Black Book's Used Vehicle Retention Index for April remained unchanged from the previous month at 112 and has declined by a mere 1% over the last 12 months, the company announced.

The Used Vehicle Retention Index is calculated using Black Book’s published wholesale average value on two- to six-year-old used vehicles, as a percentage of original, typically equipped MSRP. It is weighted based on registration volume and adjusted for seasonality, vehicle age, mileage, and condition. The index is billed as an accurate, representative, and unbiased view of the strength of the used-vehicle market values.

The monthly index has only declined once over the last four months, dropping from 113.3 to 112 at the end of February. Much of this recent strength has come from a stronger-than-average spring selling season, with a handful of car segments seeing a rise during this stretch: Since March, compact cars have increased 2.5% and are up an impressive 3.4% over the last 12 months. Mid-size cars are up 1.3% since March. On the other hand, full-size pickups, full-size SUVs and CUVs, and subcompact cars declined by 0.7%, 1.6%, and 1.2%, respectively, since March.

The Index dates back to January 2005, where Black Book published a benchmark index value of 100 for the market. During 2008, the index dropped by 14.1% while during 2016, the index fell by just 6.4%. During 2011, the index rose strongly from 113.3 to 123 by the end of the year as the economy picked up steam and used vehicle values rose higher. It continued to remain relatively stable, rising slightly until May of 2014, when it hit a peak of 128.1.

Related: Used Sporty and Midsize Cars Drive Growth