Medicare is a vital universal health insurance program for those 65 years and older. Medicare isn't easy to navigate, hardly surprising considering the Byzantine U.S. health care system. For those people who are on Medicare, we're nearing the critical open-enrollment period which runs from October 15 to December 7. Those on Medicare can change their plan if they want during open enrollment.

The evidence is compelling that it pays to research your options and do some comparison shopping. Plans change over time and so does your health. It's well worth putting in the time to investigate whether you should change plans or stick with your current plan.

Nothing is simple about Medicare, but a good starting place is understanding the two main options. Traditional Medicare is more expensive upfront since people buy Medigap insurance to fill in holes in their coverage. They also typically buy a standalone Part D plan to cover their medications. The advantage of original Medicare is you can use any health care provider in the country who accepts Medicare and very few services require prior authorization. If you are in traditional Medicare, you can change your Part D drug plan or switch to a Medicare Advantage plan during open enrollment.

The alternative to traditional Medicare is Medicare Advantage. These are private health plans paid by the federal government to provide Medicare-covered benefits. Advantage plans must cover everything that original Medicare covers, but the plans can also offer additional benefits, such as dental care and vision services. You must use the approved providers in the plan's network. There are prior authorization requirements to see specialists, get out-of-network care, and more. If you are enrolled in a Medicare Advantage plan, you can switch to another Advantage plan during open enrollment.

Traditional Medicare works best with a supplemental Medigap policy. But once past the initial Medicare enrollment period, insurers can insist on a medical underwriting test. They can turn you down for coverage if they don't like the results or charge a higher premium reflecting preexisting conditions.

Three suggestions for starting your research. Medicare offers several online tools, including the comparison-shopping Medicare Plan Finder. The Minnesota State Health Insurance Assistance Program/Senior LinkAge Line offers free assistance reviewing your options. The Medicare Rights Center is another good resource.

Chris Farrell is senior economics contributor, "Marketplace"; commentator, Minnesota Public Radio.