Twenty years ago, when Prince George's policeman Richard Shaner was a rookie, most officers were expected to pitch in an extra shift to protect shoppers and merchants at Christmas.

"You didn't get any extra compensation for it," said Shaner, now a captain in charge of the county police department's Special Operations Division. "If you wanted to be a policeman, you just did it."

Over the years, however, the department's special effort to check the increase in some types of crime that seem to come with the season has evolved from no-pay double time, to paid overtime, to the current "Christmas Assignment" detail of more than 80 additional officers doing both high-visibility and undercover work, mostly in shopping centers. Many of the officers are pulled from desk jobs and other nonpatrol functions to pound a crime beat for the period between Thanksgiving and New Year's.

Police say it is difficult to show the effectiveness of the Christmas Assignment through general crime statistics, but according to Shaner, at least one armed robbery -- at a Hyattsville convenience store -- was prevented and a would-be perpetrator was arrested. The special effort is necessary, police are convinced, because people tend to become "softer targets for crime" in the holiday rush.

"There's an awful lot of people out shopping, a lot more than usual. That includes crooks," Shaner said. He says criminals become more active because they're trying to get their goodies, too. "What happens during this time of year is that some of the resistance to crime is lost," he said, citing the armloads of packages that weary shoppers tend to leave on the seats of unlocked cars.

"It's awfully difficult for a citizen to go around constantly alert to the possibility of being ripped off," Shaner added. That is where Cpl. Frank Kobilis, who spends most of his year just talking about crime, comes in.

Kobilis, working out of a patrol car, goes in and out of shopping centers all day, checking storefronts and activities in the busy parking lots. Kobilis, an 11-year veteran who began as a patrol officer, has worked in the crime-prevention unit since 1979, lecturing senior citizens on self-protection and giving homeowners security tips, such as placing sheet-metal screws in sliding doors to keep burgulars from opening them with crowbars. He hasn't worked a car beat in seven years, but he does not seem to mind the duty.

"It's not really a demotion; I've kind of enjoyed the assignment. It's getting back out on the road again," Kobilis said during a break in his patrol.

He has not yet made any "collars," or apprehensions, but answering two holdup alarms in a busy shopping center recently reminded him of the difference between the two types of duty.

"Walking in on a holdup alarm, whether it is a good alarm or not, you don't know what's going to happen," Kobilis said, adding, "It reminds me of my old patrol days."

Those two alarms turned out to be false, but Shaner says that by Jan. 2, the usual seasonal upsurge in crime will be felt.

"There will probably still be an increase in the number of robberies, no matter what we do. I don't know the exact effect of the Christmas crime detail, but we like to think that if it doesn't stop crime, it arrests its growth," Shaner said. The special detail ends the day after New Year's.

"I suppose the season's pretty well over by Jan. 2," Shaner said. "By then everybody's pretty well recovered -- activity really drops."