Importance of Random Assignment in Experimental Design

A group of researchers is interested in understanding the effects of music on concentration levels. To test this, they designed an experiment where they asked a group of 40 high school students to complete math problems under different listening conditions: complete silence, soft instrumental music, and loud pop music. Each student spent 20 minutes in each condition, and their scores were recorded. The researchers aim to analyze whether different types of music help or hinder students' performance on the math problems.

To ensure valid results, which of the following experimental design elements is most important for this study?