Understanding “Variables” on the ACT Science Test

Time management is crucial for great scores on the ACT Science Test. Since you only have about 5 minutes per passage, that’s less than one minute per question. Spend long enough on each passage to identify just the main information, and don’t get bogged down with the confusing details. Ignore any unknown terminology and spend most of your time on the presented data and the variables.
A “variable” in math is a placeholder for an unknown. In ACT Science, this takes on a slightly different meaning and is used to describe the factors in an experiment.  Independent variables are factors that are controlled by the scientists. Did the scientists increase the heat in the experiment? Did they add or remove pressure? If the scientists have control over the variable, it is independent. Dependent variables are factors that the scientists observe changing. This is what the look for and how they record data — but they don’t control it.
You will also need to know direct variation and indirect variation. Direct variation occurs when two variables change in the same way over time. If Column A increases and Column B increases at the same time, we can say that the two columns vary directly.  If when Column A increases, Column B decreases, there is an indirect (also called inverse) variation between the two elements. In the real world, think of a pizza and the number of slices eaten. The weight of the pizza is inversely proportional to the number of slices eaten. That means that as more and more slices are eaten, the pizza weights less and less. In ACT Math, you may see direct and indirect variation in expression such as “y varies directly as x” or “y is directly proportional to x.” That means you can use the equation y = kx, where k is the constant of variation.
To understand the variables, look at the presented figures. Ask yourself: what does the figure tell me? What are the units of measurement? What changes, and when? In order to answer ACT Science questions correctly, you will need to pinpoint the relationships between the variables. Don’t try to memorize. Always go back to the data to find the correct answer. All of the information you need to get the questions correct are found the in passages, we just have to know where to look!

Much of the ACT Science data will be presented on graphs. Every graph has an x-axis and a y-axis. Pay close attention to the labels. The independent variable is most often found on the x-axis. When faced with multiple graphs, typically only one thing has changed. This is usually done by scientists to examine the results when a single variable is altered. If the scientists changed more than one variable, it usually doesn’t help them draw conclusions, since they wouldn’t know which change created the new results.


1. Find the Purpose, Method & Results for each experiment.
2. Examine the data carefully, and look for the variable relationships.
3. Go back to the passage for the correct answer.