Another bit from J.D. Power’s press release for their 2008 Vehicle Dependability Study: “on average, customers report experiencing 75 percent more problems in the third year of ownership than during the first 90 days.” They refer to this increase in reported problems as “quality deterioration.”

One big problem with this logic: the VDS covers an entire year, while the IQS covers only 90 days. In TrueDelta’s survey, problems occur fairly linearly for the first three years or so. Cover twice as many months, get twice as many repairs reported.

So a 75 percent increase is surprisingly low given that 300 percent more time is included. How might this be explained?

To begin with, JD Power includes design issues as well as reliability issues in their studies, especially in the IQS. Since design issues cannot be fixed, they’re present from the day the car is made, and never go away. They don’t increase linearly with time.

But respondent memory might also be a factor. TrueDelta conducts a monthly survey for one big reason: after a few weeks have passed, people start forgetting things. I send out emails to people with gaps in their responses. One of these people, asked to fill a hole last December, replied, “Are you kidding?” So when a survey covers an entire year, problems that were resolves earlier in the year are much more likely to be forgotten.

Finally, we have overload. J.D. Power’s surveys require much more work than TrueDelta’s, both because they cover a whole year and because they include far more questions. After reporting a problem or two, many people are likely to figure that will do and not mention problem three. Or four.

Ask every month, and you also avoid this factor.

Put all of this together, and we have far more deterioration in J.D.’s data from the IQS to the VDS than I see deterioration in the cars being studied.