The unemployment rate ticked down to 8.1 percent in the United States in April, which is the lowest it's been since President Obama took office in January 2009. One might expect cheering on Wall Street and predictions of an easy reelection for Mr. Obama. Instead, the Dow Jones stock index fell nearly 150 points in morning trading Friday and Obama looks vulnerable this fall.

Why? Because the unemployment rate is dropping for all the wrong reasons.

While the economy added 115,000 net jobs in April, some 350,000 Americans gave up looking for work. That has the effect of reducing the unemployment rate because, by the federal government's calculation, those people no longer count as part of the labor force. As a result, the share of Americans who are part of the labor force – either working or actively looking for work – has reached a 30-year low.

"The 8 percent that we see for unemployment is not a full and fair picture of unemployment," says Scot Melland, president and CEO of Dice Holdings, which runs specialized career websites in the technology, financial services, and health-care industries. If the participation rate were at normal levels, the unemployment would be above 11 percent, by one estimate.

So what's the real picture? It looks as though the economy is slowing. In January and February, the US added 275,000 and 259,000 jobs, respectively; in March and April, it added only 154,000 and 115,000. But economists suggest the outlook may not be so bleak.

For one thing, the official figures keep getting revised. When the US Labor Department announced Friday that the economy added 115,000 net new jobs in April, it also revised the February and March job numbers upward by a combined 53,000. For another thing, the abnormally mild winter weather throughout much of the country may have encouraged more-than-expected hiring in January and February, causing less-than-expected hiring in March and April.

"March and April’s results clearly raise questions, and given the numerous negatives confronting the economy, they cannot be simply dismissed as a hiccup in the data," writes Joshua Shapiro, an economist at MFR Inc., a New York research firm. "Rather, they play into the notion that an unusually mild winter combined with more aggressive seasonal adjustment than in past years probably boosted economic data in general during the winter months."

The real growth in jobs probably lies somewhere in-between. "We look for a better but still subdued pace of [monthly] job creation in the 150,000-200,000 region over the rest of the year," writes Nigel Gault, an economist with IHS Global Insight, an economic research firm in Lexington, Mass.

That's solid but uninspiring growth. Moreover, it's occurring at different speeds in different parts of the economy.

In the first four months, the private sector added 827,000 jobs; government lost 24,000. In the private sector itself, industries are moving at different speeds. Professional and business services have added 267,000 jobs; construction, only 12,000.

"This is not a balanced labor recovery: There are winning sectors and not-winning sectors," says Mr. Melland. "The skilled sectors are doing better than the not-skilled sectors." His own job search websites show no slowdown in advertising for skilled technical positions.

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Even the 30-year low in labor participation may not be as bad as it looks.

"Normally we would characterise the contraction in the labour force as a big negative, presumably a result of job seekers becoming so disillusioned they give up," writes Paul Ashworth, an economist for Toronto-based Capital Economics, in a research note. "But it is worth remembering that this is a volatile series and the labour force increased by almost 1,000,000 in the first two months of the year, so some drop back was to be expected."