You can trade price alone, but price alone may mislead you. Consider:
day 1, up 0.50, vol 20,000
day 2, down 0.75, vol 200,000
day 3, up 0.50, vol 15,000
day 4, up 0.50, vol 20,000
By price alone the stock is up 3 days out of 4.
But when volume is considered along with price, the...