How many people are killed or seriously injured because of drivers fiddling with their cellphones? Due to widespread underreporting, no one really knows.

In 2015, according to estimates of the National Highway Traffic Safety Administration, there were 3,477 deaths caused by distracted driving, including 476 involving the use of cellphones. Depending who you ask, this last number is either low or very, very low.

Fatality figures come from the Fatality Analysis Reporting System, or FARS, a federal database that relies on information from police crash reports. Unless a police report states that a driver was talking on, listening to or manipulating a cellphone when a fatal crash occurred, cellphone involvement is not coded in FARS.

Yet drivers who crash while sending a text or checking Facebook rarely admit it to police. Unlike with drunk driving, there is no breathalyzer or blood test to reveal what drivers were doing when they rear-ended someone. So unless a witness reported the driver’s use of a cellphone, that detail would not be captured in the police report. In an email to FairWarning, NHTSA officials acknowledged that their estimates “may not state the true size of the problem, since the identification of distraction and its role in the crash by law enforcement can be very difficult to measure.”

Even if it’s known that a crash was related to cellphone use, that fact still could be omitted. Although 47 states have bans on texting with a hand-held device, 26 states don’t have a space or code on their crash reports to note whether a driver was texting, according to a recent report by the National Safety Council.

Another safety council study provided additional evidence of underreporting. Researchers reviewed 180 fatal crashes from 2009 to 2011 in which there was evidence of cellphone use by drivers. But cellphone involvement was recorded by FARS in only 52 percent of those crashes.

The same study highlighted wide disparities in data from state to state. For example, it found that in 2011, more fatal crashes involving cellphones were recorded in Tennessee, 93, than in any other state. By comparison, only 22 such crashes were recorded in California, even though it had three times as many traffic deaths overall as Tennessee.