Each year Consumer Reports posts a list of models whose reliability has declined. But in some cases this “declining reliability” is a sign that they really didn’t have enough data a year ago. Many times with models that were introduced in the winter or spring they report a “better than average” result, only to report a year later that reliability has “declined.”

This affected the GMT900s this year. It affected the Honda Ridgeline last year. And, based on TrueDelta’s data, it’ll affect the 2007 Acadia and Outlook next year.

The source of the problem is that most of their surveys are returned in April, and at that time many of these late intro models have only been owned for a few weeks–or less. People haven’t had enough time to experience problems.

TrueDelta sees a similar effect in its own results when a model is very new. When the reported trips per year is under 20 per 100 vehicles, and the average odometer reading is under 3,000, there’s a very good chance that the reported repair rate is artificially low.

This is why in the notes to the August results you’ll find:

Any result under 20 repair trips per 100 vehicles is likely a result of the small sample size or cars too new to have experienced many problems, and should be expected to increase to at least 20 in future updates.

So, when looking at TrueDelta’s results, pay attention to the average odometer reading and the average number of months per vehicle. When these are very low, there’s a fair chance that future reliability will “decline.” Not because the cars have actually gotten worse, but because earlier owners had not yet had their cars long enough to experience problems.