New York ? Edgy investors sent stocks skidding Thursday as a string of bombings in Madrid overshadowed mostly good economic news and bullish forecasts from several companies.

The Dow Jones industrial average nosedived nearly 170 points, giving the blue chips their biggest four-day drop in nearly 18 months.

The Dow sank 168.51, or 1.6 percent, to 10,128.38, leaving it down 3.1 percent for the year. So far this week, the Dow has lost 467.17, giving the index its largest four-day point tumble since early October 2002, when it slid 515.95.

It also was the Dow’s biggest two-day point drop since the end of January 2003, when it fell 379.91, or 4.54 percent.

The broader market also fell sharply. All three indexes were at their lowest points since December.

“This has been quite a day,” said Michael J. Cuggino, president and manager of the Permanent Portfolio Fund. “This is why you need to be exposed to a lot of different asset classes that can protect you from the different scenarios that come up in a volatile global political climate.”

Beyond the terrorism concerns, the market’s own dynamics were causing worry among analysts. Usually solid performers were lagging the tech-heavy Nasdaq, which has been the most temperamental gauge in recent months.

National Semiconductor closed up 88 cents at $39.08 after posting quarterly profits that beat expectations on strong sales in the personal computing and wireless markets.

The news offered some short-term support for tech shares; computer maker Dell Inc. ended the day up 35 cents at $31.99, but chip-maker Texas Instruments lost 48 cents to $29.20 after an earlier rise.

Aluminum producer Alcoa Inc. added 7 cents to $34.27 — the only stock in the Dow 30 to end the day in positive territory.