Hi,

This is called a census if you are attempting to collect information/data from every single member of your study target population.

Wish you all the best with your research,

It depends on what you consider your universe to be. If that school has only 150 students and if you are selecting all the 150 students, then it is census sampling.

However, if your universe is much larger, say, 300 or 400 students in that school and if you are selecting 150 students from three grades because they have some characteristics that you are interested in studying, then its purposive sampling.

If you are surveying all 150 students in three different grades in the school, it's referred to as a "census" or "complete enumeration," where the entire population is included in the study. This is in contrast to sampling techniques, where only a subset of the population is selected for the study.

Thanks for all of the valuable answers to my question regarding the sampling technique.