Susan used to work for a large corporation where the premium for employment-based group health insurance was $500 per month. Of this, she contributed $100 and her employer contributed the remaining $400. When she left this job, her new employer did not offer group health insurance. She discovered than purchasing an equivalent quality individual health insurance plan would cost her a monthly premium of $800. Explain why there was such a difference in the premiums. (Assume this was before passage of the Affordable Care Act)