The FBI’s Uniform Crime Report shows that murder rose nationally for the second straight year in 2016, and we can feel fairly confident in that assessment because murders rarely escape the attention of the authorities. But for many other types of crime, including theft and rape, the UCR provides only an incomplete picture — it relies on police reports, and not everything gets reported to the police.

That’s why the National Crime Victimization Survey — released today by the Bureau of Justice Statistics — is important. The NCVS is a survey of individuals rather than police reports; it began in 1973 with the goal of quantifying non-fatal crimes that both are and are not reported to the police. Roughly 160,000 people from about 90,000 households are surveyed every year in order to estimate national crime trends.

The 2016 survey found that there were about 21.1 violent crime victimizations per 1,000 persons age 12 or older. On its face, that would suggest a big rise in violent crime from the roughly 18.6 victimizations per 1,000 people identified in the 2015 NCVS. And the UCR released relatively similar findings in September, which appears to confirm that there was a rise in violent crime nationally in 2016.

“Appears” is the key word there, however. In fact, there are a number of reasons to be cautious about interpreting this year’s NCVS, as well as the UCR.

Part of the NCVS involves estimating what percentage of certain crimes are reported to law enforcement. The 2016 NCVS found that just 42 percent of violent crimes and 36 percent of property crimes were reported to police last year. Ultimately, if less than half of (non-murder) violent crimes are reported to the police, then year-to-year trends highlighted in the UCR may or may not be accurate — it’s possible that they’re more a reflection of fluctuations in how many people report a crime each year rather than changes in how many crimes are actually committed. And the UCR is one of our main pieces of evidence that violent crime has increased.

Share of crimes reported to the police TYPE SHARE REPORTED Auto theft 79.9% Aggravated assault 58.5 Robbery 54.0 Burglary 49.7 Theft 29.7 Rape or sexual assault 22.9 Among major crimes in the 2016 Uniform Crime Report Source: Bureau of Justice Statistics

Crimes go unreported for all sorts of reasons, including fear of repercussions, lack of trust in the police, long waits for police to respond to a call, or simply disinterest in involving the police in a minor incident. The NCVS shows clearly that the rate at which aggravated assault, rape, robbery, burglary, theft and auto theft are reported to the police varies significantly by crime type. For example, nearly four out of every five auto thefts in 2016 were reported to the police, but less than a quarter of rapes and sexual assaults were.

It makes sense that auto thefts and aggravated assaults are reported to the police at relatively high rates. Stolen vehicles are expensive to replace and are more likely to be recovered by police than other stolen items. In addition, the vast majority of cars are insured and most insurance policies require a police report before companies will pay out a claim on a stolen vehicle.

Aggravated assaults, for their part, usually involve a weapon and always involve the intent to inflict severe injury, according to the FBI. As such, these crimes are more likely to result in hospitalization, and most states require hospitals to report suspicious injuries to law enforcement, though requirements vary from state to state.

The other reason to question whether NCVS points to a real rise in violent crime is that the survey changed how it constructs its sample of respondents, which means violent crime rates recorded in the 2016 survey are not comparable with findings from previous years. This was a “routine redesign” of the survey sample, which the NCVS performs every 10 years to keep up with the population changes identified since the most recent U.S. Census.

The NCVS measured a jump in overall violent crime victimization from 2015 to 2016, but the survey found no measurable change in crime rates “among counties that remained in the sample from the previous design.” To John Pfaff, a professor at Fordham University who specializes in criminal and sentencing law, the main takeaway is that “the increase is likely entirely the product of changes in how the [Bureau of Justice Statistics] gathered the data.” In other words, we simply don’t know whether violent crime victimization rose last year, even though the survey shows an increase. It’s possible the jump in violent crime between 2015 and 2016 just reflects an increase in the number of crimes that victims are reporting to NCVS interviewers rather than an increase in actual crimes.

Pfaff says NCVS and UCR are “measuring crime in two different ways and providing two different images of an underlying problem. It’s less about saying which one is right and more about trying to understand what they say together.”

Ultimately, both the crime victims survey and the FBI crime report are in agreement that violent crime has fallen significantly since the 1990s. There is also little doubt that the national murder rate has risen about 20 percent since its lowest point in 2014. But any general claims of rising violent crime in America should be taken with a grain of salt: We simply cannot say for sure whether violent crime increased, decreased or stayed the same last year.