The sampling method used by the band director is known as cluster sampling.
In cluster sampling, the population is divided into separate groups, known as clusters.
A random selection is then made of these clusters, and all members of the chosen clusters are surveyed.
In this case, the director divided the band into clusters based on their grade - freshmen, sophomores, juniors, and seniors.
After that, one group was chosen randomly, and every member within this group was surveyed.
This approach differs from stratified sampling, where samples would be taken from each stratum (grade level in this context) rather than selecting all members from a randomly chosen single stratum.