When Otto Eckstein, the former Johnson administration economist who now heads Data Resources Inc., found in late December that the economy was shifting more rapidly than he had predicted earlier, he engineered what may turn out to be theultimate in forcasting flexibility:

Sure, Eckstein conceded, the unexpectedly robust statistics right then showed little in the way of hard evidence that pointed to a recession.But he forecast a downturn anyway - on the grounds that something almost certainly would happen to make things go wrong.

Although there was more sanity than may be apparent in Eckstein's wry forecasting maneuver, the incident serves to illustrate a point: Economic forecasts - and interpretation of monthly economic statistics - often seem to depend more on who is doing the forecasting than on what the figures are.

Indeed, for some economists, the current recession seems only the fulfillment of the law of averages: They've been predicting the downturn for so long that it had to come true eventually. For years, the joke has gone: Lay all the economists end to end around the world and you still won't reach a conclusion.

Why does the "dismal science" often seem so unscientific? Why do economic forecasts often vary so widely? Why do they often seem so far off base? And, if economics actually is that tenuous, why should policymakers - and the public - pay any attention?

In short, what's or ordinary citizen to do?

Well, first, in fairness to the economists, the fact is that they haven't been all that far apart in their forecasting over the past few years. Last January's forecasts for 1979, for example, were all ranged from mild slowdown to recession. The semantics were a matter od degree.

If anything, critics of the art say the forecasters probably have been too close together for the good of the economics business and the public. There seems to be a pack instinct working. Where a few bigname forecasters lead, the rest - too often - seem content to follow.

Second, when economists do vary more widely in their forecasts or their interpretations of specific economic statistics, it more often reflects a difference in their individual philosophical bents rather than a significant dispute over anything very remotely "scientific."

Economists, like people, have divided themselves into various doctrinal categories. To oversimplify, monetarists believe the economy is most heavily influenced by the rate of growth in the nation's money supply, while Keynesians allege the controlling factor is fiscal policy.

There also are Republican and Democratic economists; those who have faith in wage-price guidelines and those who don't; some who pay more attention to "micro" (or industry-by industry) trends; some who weigh tight-money policies more heavily than others; and a few who consistently are more gloomy.

There also are some who base their forecasts almost entirely on computer models - which often misinterpret new developments that haven't been part of economic history. And there are a few ultimate pragmatists, who don't give a hoot about any of this dogma.

Anyway, to oversimplify again, an economists's view of the world often is colored partly by his theological bent. As a result, a monetarist economist can look at a set of figures and predict a deeper and more prolonged recession than a nonmonetarist - because the nation's money supply is on the decline.

Another economist might be more optimistic on inflation because he has high hopes that the guidelines will prove effective. And still a third may favor more economic stimulus because his political leanings make him more reluctant to accept a modest rise in unemployment than his more conservative colleagues.

Admittedly, economists have their conspicuous shortcomings. Most of the fraternity, for example, didn't foresee the economic overheating that occurred last fall and winter. And they completely blew predictions on this year's inflation rate: It turned out to be double what most of them foresaw.

There's a good deal of truth to be charge that economists too often have underestimated both inflation and the economy's resiliency - often contributing to costly policy errors. President Carter more than once has publicly voiced his exasperation over the economic advice he's gotten.

And, occasionally, economists are just plain wrong on month-to-month developments. Only last spring, for example, the administration's top economic policymakers called for higher interest rates to dampen alleged overheating - when the economy actually was starting to turn down.

Sometimes forecasts are thrown off track by political developments or natural disasters - the worldwide grain shortage of the early 1970s, for example, or the political turmoil in Iran that paved the way for this year's extraordinary increases in crude oil prices. No forecaster can cope with these.

But most often, it's the politicians - and not the economists - who make the mistakes. There were enough warnings about potentially high inflation in the mid-1960s, but President Johnson refused to propose raising taxes or cutting spending, setting off the wage-price surge that continues today.

Similarly, economists warned the Carter administration about the potential for a new wage-price spurt in 1977, but the White House went blithely on as if that development didn't exist. And most analysts foresaw the current recession well in advance (although some of them had been predicting it for two or three years).

Murray L. Weidenbaum, a Republican economist now at Washington University's Center for the Study of American Business, contends that, although forecasters have "made our fair share of goofs, we've usually gotten the basic direction of the economy correct" - which these days is no small feat.

Weidenbaum's advice - and that of many other economists - is to regard forecasts as just predictions, not as history chiseled in stone.

What seldom comes out in the forecasting business is that the prognostications are offred as probabilities - a 55 percent chance of recession, for example - like the weather service's recent adoption of "10 percent chance of rain."

Like the weathermen, the economists sometimes turn out to be all wet in spite of themselves. The trick may be to keep an umbrella - or an alternative economic policy - in the office just in case.