The IIHS uses four ratings for each category, Good (best, green G), Acceptable (yellow A), Marginal (orange M) and Poor (worst, red P). Vehicles which score Good in all the various rating categories, or which have only one Acceptable category, are given Best Pick designations.
As with NHTSA's New Car Assessment Program testings, vehicles across different categories may not be directly compared.
The IIHS recenlty introduced a side impact test. The NHTSA has a New Car Assessment Program test's low barrier. The IIHS uses an elevated barrier to simulate the impact of an SUV (approximately half of all new cars sold) into the side of the vehicle being tested. This is a very demanding test of both the structural integrity of the vehicle, as well as the restraints. While most new vehicles achieve 4-5 stars from the NHTSA (where head injuries are not part of the rating), many do not score well in the IIHS side impact test.
The IIHS also evaluates vehicles' bumpers in a series of 5 mph (8 km/h) impacts, as well as seat and head restraint designs in relation to rear-impact protection, using the same Poor-Good rating system.
The IIHS and NHTSA tests can differ wildly. For example, the NHTSA graded the Chevrolet Venture (also marketed as Oldsmobile Silhouette, Pontiac Montana/TransSport) as 4/5 stars, but the IIHS graded it "Poor" for its poor structural integrity which becomes apparent in the offset crash test. The same applies for the 1997-2003 Ford F-150.