When Massachusetts expanded health insurance a decade ago, state officials unknowingly created an experiment. It’s turned out to be an experiment that offers real-world evidence of what would happen if the House Republicans’ health bill were to become law.

The findings from Massachusetts come from an academic paper being released Thursday, and the timing is good. Until now, the main analysis of the Republican health bill has come from the Congressional Budget Office, and some Republicans have criticized that analysis as speculative. The Massachusetts data is more concrete.

Unfortunately for those Republicans, the new data makes their health care bill look even worse than the C.B.O. report did. The bill could cause more people to lose insurance than previously predicted and do more damage to insurance markets. The $8 billion sweetener that Republicans added to the bill on Wednesday would do nothing to change this reality. President Trump and Speaker Paul Ryan are continuing to push a policy that would harm millions of Americans.

Here are the basics of the new study, and why it matters:

The Massachusetts law subsidizes health insurance for lower-income households, and does so via four different income categories. Everyone in a category — for example, a family of four earning between $44,700 and $55,875 a year — would pay the same price for insurance. A family earning less would pay less, and a family earning more would pay more.