Is it possible to quantify the probability of sudden big movements for a high-volume stock?

Certainly no one knows in advance how much a stock is going to swing around. However, there are measures of how much it has swung around in the past, and there are people who will estimate the probability.

First of all, there's a measure of an individual stock's volatility, commonly referred to as "beta". A stock with a beta of 1 tends to rise and fall about as much as the market at large. A stock with a beta of 2, in the meantime, would rise 10% when the market is up 5%. These are, of course, historical averages. See Wikipedia:

Secondly, you can get an implied measure of volatility expectations by looking at options pricing. If a stock is particularly volatile, the chance of a big price move will be baked into the price of the stock options. (Note also that other things affect options pricing, such as the time value of money.) For an options-based measure of the volatility of the whole market, see the Volatility Index aka the "Fear Gauge", VIX.



Looking at individual stocks as a group (and there's an oxymoron for you), individual stocks are definitely much more likely to have big moves than the market. Besides Netflix, consider the BP oil spill, or the Tokyo Electric Power Company's Fukushima incident (yow!). I don't have any detailed statistics on quantitatively how much, mind you, but in application, a standard piece of advice says not to put more than 5% of your portfolio in a single company's stock. Diversification protects you. (Alternatively, if you're trying to play Mr. Sophisticated Stock-Picker instead of just buying an index fund, you can also buy insurance through stock options: hedging your bets. Naturally, this will eat up part of your returns if your pick was a good one).

The P/E is currently 20. In hindsight, it's easy to see that when it was 50, not long ago, it was very overpriced. They were not adding customers or increasing revenue as they should have to sustain that P/E level.

Probability? I suppose this can happen with any company that has both a high P/E and non-diversified business. Why did you think this company was large and stable? Their marketing blunders simply pricked the bubble level pricing these guys had.

(Disclaimer - I am actually a happy customer of Netflix. For $8/mo, I get 6-8 DVDs and neither spend gas nor time to get them. Others who grew used to free streaming feel otherwise)

This is a classic correlation does not imply causation situation. There are (at least) three issues at play in this question:

  • The resolution of price changes (A 14 cent stock only has 13 marks between its price and zero).
  • The volume of stock being traded. (See this question)
  • The actual viability of the business.

If you are swing- or day-trading then the first and second issues can definitely affect your trading. A higher-price, higher-volume stock will have smaller (percentage) volatility fluctuations within a very small period of time.

However, in general, and especially when holding any position for any period of time during which unknowns can become known (such as Netflix's customer-loss announcement) it is a mistake to feel "safe" based on price alone.

When considering longer-term investments (even weeks or months), and if you were to compare penny stocks with blue chip stocks, you still might find more "stability" in the higher value stocks. This is a correlation alone — in other words, a stable, reliable stock probably has a (relatively) high price but a high price does not mean it's reliable.

As Joe said, the stock of any company that is exposed to significant risks can drop (or rise) by large amounts suddenly, and it is common for blue-chip stocks to move significantly in a period of months as changes in the market or the company itself manifest themselves.

The last thing to remember when you are looking at raw dollar amounts is to remember to look at shares outstanding. Netflix has a price of $79 to Ford's $12; yet Ford has a larger market cap because there are nearly 4 billion shares compared to Netflix's 52m.