Methodology

Sources

• Owner complaints — NHTSA Office of Defects Investigation, the bulk complaint file. One complaint is counted once, by its ODI number, even when it names several parts.
• Recalls — the ODI recall files, including do-not-drive and park-outside warnings.
• Federal investigations — the ODI investigation file, open and closed.
• Manufacturer communications — the technical service bulletins manufacturers file with NHTSA.
• Crash tests — NHTSA's New Car Assessment Program.
• Fuel economy — the U.S. EPA / DOE fuel economy data.

What a page is about

A page covers one model year of one model, as the EPA groups it. The names NHTSA receives complaints under are matched to that group, so a complaint filed against a hybrid or a trim name is counted with the model it belongs to.

The two judgments

Within the generation. Model years cannot be compared by their running totals, because an older year has had more time to collect complaints. Each year is therefore counted over the same span: complaints received up to two calendar years after its model year. A year is compared only when that span is complete and holds at least 50 complaints, and only against the other years of its own generation, never against another model. A year is named when it holds at least 1.5 times the median of its generation, or at most the median divided by 1.5.

The defect pattern. Complaints name a system. When one system holds 40% or more of the complaints that name any system, the page says so. This catches faults that appear after the three-year window closes, which the comparison cannot see.

A page is published when four sources cover the year, it holds at least 50 complaints, and it carries at least one of those two judgments.

What this site does not measure

Complaint counts are not failure rates. They are not divided by how many cars were sold, because no public source reports U.S. sales by model year. A model that sells more draws more complaints, and owners file more after news coverage or a recall. Comparing years inside one generation reduces that effect but does not remove it.

Owner complaints are unverified accounts, quoted as filed. A service bulletin is not a recall and not a defect finding, and most bulletins cover several model years at once. Whether a recall has been carried out on a particular car depends on its VIN.

Generations

No data source publishes model generations, so the boundaries are entered by hand from public model history, for the U.S. market. The U.S. calendar often differs from the manufacturer's global one: the Ford Focus kept its first generation here through 2007 while Europe moved on in 2004, and the fourth generation was never sold in the United States at all.

Corrections

The federal files are reloaded and the pages rebuilt on a schedule; every page carries the date of the data it was built from. If a page misreads the record, the correction is made in the rule that produced it, not in the page alone.