves in water. Worried that a single stray reading could mislead her, she runs each temperature three times and averages the three dissolving times before recording a value. Why did she run each temperature three times?
Experiment 1: a scientist measures how quickly bread mold grows on slices kept at several humidity levels, all in the light. Experiment 2: she repeats the exact procedure and humidity levels, but keeps every slice in complete darkness. All other conditions match Experiment 1. What can she learn by comparing the two experiments that she could not learn from Experiment 1 alone?
Mixing up the independent and dependent variables — calling the measured result the "changed" factor.
Ask which column the experimenter set on purpose (independent) versus which column they recorded as an outcome (dependent). The values that count up in a planned pattern are almost always the independent variable.
Answering a design question from the data table instead of the method. "Which variable was controlled?" is not a number you can read off.
Find the column that stays the same in every trial while another is deliberately varied — that constant column is the controlled variable. Read the method paragraphs, not just the results.
Overthinking "why was this step done?" and picking an elaborate reason the passage does not support.
Match the step to a standard design goal: isolate a variable, improve reliability by repeating, or extend the tested range. The simplest matching purpose is almost always correct.
Failing to recognize the control group because it is not labeled "control" — it is just the trial with a zero or baseline treatment.
Scan the independent-variable column for the value that means "none" (0 g, no additive, ordinary conditions). That untreated trial is the control that the others are compared against.
A student investigates how the length of a pendulum affects how long it takes to complete one swing. She uses strings of different lengths, keeps the bob mass the same, and times one full swing for each.
| Trial | String length (cm) | Bob mass (g) | Swing time (s) |
|---|---|---|---|
| 1 | 20 | 50 | 0.9 |
| 2 | 40 | 50 | 1.3 |
| 3 | 60 | 50 | 1.6 |
What is the independent variable in this experiment?
A researcher shines light of different colors on identical solar cells and records the current each cell produces, keeping the light's brightness the same for every color.
| Trial | Light color | Brightness (lux) | Current (mA) |
|---|---|---|---|
| 1 | Red | 800 | 22 |
| 2 | Green | 800 | 31 |
| 3 | Blue | 800 | 40 |
What is the dependent variable?
A team studies whether a coating slows rusting. They leave iron nails in salt water: three nails have different coating thicknesses and one nail is left uncoated. Every nail sits in the same salt water at the same temperature.
| Nail | Coating thickness (µm) | Mass lost to rust (mg) |
|---|---|---|
| 1 | 0 | 44 |
| 2 | 10 | 26 |
| 3 | 20 | 15 |
| 4 | 30 | 8 |
Which nail serves as the control group?
In the rusting study above, the team kept all four nails in the same salt water at the same temperature. Why did they keep the salt water and temperature identical for every nail?
A chemist measures how fast sugar dissolves at several water temperatures. Concerned that one careless reading could distort her data, she repeats each temperature five times and records the average dissolving time. Why did she repeat each temperature five times?
Experiment 1: a scientist measures how much gas a yeast culture produces at several sugar concentrations, all at 30 °C. Experiment 2: she repeats the identical procedure and sugar concentrations, but runs every culture at 10 °C instead. All other conditions match. What does comparing Experiment 1 with Experiment 2 allow her to determine?
How to read values, units, and axes off ACT Science tables and graphs — and how to pull a single fact out of a busy figure without needing any outside science knowledge.
Judge whether ACT Science data actually supports a stated conclusion, tell a supported claim from an overreach, and name the new evidence that would confirm a result.