What should one check when a share price starts falling considerably

What data, sources of information,websites, parameters should we track when a stock/share price starts falling considerably, to decide whether to hold the stock or sell it?

 

THINGS TO LOOK AT BEFORE AVERAGING

So, if a stock you have bought falls lower than the purchase price, do a fundamental check on the company - balance-sheet strength, valuation, management quality and the core proposition. Study the fundamentals of the sector to see if there is any chance of a re-rating.