A contractor that has been compiling Oahu home sale statistics for the Honolulu Board of Realtors since March 2010 has been overcounting condominium sales but maintains that the impact has been relatively small.

The firm, 10k Research and Marketing, has been including items such as storage units, offices and parking stalls among its monthly tally of residential condo sales. The company also has been counting hotel-condominium units and new condos sold by developers.

Historically, the Honolulu Board of Realtors filtered out all such transactions in its reports of previously owned condo sales. But 10k recently acknowledged that it didn’t continue the same practice, meaning that condo sales totals since March 2010 have been overstated.

Company spokesman Greg Sax said 10k plans to begin excluding such sales within the next month or two. "We’re changing our methodology," he said in an email. "Now that we know how to find them, we will snuff them out."

Sax said the inaccuracy hasn’t been big enough to be statistically significant, and won’t result in major revisions to previously reported sales totals.

According to Prudential Locations, a local real estate firm that maintains its own set of sales data that exclude new and nonresidential units, there were 3,833 condo sales last year — 101 fewer or 2.6 percent less than the 3,934 reported by 10k for the Honolulu Board of Realtors.

The difference between each firm’s condo sale numbers suggests that no directional trends in Oahu’s condo market would be reversed had 10k’s count been more accurate.

However, if 10k continued with the overcounting, there could have been large distortions during a real estate boom involving development of new condo towers or conversions of hotel rooms to hotel-condo units.

For instance, 10k recently revised several years of old Honolulu Board of Realtors data and inflated the number of condo sales in 2006 from 6,380 to 8,234.

These revisions are expected to be eliminated or redone.

While 10k has had a problem with condo data, the same difficulties don’t exist for single-family home resales.

The contractor revised Honolulu Board of Realtor single-family home sale data from 2006 to 2009, but the adjustment ranged from three more sales in 2008 to 105 more in 2009.

Difficulty counting condo sales stems from how real estate agents use the Multiple Listing Service, an electronic system used to sell homes.

A parking space or storage unit in some cases can be sold apart from a living space. But agents sometimes muddy distinctions. For instance, Unit S-206 in the luxury Ko‘olani tower in Kakaako is listed for sale in the MLS as a "studio" with 72 square feet of living space. It’s actually a storage space in the parking garage priced at $30,000.

Hotel-condo units present another challenge. Most hotel-condo units are designed and used as visitor accommodation units sold to investors.

During the last real estate boom, several thousand of these so-called condotel units were created in Hawaii, and continue to be resold. For instance, four units in the Ala Moana Hotel listed in the MLS as condos sold in June for between $78,000 and $120,000.

Sales of new condos are another issue. Sometimes brokers for developers load such sales into the MLS. The Honolulu Board of Realtors excluded new condo sales, which can distort market activity when hundreds of units in a high-rise building are sold for the first time.

To be fair, sales data previously counted by the Honolulu Board of Realtors wasn’t perfect.

Homes sold by owners and not listed on the MLS aren’t counted. And to speed delivery of reports, the trade organization counted some sales a month after they occurred in cases where brokers sold a property at the end of the month but were slow to enter the sale in the system. To correct this problem, 10k waits several days each month before reporting data.

The Honolulu Board of Realtors had compiled its own data going back to 1985, but began outsourcing the work after its longtime data cruncher Harvey Shapiro died in late 2009.

The trade association continued Shapiro’s work that included weeding out misidentified condo sales for several months before contracting with 10k, which is an affiliate of the Minneapolis Area Association of Realtors that produces statistical reports for multiple real estate agent trade associations.