The booster seat crash tests used a specially-designed dummy that was about the size of a 6-year-old.
"The main thing boosters are supposed to do is provide good belt fit. So it is somewhat surprising that some boosters aren't doing that very basic job," said Anne McCartt, Institute senior vice president for research.
A bad fit at the shoulder can fling kids foward, and at the waist can cause internal organ injuries.
A good booster routes the lap belt flat across a child's upper thighs and positions the shoulder belt at midshoulder. The Institute released its first booster ratings last year, evaluating 41 seats.
"Parents can't tell a good booster from a bad one just by comparing design features and price," said McCartt. "What really matters is if the booster you're considering correctly positions the safety belt on your 4-8 year-old in your vehicle. Our ratings make it easier to pick a safer booster for kids who have outgrown child restraints."
The new ratings cover almost all models sold in the United States right now. Manufacturers provided seat model numbers, and the Institute conducted its own check of retail inventories before purchasing seats.