NEW YORK -- The purchasing managers' composite index rose to 60.7 percent in September, indicating economic growth in the month, said the National Association of Purchasing Management's Business Survey Committee.

The index, a barometer of economic health, is based on interviews with 250 U.S. industrial purchasing managers on new orders, the rate of production, vendor deliveries, inventories, prices and employment.

Readings above 50 percent indicate that the economy is generally expanding; readings below 50 percent signal an overall decline.

"The economy ended the third quarter with what must be characterized as robust growth," said Robert J. Bretz, chairman of the association's Purchasing Management's Business Survey Committee, as well as director of Materials Management at Pitney Bowes Inc.

"All indicators were positive," Bretz said. "Furthermore, the healthy increase in new orders is a signal that the fourth quarter will begin on a high note."

The indicator has exceeded 50 percent for the past 14 months, the association said. In August, it registered 59.9 percent.

"The purchasing index has averaged 56.6 percent for the first nine months of 1987," Bretz said. "Based on past experience, if this average were to continue for the remainder of 1987, it would be consistent with real GNP growth of 3.8 percent."

In September, the rate of production grew at the highest rate in more than three years. The survey found 43 percent of the managers reported higher production, while 9 percent said it slowed. In August, 33 percent of respondents had an improved rate of production, while 8 percent reported lower production.

During the month, the speed of vendor deliveries slowed, another sign of growth in production. Some 28 percent of respondents said they had slower deliveries, while 4 percent said they had faster deliveries in September. In August, 20 percent had slower deliveries and 2 percent reported quicker deliveries.

Inventories were flat after growing slightly in August, indicating that the growth in the production growth rate was not based on an inventory buildup. In both August and September, 19 percent of managers reported higher inventories.

New orders expanded for the ninth month in a row as 43 percent of respondents reported higher new orders and only 12 percent said they experienced a decline in new orders. This compares with 36 percent who had more new orders and 13 percent who had fewer new orders in August.

The survey also found that employment rose for the sixth time in the past seven months, with 19 percent reporting higher employment and 13 percent reporting lower employment. This compares with 16 percent who had higher employment and 13 percent who had lower employment in August.

Also in September, purchasing managers reported more price increases than decreases, for the 13th month in a row. The survey said 54 percent reported higher September prices while 2 percent said they had lower prices in the month. In August, 52 percent had higher prices, while 3 percent had lower prices.

In other results, the survey concluded that lead times for production materials declined in September. This result followed two consecutive months of growth. The survey said 58 percent had lead times of 30 days or less, up from 54 percent in August and 56 percent in July.

It also found that lead times for capital expenditures were longer in September, with 52 percent reporting lead times of six months or more. This is up from 50 percent in August. Lead times for maintenance, repair and operating supplies lengthened also, with the number of members reporting lead times of 30 days or less, falling to 82 percent from 86 percent in Augus