Opioid-related overdose deaths are being undercounted by 20 to 35 percent, a new study concludes.

The reason is widely varying standards across the country for investigating and reporting on overdoses, NPR reports. Some opioid overdose deaths are not being captured in state and county data reported to the federal government, the study found.

The Centers for Disease Control and Prevention reported more than 42,000 people died from opioid-related overdoses in 2016. Christopher Ruhm, lead author of the new study, says the real number is closer to 50,000. His analysis included overdoses that were not linked to specific drugs. Ruhm says many of these deaths were likely opioid-related.