Science 25 min

Scientific Investigation

Experimental design: variables, controls, methods and what an experiment tests.

These questions are about how experiments are built: what changed, what was measured, and what stayed the same. Nail those three and you can answer almost anything.

What it tests

Scientific Investigation questions ask about how an experiment was designed and carried out: which variable was changed, which was measured, what was held constant, why there’s a control, and how you could change the experiment to test a new question.

How it shows up

The Science test is 40 questions in 40 minutes — one minute per question. On the enhanced ACT it is optional: it isn’t part of your composite (English + Math + Reading), but it gets its own score and counts toward the STEM score, and many colleges still like to see it. Passages show data (tables and graphs), describe experiments, or present competing viewpoints.
Scientific Investigation is a mid-sized Science category. It shows up most in “research summary” passages that describe Experiment 1, Experiment 2, and so on.
Typical stems: “In Experiment 2, the independent variable was…”, “Which factor was held constant?”, “Why did the students include Trial 5?”, “Which additional trial would best test whether…?”.

Key rules & concepts

The content you actually need to know.

Variables

Independent variable: what the experimenter changes on purpose (often the first column of a table or the x-axis).
Dependent variable: what is measured as a result (often the last column or the y-axis).
Controlled variables (constants): everything kept the same so the test is fair.

Controls and fair tests

A control group gets no treatment (or the standard condition). It’s the baseline for comparison.
A fair test changes only ONE variable at a time. To test the effect of X, compare trials that differ only in X.

Methods

Repeated trials and averaging reduce the effect of random error.
Larger samples make results more reliable.
Know basic tools: a balance measures mass; a graduated cylinder measures volume; a thermometer measures temperature.

Designing a new test

To answer a new question, change the variable in that question and keep everything else the same as before.
Eliminate designs that change two things at once — you couldn’t tell which caused the result.

Strategy & traps

Step by step

1For each experiment, fill in: “They changed _, measured _, and kept ___ the same.”
2Compare the experiments: what’s different between Experiment 1 and Experiment 2?
3For “which new trial” questions, pick the one that changes only the variable in the question.
4Use the passage’s own procedure details — don’t assume a factor was controlled if the passage doesn’t say so.

Swapping independent and dependent

The one they choose values for is independent; the one they record is dependent.

Two changes at once

A design that changes both temperature and sugar can’t show the effect of either one.

Overreaching conclusions

Results only apply to the conditions actually tested.

Board Buddy tip: Say it in one line: “They changed ___, measured ___, kept ___ the same.” Most SIN questions answer themselves after that.

Worked examples

Try each one first, then reveal the solution.

Rolling Cars

1A student released a toy car from the top of a ramp and measured how far it rolled across a carpeted floor after leaving the ramp. Each setup was run 3 times, and the average distance was recorded.

2Experiment 1: The ramp height was kept at 30 cm. Clay was added to the car to give it masses of 50 g, 100 g and 150 g.

3Experiment 2: The car’s mass was kept at 100 g. The ramp height was set to 10 cm, 20 cm and 30 cm.

Experiment 1 (ramp height 30 cm)

Car mass (g)	Average distance (cm)
50	142
100	139
150	141

Experiment 2 (car mass 100 g)

Ramp height (cm)	Average distance (cm)
10	48
20	95
30	139

Example 1

In Experiment 2, the independent variable was:

Acar mass.
Bramp height.
Cdistance traveled.
Dfloor surface.

Example 2

Which factor was held constant in BOTH experiments?

ACar mass
BRamp height
CFloor surface
DDistance traveled

Example 3

The student ran each setup 3 times and averaged the results most likely in order to:

Atest more than one variable at a time.
Btest a second hypothesis.
Cmake the car roll farther.
Dreduce the effect of random measurement error.

Practice

Pick an answer for instant feedback. Answers lock once chosen.

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Yeast and Sugar

Yeast produces carbon dioxide (CO₂) gas as it uses sugar. Students tested how sugar concentration affects yeast activity.

In each trial, 5 g of yeast was mixed with 100 mL of a sugar solution in a flask kept in a 35°C water bath. A balloon was stretched over the top of each flask. After 30 minutes, the students measured the circumference of the balloon as an indicator of how much CO₂ had been produced. Trial 5 used yeast and plain water with no sugar.

Table 1

Trial	Sugar (g per 100 mL)	Balloon circumference after 30 min (cm)
1	5	18
2	10	24
3	20	29
4	40	22
5	0	9

Question 1

In this study, the dependent variable was:

Question 2

Trial 5 most likely served as:

Question 3

Which factor was NOT kept the same in all five trials?

Question 4

The students want to test whether temperature affects yeast activity. Which set of new trials would best do this?

Question 5

Which conclusion is best supported by the results in Table 1?