Which of the following best describes a census in the context of data collection?

A researcher collected responses from $180$ employees at a manufacturing company to assess job satisfaction. The company has a total of over $3,500$ employees. Is this data set a population or a sample? Explain your reasoning.

A school district is testing a new math curriculum. They select $50$ classrooms from different schools. The classrooms are randomly assigned to use either the new curriculum or the standard curriculum for one semester. At the end of the semester, students' math scores are compared. What are the experimental units and the treatments in this experiment?

You design an online survey about commuter students' use of campus transportation. You e-mail the survey to a convenience sample of $150$ students and receive $42$ completed surveys. Which of the following is the correct response rate and the most likely nonsampling error affecting representativeness?

A national research group now recruits survey participants through a smartphone app and by sending SMS invitations instead of using landline calls. Explain how these recruitment methods can introduce nonsampling error, and identify the main types of nonsampling error likely to arise.