A statistical index designed to fortell changes in the performance of the economy declined slightly in March, the government reorted yesterday.

The Commerce Department's index of leading indicators slid a scant 0.1 per cent in March, the agency announced - down from an 0.5 per cent rise in February and a drop of 1.3 per cent in January.

At the same time, the department reported that construction spending rose 3.5 per cent in March, continuing the economy's rebound from the lull this past winter following the coal strike and cold weather.

Although the index of leading indicators is designed to be an aid in forecasting, it frequently is distorted by special factors. Preliminary figures for February were revised up sharply after first showing no change.

Yesterday's figures were thrown off in part by the coal strike, and partly by unusually large fluctuations in key financial indicators, which often have little to do directly with the performance of the economy.

Two of the key components that registered declines were stock prices and money supply growth. Stock prices already have staged a vigorous rebound, and the growth in the money supply, while slowing, has varied week by week.

William A. Cox, the department's deputy chief economist, said he did "not think the figures should be interpreted" as significant. Analysts generally distrust any movement in the index unless it continues for months.

The rise in construction expenditures in March followed a hefty 4 per cent jump in February, bringing overall spending in the industry to a seasonally adjusted annual rate of $184.5 billion.

Although private construction rose a sizeable 3.7 percent, building in the public sector jumped 2.9 percent Overall, construction outlays were up 12.6 per cent from their levels of a year before.

In the figures on leading indicators, six of the 10 components available in the preliminary tabulation declined, while three rose and one remained unchanged.

Along with stock prices and money supply growth in 1972 dollars, those declining included total liquid assets, change in sensitive prices, contracts and orders for plant and equipment in 1972 dollars and new orders in 1972 dollars.

The three indicators whose performance improved were the length of the average work week, vendor performance and building permits. The component that showed no change was the layoff rate.

The department said the indicator that contributed most to the March decline was the change in total liquid assets. The March drop left the index at 134.1 per cent of its 1967 average.

Separately, the department reported that the nation's foreign trade balance, calculated on the so-called "balance of payments basis," which excludes military trade, posted an $11.2 billion last quarter - an all-time record.

The figure compared with a $9.9 billion deficit figure for the fourth quarter of 1977. The measure is important primarily to analysts. By contrast, the merchandise trade deficit, the more widely-used figure, was $2.78 billion for March alone.