Nearly half of the children whose cases were part of national study on kidnaping were abducted by trickery or enticement and not by force, according to a report released yesterday by the National Center for Missing and Exploited Children.

Center officials who examined the method of abduction in 820 cases said the most popular "lures" were the offer of a ride, the promise of a gift or an invitation to a party. Many abductors also impersonated a police officer or a friend of the child's parents.

The findings were part of a national study of 1,299 kidnaping cases from 1979 through 1985. The study included 211 kidnapings -- the total number reported in 1984 -- from law enforcement records in Houston and Jacksonville. None of the cases involved children taken by a family member.

The study exploded a number of myths about child kidnaping, including the belief that it is usually a crime against the very young, according to Jay Howell, the center's executive director. Almost half the children were more than 10 years old, and the largest category were 11 to 14 years old, he said.

Howell said at a news conference that he was most startled by finding that more than 83 percent of the children kidnaped in Jacksonville and 96 percent in Houston were sexually assaulted. "I did not suspect in my darkest imaginings that so many were victimized by a second crime," he said.

Howell said that by providing insight into the crime the study offered hope for prevention through new strategies, and he warned against what he called the "worn-out, outdated" admonition that children should not take candy from strangers.

Children think of "strangers" as "bizarre" or "unusual," men wearing raincoats and lurking in dark corners, Howell said. To a child, someone who acts friendly or merely introduces himself is no longer a stranger, according to Howell.

He said children need to be told that they do not have to go anywhere with someone because he says he is a policeman or a friend of the family, nor do they need to give directions or other assistance to an adult who asks, which is another common lure.

"Talk to children . . . and give them straightforward, nonalarming facts," said Howell. "Kids ask kids for help. Adults ask other adults."

The study also found:

*Children were most vulnerable when simply walking down the street, with more than 43 percent abducted at that time.

*About twice as many girls as boys were kidnaped.

*The national figures, which were drawn from cases that came to the center's attention through various means, showed that 22 percent were slain, a figure officials said might be skewed because homicides are more likely brought to the center's attention. Two of the Jacksonville children were murdered, as were four from Houston.

Several speakers at the news conference spoke of how children, even those safely returned home, are traumatized, and a mother, introduced only as "Judy," brought that point home. She said her child was chased down the street last year by a man who dragged her into a van and tried to stuff her into a tool box. The 10-year-old girl swung at him with a tool and escaped, the mother said.

However, she said, her child has sometimes come home "hysterical" believing she has seen the man again, and is still afraid to sleep alone. The family moved to another neighborhood and transferred the girl to a new school.