What is the question human?
This is just the basic random sampling. 120 students were chosen at random from all classes and every student had the same chances of getting chosen. Things like social background or race were not taken as a criteria for getting chosen, nor did things like gender. Everyone was chosen at random.
Women usually get paid less
About 3,000 years ago they simply began a huge migration east word and north word, bringing with them their language, culture and farming methods. The migrating Bantu population eventually displaced the local hunter gather peoples causing them to blend into the population.
US Bureau of Labor Statistics