Read the following research description and answer the questions that follow.

Description:

The registrar's office of a small college was asked to prepare a report for a national association of colleges regarding the job placement of their recent graduates. The college registrar conducted a mail survey of alumni who graduated in the last two years. A brief questionnaire asking about the employment status of the graduates was constructed and mailed to the 682 graduates who appeared on the graduation rolls in the last two years. A total of 504 completed questionnaires were returned. A tabulation of the survey results revealed that 88 percent of the graduates indicated they were employed in a field of their choice. The average time reported to obtain a first job after graduation was 1.2 months. The registrar was quite enthusiastic about these results and quickly forwarded them to the national association. In his summary letter to the association, he underscored the fact that at his college nearly 9 out of 10 graduates are successfully employed within 6 weeks!

1		Was this survey study based on a probability sampling procedure or a nonprobability sampling procedure?
2		What survey research problem is possible because of the return rate in this study?
3		How does a potential response bias problem likely affect the conclusion that nearly "9 out of 10" graduates had jobs?
4		Although it is possible that the actual percentage of recent graduates from this school who have jobs in fields of their choice is 88%, the percentage is likely somewhat less than 88%. If we assume that those graduates who responded to the survey answered truthfully, what could be the actual percentage of graduates holding jobs?