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Two boys abducted in Missouri were found in a St. Louis suburb last Friday, in the home of a pizza shop employee. One had been missing for less than a week, the other for more than four years. News reports cited a statistic that 800,000 children disappear every year—or about 2,000 a day. Seriously? How reliable are these numbers?

Reliable enough, but easily misinterpeted. Like most crime statistics, abduction numbers are fungible since they depend so much on whether the crime gets reported and how you define abduction. Saying a child is “missing” can mean any number of things; a child who has run away from home counts the same as a kidnapped murder victim. For officials, the total number includes those who fall into several different categories: family abduction, nonfamily abduction, runaways, throwaways (abandoned children), or lost and “otherwise missing” children. Local police departments register missing children with the federal National Criminal Information Center database, specifying what type of abduction it is.

When the categories get conflated, the statistics can become confusing. Take the number 800,000: It’s true that 797,500 people under 18 were reported missing in a one-year period, according to a 2002 study. But of those cases, 203,900 were family abductions, 58,200 were nonfamily abductions, and only 115 were “stereotypical kidnappings,” defined in one study as “a nonfamily abduction perpetrated by a slight acquaintance or stranger in which a child is detained overnight, transported at least 50 miles, held for ransom or abducted with the intent to keep the child permanently, or killed.” Even these categories can be misleading: Overstaying a visit with a noncustodial parent, for example, could qualify as a family abduction. Some individuals get entered into the database multiple times after disappearing on different occasions, resulting in potentially misleading numbers.

But in other ways, the NCIC may understate the figures. Many missing persons aren’t reported at all—a 1997 study estimated that only 5 percent of nonfamily abductions (in which a nonfamily member detains a child using force for more than an hour) get reported to police. Some police departments may not even bother filing a report when a kid runs away from home for a few days. It’s also easy to lose track of abduction cases, since some of them get filed away under associated crimes, like homicide or sexual assault.

Until the early ‘80s, investigating cases of missing children was left entirely up to local officials, who didn’t have an alert system in place or a central database to keep records. But after a series of high-profile abductions in the late 1970s and early ‘80s, like those of 6-year-olds Etan Patz and Adam Walsh (son of America’s Most Wanted host John Walsh), Congress passed legislation creating the National Center for Missing and Exploited Children, an organization that monitors the FBI’s database of missing children and collaborates with local law enforcement to get the word out. In recent years, states implemented “Amber laws,” named after 9-year-old murder victim Amber Hagerman, setting up an alert system for missing children.

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Explainer thanks Paula Fass of the University of California, Berkeley and Connie Marstiller of the National Criminal Information Center.