I've no experience of internships, but you might want to consider also including Apprenticeships in this study, where people are employed on training contracts for the duration of their study for an NVQ/Apprenticeship/Advanced Apprenticeship, and are paid a fixed rate on the training contract. Once the qualification is obtained, the contract ends with no obligation on the employer to keep them on. (In the health sector in my experience, some employers do offer a permanent contract if the process has worked for them - others don't).