The "chances are 50-50" there will be an East-West agreement next year on reducing the risk of war in Europe, according to James E. Goodby, former head of the U.S. delegation to the Stockholm conference working on such an agreement.

It could be the first post Geneva-summit agreement, Goodby said, noting that the joint statement by President Reagan and Soviet leader Mikhail Gorbachev described the Stockholm negotiations as the only U.S.-Soviet talks that were making progress. The two leaders called for "an early and successful completion of the work" and "reaffirmed the need for a document" that would include confidence-building measures as well as an expression of "the principle of non-use of force."

Called the Stockholm Conference on Confidence- and Security-Building Measures and Disarmament in Europe, it is being conducted among NATO and Warsaw Pact nations along with European neutral nations. The present session is scheduled to end next week and resume early next year.

Goodby told a group at the Wilson Center of the Smithsonian Institution yesterday that the Stockholm talks have not gotten the public attention they deserve because most people are concerned with the Geneva negotiations on reducing nuclear weapons and preventing an arms race in space.

Stockholm "is not an alternative to arms control," he said, "but is a complement to it." Expanding on that theme, he said the aim of the Stockholm talks is "risk reduction" as against "arms reductions," which are the goal in Geneva.

On their own, Goodby argued, the Stockholm talks are worthwhile because they are aimed at preventing an unintended conventional war in Europe, which both sides often cite as the action that would lead to a nuclear exchange. Thus, he said, the talks could be instrumental in preventing nuclear war.

Goodby, who is on leave from the State Department and teaching at Georgetown University, said conclusion of the "document" called for in the summit statement should be "possible by the fall of next year."

He said he expected it would involve information exchanges about NATO and Warsaw Pact military activities, including an annual forecast, a year in advance, of major military exercises; advance notification of military movements; permission for observers from both sides to view some of those exercises, and "verification in some form," a major goal of the neutral nations.