The stock market is getting whacked. The question now for many investors is, what to do about it.

The first thing to do is to understand what's going on -- in part because simply saying the stock market is getting whacked doesn't tell the whole picture.

So let's sketch that in and then decide whether to sell, sell a little, do nothing or buy.

First, the Dow Jones industrials fell 143 points on Friday to 16,027. The Standard & Poor's 500 Index dropped 17 points to 1,816 --and the Nasdaq Composite Index slid 54 points to just under 4,000.

In addition, the Nasdaq-100 Index tumbled 41 points to 3,447. The Russell 2000 Index tumbled 15 points to 1,113, and the Dow Jones Transportation Average gave up 69 points to 7,362.

Related: Biotechs Struggling To Find A Bottom

For the week, the Dow lost 2.4 percent, with the S&P 500 off 2.7 percent and the Nasdaq down 3.1 percent. The Nasdaq-100 tumbled 2.6 percent. The Russell was down 3.1 percent and the Dow transports 2.8 percent.

A Correction?

The real damage is visible if you look at declines -- since these indexes hit either all-time highs or multi-year highs in the last few weeks. Dow and S&P 500 are down 3.6 percent and 4.3 percent. The Dow transports are down five percent. The Russell 2000 is off 8.3 percent. The Nasdaq has lost 8.5 percent, and the Nasdaq-100 is down a whopping 12.1 percent.

The index losses don't mean the overall market is in a correction yet; a correction being popularly defined as a decline of 10 percent or more from a recent high.

The Nasdaq and Russell 2000 indexes very nearly are, however, and the Nasdaq-100 definitely is. Twitter (NYSE: NFLX) is down 28.7 percent since hitting an all-time high of $458 on March 6. (NADSAQ: AMZN) has dropped 23.6 percent since peaking at $408.06 on Jan. 22.

Watching The Momentum Stocks

Biotechnology stocks, meanwhile, are getting slaughtered. Gilead Sciences (NASDAQ: HALO), which has been working on a drug to treat pancreatic cancer, is off 59 percent since peaking on Jan. 24.

The damage is being suffered most by momentum stocks: the stocks that screamed for attention as they posted huge gains in 2013 and into this year. Computerized trading tools have decided the stocks were overbought, which they were, and will drive the prices lower until they get too low.

But keep this in mind: Thirteen of the 30 Dow stocks are still ahead on the year, led by Merck (NYSE: CAT), both up about 11.7 percent. Goldman Sachs (NYSE: GS), Visa (NYSE: V) and aerospace giant Boeing (NYSE: BA) are the laggards -- down 13.8 percent, 11.7 percent and 10.6 percent, respectively. More than 230 S&P 500 stocks are still ahead on the year.

Time To Pay Attention

So what should you do? If you've been playing the hot stocks and made a bunch of money, it's probably time to sell at least some of your stake. If you haven't thought about what will trigger a sell, now's the time. A 10 percent decline from, say, Friday's close is probably appropriate. That way you've limited your downside. If the stock doesn’t hit your trigger but recovers, then you're in the clear.

If you've just bought in, decide what price is appropriate to limit your losses.

But if you're a mutual fund or exchange-traded fund investor, selling in a panic is probably the wrong thing do. The blow-off in stocks has been nasty, but it is possible the selling has been extreme. Momentum measures suggests a bottom is starting to form. It may take a while to emerge, however. And even if a bottom is formed, that doesn't mean those stocks are going to shoot higher.

The market is selling off after a fabulous run-up since March 2009. There's been too much froth generated by a hot market for initial public offerings. But a successful IPO does not mean a successful stock. Meanwhile, there are worries about China, Japan and Russia and Ukraine. Bond yields are falling because of an undefined unease about the domestic and global economies.

Spring is often an uncomfortable time for the stock market. Therefore, it will pay to pay attention.

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