THE Christian siblings were doing their homework when the police arrived. Two officers entered the house, guns drawn, pursuing what was evidently a prank tip-off about a captive being held at their address. The guns stayed out even when the mistake became apparent. The officers ran the details of the children’s father—who, like them, is black—through the police system on the off-chance of turning something up.

The family was traumatised. The incident, in 2013, brought home to Ima Christian, now 18, that Americans could be vulnerable to rough policing “no matter where you live, or who you are”; her sister Asha, who is 16, says it is “not until you are face to face with an officer that you realise what the deal is.” The sisters—from Stone Mountain, just outside Atlanta—didn’t get even, exactly. Instead, with their brother Caleb (now 15), they developed an app, called Five-O, intended to help improve police behaviour and community relations. It lets citizens rate their experiences with officers, record both parties’ race and sex and the purpose of the interaction, and find aggregate scores for county forces.

Five-O (a slang term for cops) was launched in 2014, but will get a boost this spring from the €20,000 ($22,000) prize it won at an international contest for justice-related initiatives, organised by a think-tank in the Netherlands. The money will go towards marketing the tool in Baltimore and Chicago. Attracting input from broad cross-sections of such communities is one of the ways the Christians believe they can neutralise an obvious potential bias—ie, that the ratings will be skewed by the aggrieved, legitimate as those grievances may sometimes be. That composite picture, combining good and bad feedback, is, they reckon, one of the ways their product differs from other police-related apps, which concentrate on uploading video. They also want to extend its availability from Android to iPhones. The long-term plan is to include Britain, Brazil, Canada and Russia, making Five-O, as Asha puts it, “a global repository of unbiased police data”.

That is an ambitious goal for teenagers who mostly taught themselves to code. (Their parents used to work for an internet start-up and, Caleb recalls, noticed youngsters “getting paid insane amounts of money” for programming.) In 2016 they aim to launch another app through their firm, Pinetart Inc: this one, Coily, lets women rate hair-care products, and so avoid shower-stall accumulations of half-empty bottles. Studies permitting, that is. Ima is a freshman at Stanford University; Asha—who is finishing high school online, to free up time for enterprise—hopes to join her or go to Columbia. “I’m very proud of them,” says their mother Karen.