[john_b]

+/-3% is a common benchmark for the market's definition of "big swing". There's nothing really formal about it, and it's a more accurate measure when applied to the market as a whole rather than individual stocks. For a company like AAPL which is famous for not providing a lot of detailed information to investors, it might not be a big swing. For a different company, say P&G, it might be a huge swing.

[bradly]

Actually, there is a formal measurement of a stocks volitiliy called it's beta.

http://en.wikipedia.org/wiki/Beta_(finance)

[john_b]

Beta is one of many measures of volatility. Since it's a normalized correlation between the stock's rate of return and that of the market, it only really has meaning in the context of a stock's performance over a long time or in relation to the market as a whole. A stock's performance post-earnings is about as market-independent as you can get, however.

[foobarqux]

The formal measurement of a stock's volatility is its volatility.