The Portland Police Bureau has struggled to fill vacancies in its ranks and officials expect 100 more retirements this year, according to PPB budget documents.

Intuitively, the staffing shortage would seem to contribute to overtime for eligible officers and sergeants, who get paid time-and-a-half for overtime hours. But an audit released this morning says PPB has loose grasp of exactly why its people are working so much overtime.

"We found that inadequate data collection and reporting limited police supervisors' ability to effectively control overtime for patrol officers," the audit says. "The Police Bureau blames the majority of overtime on staffing shortages, but it could not verify that staffing shortages were the main driver of overtime."

Here's how many hours of OT the bureau, which accounts for by far the largest share of the city's $682 million general fund budget, incurred in 2017-18 and prior years.

The bureau spent $15.7 million on overtime in 2017-18, the audit says, which was 8.8 percent of PPB's overall budget.

The auditors did not buy the bureau's explanation for why overtime expenses are so high.

"Bureau staff at all levels said there was no sense in looking for ways to limit overtime because of the existing personnel shortage," the auditors wrote. "We found that reasoning to be based on faulty assumptions that overtime data were reliable and management decisions about when to use it were sound."

Chief Danielle Outlaw responded positively to the audit's findings, saying the bureau is taking several steps to reduce its reliance on overtime.

“We are currently in the process of changing our shift schedules and minimum staffing levels based upon demand for service and public safety needs, and will continue to evaluate how efficiently we are meeting these demands in the future,” Outlaw wrote to Auditor Mary Hull Caballero.

"In most instances, the Bureau either has already or is in the process of implementing strategies along the lines of the suggestions put forth in the report."