A characteristic of a healthy bull market is that it makes higher-highs and higher-lows. This indicates a continual upward shift in expectations and the supply/demand lines. The amount that prices retreat following a higher-high can be measured using a technique referred to as "percent retracement." This measures the percentage that prices "retraced" from the high to the low.
For example, if a stock moves from a low of 50 to a high of 100 and then retraces to 75, the move from 100 to 75 (25 points) retraced 50% of the original move from 50 to 100.
Measuring the percent retracement can be helpful when determining the price levels at which prices will reverse and continue upward. During a vigorous bull market, prices often retrace up to 33% of the original move. It is not uncommon for prices to retrace up to 50%. Retracements of more than 66% almost always signify an end to the bull market.
Some investors feel that the similarities between 33%, 50%, and 66% and the Fibonacci numbers of 38.2%, 50%, and 61.8% are significant. These investors will use Fibonacci Levels to view retracement levels.
I labeled the following chart of Great Western at three points (labeled "A," "B," and "C").