For more than a year, Tesla has defended its semiautonomous Autopilot as a vital, life-saving feature. CEO Elon Musk has lambasted journalists who write about crashes involving the system. “It's really incredibly irresponsible of any journalists with integrity to write an article that would lead people to believe that autonomy is less safe,” he said during a tumultuous earnings call this week. “Because people might actually turn it off, and then die.”

This wasn’t the first time Musk has made this argument about Autopilot, which keeps the car in its lane and a safe distance from other vehicles but requires constant human oversight, and has been involved in two fatal crashes in the US. “Writing an article that’s negative, you’re effectively dissuading people from using autonomous vehicles, you’re killing people,” he said on an October 2016 conference call.

Wednesday’s haranguing, however, came a few hours after the National Highway Traffic Safety Administration (NHTSA) indicated that Tesla has been misconstruing the key statistic it uses to defend its technology. Over the past year and a half, Tesla spokespeople have repeatedly said that the agency has found Autopilot to reduce crash rates by 40 percent. They repeated it most recently after the death of a Northern California man whose Model X crashed into a highway safety barrier while in Autopilot mode in March.

Now NHTSA says that’s not exactly right—and there’s no clear evidence for how safe the pseudo-self-driving feature actually is.

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The remarkable stat comes from a January 2017 report that summarized NHTSA’s investigation into the death of Joshua Brown, whose Model S crashed into a truck turning across its path while in Autopilot mode. According to its data, model year 2014 through 2016 Teslas saw 1.3 airbag deployments per million miles, before Tesla made Autopilot available via an over-the-air software update. Afterward, the rate was 0.8 per million miles. “The data show that the Tesla vehicles' crash rate dropped by almost 40 percent after Autosteer installation,” the investigators concluded.

Just a few problems. First, as reported by Reuters and confirmed to WIRED, NHTSA has reiterated that its data came from Tesla, and has not been verified by an independent party (as it noted in a footnote in the report). Second, it says its investigators did not consider whether the driver was using Autopilot at the time of each crash. (Reminder: Drivers are only supposed to use Autopilot in very specific contexts.) And third, airbag deployments are an inexact proxy for crashes. Especially considering that in the death that triggered the investigation, the airbags did not deploy.

Tesla declined to comment on NHTSA’s clarification.

The statistic has been the subject of controversy for some time. The research firm Quality Control Systems Corp. has filed a Freedom of Information Act lawsuit against NHTSA for the underlying data in that 2017 report, which it hopes to use to determine whether the 40 percent figure is valid. NHTSA has thus far denied its FOIA requests, saying it agreed to Tesla’s requests to keep the data confidential, and that its release could threaten the carmakers’ competitiveness.

Tesla’s oft-touted figure is flawed for another reason, experts say: With this data set, you can’t separate the role of Autopilot from that of automatic emergency braking, which Tesla began releasing just a few months before Autopilot. According to the Insurance Institute for Highway Safety, vehicles that can detect imminent collisions and hit the brakes on their own suffer half as many rear-end crashes as those that can’t. (More than 99 percent of cars Tesla produced in 2017 came equipped with the feature standard, a higher proportion than any other carmaker.)