Steven Marston, an economist with the Federal Trade Commission, had just returned from a meeting of economists in New York City. His wife, Martha F. Riche, was looking forward to seeing her new statistics newsletter published this week.

Then, Sunday night, Marston, 36, was shot dead, apparently by one of three robbers who invaded their home in Alexandria's quiet Parkfairfax condominium community not far from Shirley Highway. Riche, shot in the chest and white with shock, ran screaming to a neighbor's home.

Riche was reported in fair condition yesterday at Alexandria Hospital.

Neighbors said the couple, who moved to the two-story town house they rented at 3603 Greenway Pl. from Ithaca, N. Y., last summer, was preparing to move to a new home in another Washington suburb. At about 9 p.m. Sunday, police say, three men entered, apparently following Marston in through an open door.

Few details of the incident were available yesterday beyond the police statement that Marston and his wife were shot during a robbery attempt. Marston, a former economics professor at Cornell University, was pronounced dead at the scene.

Riche's desperate run to a neighbor's house was recalled by Bruce Dell, who lives at 3614 Greenway Pl: "I heard her screaming. It was a weird sound, like a little kid at first. . . All she said was, 'I've been shot.' " Dell said he supplied a blanket as another neighbor held the bleeding woman until an ambulance arrived.

"There has been a penny-ante thief in the neighborhood--kids breaking in to steal a TV--but never a murder," Robert Haley said of the Parkfairfax community that was home to Richard M. Nixon and Gerald R. Ford years before their presidencies.

"There's never been anyone killed," chimed in another neighbor as he stared across the street at the town house where the shooting occurred.

Following the shootings, Haley said, much of the neighborhood stood for hours outside their homes in disbelief. "People were upset. They weren't scared. People didn't know what to do," he said.

"No arrests have been made and the investigation is continuing," said Alexandria police spokesman Kathy Salvas. "We have nothing new."

Riche, 44, is associate editor of American Demographic Magazine in Ithaca and was recently named editor of a new publication called The Numbers News, according to publisher Peter Francese.

"Her home was our Washington office where she had an Apple computer with transmission capabilities" to the magazine's Ithaca office, Francese said, adding that the first edition of the national monthly newsletter is due out this morning.

According to Cornell University, Marston was an assistant professor of economics there from 1976 until last summer when he left for a job in the FTC's bureau of economics.

A statement issued by Mukul Majumdar, chairman of the school's economics department, described Marston as talented and a "popular colleague, very pleasent and charming to work with."

His death, the statement said, "brings into sharp focus some of the problems that our society faces.