Scientific Method

The Scientific Method is the process by which scientists attempt to construct an accurate representation of the world. This process is fundamental to scientific investigation and acquisition of new knowledge based upon actual physical evidence and careful observation. The Scientific Method is a means of building a supportable, documented understanding of our world.

The Scientific Method includes four essential elements:

  1. Observation
  2. Hypothesis
  3. Prediction
  4. Experiment

The passages included on the ACT Science Test have been written with the Scientific Method in mind. You can often use common sense along with a basic understanding of the process to answer many of the questions.

During the observation phase, the experimenter directly observes and measures the phenomenon that is being studied. Careful notes should be taken and all pertinent data should be recorded so that the phenomenon (the thing observed) can be accurately described.

The experimenter then generates a hypothesis to explain the phenomenon. He or she speculates as to the reason for the phenomenon based on the observations made and recorded.

Next, the experimenter makes predictions to test the hypothesis. These predictions are tested with scientific experiments designed to either prove or disprove the hypothesis. The Scientific Method requires that any hypothesis either be ruled out or modified if the predictions are clearly and consistently incompatible with experimental results.

If the experiments prove the hypothesis, it may come to be regarded as a theory or law of nature. However, it is possible that new information and discoveries could contradict any hypothesis at any stage of experimentation.

Experimental Design

When scientists design experiments to test their hypotheses, they have to be careful to avoid confounding of variables. This means that they have to isolate, as much as possible, one variable at a time so that they can reveal the relationships between the variables, if any.

An independent variable (manipulated by the experimenter) is under the control of the scientist. As the scientist changes the independent variable, it is hoped that the dependent variable (observed by the experimenter) will change as a result, and that a relationship can be established.

A control is an element of the experiment that is not subjected to the same changes in the independent variable as the experimental elements are. For example, if you want to find out how the consumption of sugar impacts the fatigue level of test takers, you would need at least a few test takers who do not consume any sugar so that you can measure the “baseline” or “natural” fatigue level for comparison to the group who consumes sugar.

If there were no control group, you wouldn’t be able to say for sure that sugar has any impact on the fatigue level of test takers. If all of the test-takers consumed sugar, and if all of them were sleepy, you would face a confounding of variables situation because the sleepiness could be caused by any other factor that the group had in common.


Some of the ACT Science Passages refer to "studies" rather than experiments. An experiment is an artificial situation that is created by the researcher. A study is characterized by careful, documented observation. Nevertheless, studies can include some of the elements of experiments, such as control groups.