One of the nation's most prominent public-policy think tanks was wiping egg from its face Monday after retracting a study it released last month challenging law enforcement's oft-advanced claim that medical marijuana dispensaries boost neighborhood crime.

RAND Corporation's preliminary study of 600 marijuana dispensaries -- some of which shut down, some of which stayed open -- over a three-week period in 2010 indicated that crime rises in surrounding neighborhoods when dispensaries close. The report was immediately touted by medical marijuana advocates from coast to coast as evidence that police complaints of heightened crime at or near dispensaries were merely anecdotal; law enforcement replied the study examined was too small a sample over too short a time.

But RAND announced Monday that questions raised after publication prompted the Santa Monica-based think tank to take another look at the study. In fact, the study's data described as covering the city of Los Angeles and surrounding areas did not include crime data reported by the Los Angeles Police Department.

"The data came from a commercial crime-mapping service, and they pull in information from multiple jurisdictions -- including, in this case, the L.A. sheriff's data and other neighboring jurisdictions" but not LAPD, said Debra Knopman, vice president of RAND's Infrastructure, Safety and Environment division. "We're still doing a review of the data, so I can't give you a more complete answer than that."

All RAND reports are subject to pre-publication scrutiny by peer reviewers inside and outside the organization, she said. "We followed all of those processes but we obviously made a mistake here. That's what we're trying to find out in the review process, where the error occurred."

The retraction comes even as California's federal prosecutors stage a crackdown on for-profit dispensaries across the state.

California Police Chiefs Association President David Maggard Jr., Irvine's police chief, issued a statement saying his group appreciates "RAND's acknowledgment that the data and research were insufficient to reach the conclusions it did and agree(s) with their decision to retract the paper."

"As leaders responsible for providing law enforcement services to 78 percent of California, we have a keen perspective on this issue," he said, noting his association's 2009 report showed crime and other quality of life problems are common to areas near marijuana dispensaries.

But that report offered anecdotes of crime in and near dispensaries, not a statistical analysis.

Kris Hermes, spokesman for Oakland-based Americans for Safe Access, said his national medical-marijuana advocacy group has done its own studies showing crime drops or remains stable around dispensaries, and Los Angeles Police Chief Charlie Beck last year said his department's 2009 data showed banks were bigger crime targets than dispensaries. Hermes said his group is concerned that the RAND report's retraction "appears to be politically motivated."

"The L.A. city attorney's office had very negative things to say about the study," he said. "For RAND to say they didn't consult with the LAPD to get their own crime statistics is pretty implausible. If that's the case, I think their research department has some explaining to do."

The San Jose City Council last month decided to allow only 10 dispensaries, ending nearly two years of debate as nearly 12 dozen dispensaries spread across the city. San Jose Police spokesman Sgt. Jason Dwyer reiterated Monday what he'd said when the RAND report first was released.

"The issue we had with the study was that it's very short in duration," covering only a 21-day period, he said. "This is something that has to involve more long-term monitoring, to come up with some data compiled over a year or so."