When employers are hiring, interviewing candidates is pretty much a given. Yet that practice may be overrated. Research has shown that unstructured interviews, in particular, do not inform an employer much and can actually hurt if one already has more objective data such as standardized test scores. A new study reports that interviews do not just make us less accurate at predicting how qualified a candidate will be, they increase overconfidence in predictions, compounding the problem.

Edgar Kausel, a psychologist at the Pontifical Catholic University of Chile, once worked in retail and recalls that hiring managers did not ask consistent—or necessarily relevant—questions in interviews: “They might ask some people about their favorite celebrities to infer their personality and work ethic, then ask other people questions about their favorite color.” Such unstructured interviews create noise and bias, diluting more useful information. In 2013 Jason Dana of Yale University and his colleagues reported that when people predicted college students' semester grades, interviewing them in addition to seeing their cumulative grade point averages reduced accuracy.

In the new study by Kausel and his collaborators, published in the November issue of Organizational Behavior and Human Decision Processes, hiring managers saw pairs of profiles based on actual airline employees and then attempted to select the better candidate. Everyone saw scores on tests of intelligence and conscientiousness, but some also saw interview scores. Surprisingly, when the raters' predictions were graded against actual job performance, managers with access to interview scores did worse.

In another experiment, undergraduates performed a similar task evaluating job candidates but could bet points on the accuracy of their forecasts. Students with access to interview scores were more overconfident in their judgments and lost more of their bets. In the real world, a human resources representative might select the wrong people, then bet big on them. “Managers generally perceive that their gut instinct is efficient and accurate when hiring,” Kausel says.

Of course, these findings do not necessarily apply to all jobs or employees, nor are all interviews created equal. Kausel recommends using measures of actual performance, and when you do an interview, take a structured approach by asking everyone the same, job-specific questions and scoring answers individually rather than relying on a general impression. Then, trust the numbers.