Hamilton police say the official statistics that show it considers almost one in three sexual assault reports as unfounded are wrong.

The service says it has internal statistics that are different from the official ones recorded for national crime reporting. It says those numbers show it is faring much better in how often victims are believed when they come to them reporting a sexual assault.

However, the system used to record official numbers is based on a national standard, and has been adopted by every police service across the country for decades.

The discrepancies raise questions about statistics gathering at Hamilton police as a whole, and just what numbers are being used when making decisions about policing.

Even with the new numbers, it's not clear what the service's actual rate has been.

The police service says that in 2015, it launched a review in response to a request for information for a Globe and Mail investigation that revealed police in Hamilton and Halton dismissed 30 per cent of sex assault claims over a five-year period as "unfounded," which is a number far higher than the national average of 19 per cent.

When an allegation is made to police and it is deemed unfounded, that means the investigator doesn't believe a criminal offence was attempted or occurred.

'Discrepancies' in the data

Insp. Dave Hennick told CBC News Tuesday that after police received the Globe's request for statistics in 2015, they started reviewing the service's numbers.

"We undertook a review [in 2015] for a closer look at how sexual assaults were being investigated," Hennick said. That involved looking at the service's numbers, but also its practices when it comes to how officers handled sexual assault complaints.

As an example, Hennick says the numbers released to the Globe say Hamilton police received 470 sex assault complaints in 2014, but after review, that the number should actually be 683.

Hennick said they found that statistics for the whole police service inputted into the Uniform Crime Reporting Survey (UCR) didn't capture all the cases that had been dealt with on a "granular level."

"We have recognized there are limitations in the UCR reporting," he said.

But it's not clear why the service is not including these cases in its official reporting. Both Hennick and a police spokesperson did not answer repeated requests for clarification on the issue.

UCR reporting has been the standard way that police services across the country have been reporting crime statistics since 1962, said Alex Smale, a senior technical officer for Statistics Canada.

"All police services in Canada are on board with the UCR," he said.

Though police across the country have to use UCR to report cases, Statistics Canada doesn't require them to send in cases that were determined to be unfounded.

In a statement, the organization said that's because the "quality of data presents huge inconsistencies to a point where it would be impossible to interpret [it]."

The importance of good data

The question then remains, why do the internal numbers that Hennick says the sexual assault unit has differ from what is being reported officially?

Hamilton police did not respond to follow-up inquires on the issue Wednesday.

In an interview on Tuesday, Hennick attributed the inconsistencies to the UCR system, which he says lacks the ability to dig far down into the data, which is why the sexual assault unit keeps its own statistics. Between the two sets of numbers, there are "discrepancies," he said.

Lenore Lukasik-Foss, the director of the Sexual Assault Centre of Hamilton, told CBC News that she was "quite surprised" to hear about the discrepancies in the numbers.

"It speaks to the importance of good data," she said. "We can't make good policy decisions if we don't fully know what's going on."

In the wake of the Globe's investigation, police services in London and York Region, as well as the OPP, have committed to a review of previous sexual assault cases.

adam.carter@cbc.ca