WASHINGTON (CNN) - Only a handful of child safety seats won "A" ratings in all categories, including securing the child in place, the government said Wednesday in its first-ever rating of how easy the seats are to use.

The National Highway Traffic Safety Administration (NHTSA) rated 68 seats -- about 95 percent of those on the market -- as A, B, and C. Thirty-nine seats got an overall rating of "A" but only three won that ranking in all categories. (For more, check the NHTSA Web site www.nhtsa.gov or for the full list click here.)

The overall seat grades were based on five categories: whether assembly was required, whether clear labeling was on the seat, ease of following instructions, ease of securing the seat, and whether there were special features to make the seat easy to install.

"Overall the ratings are positive, but there is room for improvement," said NHTSA administrator Jeffrey Runge. "Ultimately, we hope all seats will achieve an 'A' rating."

NHTSA rated all categories: infant, convertible, forward-facing only, combination, and booster. Two categories -- convertible seats and combination seats -- were rated for both forward- and rear-facing and booster mode.

Joining NHTSA at the news conference were Consumers Union, the publisher of Consumer Reports, and the Insurance Institute for Highway Safety.

The insurance group tested the installation of child safety seats using the federally mandated Latch system (Lower Anchors and Tethers for Children), finding that while the system makes installation easier, it isn't perfect.

"Latch doesn't always make it a simple click-in operation to install a restraint," said Susan Ferguson, the institute's senior vice president for research. "Before buying one, parents should be sure to try fitting it in the vehicle they plan to use it in, because not every restraint is going to fit in every vehicle."

The institute used a sample of six child seats and a sample of 10 vehicles for its study. The easiest installation was in the Chevrolet Trailblazer, Dodge Grand Caravan, and Toyota RAV4. The most difficult installation for a car seat was in the Hyundai Santa Fe and Cadillac CTS.

"With literally thousands of combinations of vehicles and seats, it is understandable that some compatibility issues would arise," said NHTSA's Runge.

While the groups agreed that Latch was an improvement, NHTSA plans to meet with automakers and seat manufacturers to improve the current system.