Doc, in the absence of news or any other extraneous factor that might cause a stock to keep rising or falling, how common is it for a stock to "fill a gap"? And why does a gap have to be filled in the first place, what makes it a gap and why so important? Thanks.
Well, the first question is difficult, because some gaps will never be filled, others will take months, others years, etc, etc. The gap's theory says that about 90% - 95% of the gaps are filled, but, depends of the gaps, for example this gap
http://stockcharts.com/h-sc/ui?s=BPAX&p=D&b=5&g=0&id=p79657395053 may take months to be filled, maybe years, because the distance is too big, but, for example, this gap took almost five moths to be filled
http://stockcharts.com/h-sc/ui?s=BPAX&p=D&b=5&g=0&id=p79657395053 , curiously with another gap, bigger than the previous one
, but, this one
http://stockcharts.com/h-sc/ui?s=CRYP&p=D&b=5&g=0&id=p30255388249 , may take only a few days, because the distance is small, from 2.05 to 1.75 is about a 10% move to the down side, so this stock may go higher, but, first it will fill the gap, almost sure.
What makes a gap is in 99% of the cases, extremely buy or sell pressure caused by news, very good or very bad, so the pressure is so high that causes a panic buy or sell before the market open, and, when it opens, causes a gap in the charts, the BPAX example, it was bad news, the pharma companies are the ones with more gaps, due to approval or disapproval of new drugs and medications by the FDA.
A gap is very important, because the market follows the theory of the gaps, that they will or need to filled, sooner or later, so the gap acts like a magnet, something that attracts the price of the stock to that level.
Hope I was clear enough, if not, just let me know.