JSL,
Probably the most important thing to learn as a trader or investor is the reason does not matter. THC is a good example. Two weeks ago it was a $50 per share, Friday $14.
PE can be a reason, but a cyclical stock like GM would be the most dangerous when the PE was low. Reason Being, earnings are at a peak. However, a biotech company may not have earnings for 5 years, but the promise of a miracle drug can move the stock.
Volume matters, a thinly traded stock would move quite a bit if it traded 10K shares a day and someone wanted to sell 100K shares in one day.
News matters a lot, but it depends on what people were expecting. But this can be deceiving. People could be expecting great earnings from XYZ, so they buy the stock ahead of the earnings, and as soon as the great earnings hit they sell and the stock goes down.
Finally, analyst opinions matter if the street sees them as valid. For instance, if Dan Niles upgrades INTC, that in my opinion will move it a lot more than if Abbey Joseph Cohen upgrades it. conversely in 99, I bet Cohen's upgrade carried more weight.
There is not a concise answer to the question.
