These researchers conducted surveys at a nursing conference. What type of sample is this? How does this affect the generalizability of the results?

In a study by Vassiliadou et al. (2008), the role of nursing in sexual counseling of myocardial patients was examined. The authors examined professional nurses and collected data about their gender, age, education, unit of employment, and experience in cardiac clinics. They then compared, among other things, the knowledge and comfort level nurses had with regard to sexual counseling.