When and if to sell depends on the companies outlook for the future, if you can trust them. Maybe it's a one time event but if they project several quarters of trouble it might be best to get out quickly. We don't claim to be an expert at timing the market, in fact, no one is able to do it consistently. The best friend of a long-term investor is time and patience. Making some mistakes is normal and acceptable.
Nobody gets it right all the time.
That's one purpose of diversification. Almost everyone knows the best way to make money in the market it to buy quality companies and hold the stocks for many years. We highly recommend being a long-term investor in companies, not playing the stock market just to make a quick buck . Most good companies that fall out of favor or have some temporary troubles often come back within a year if market and economy conditions are favorable. At the time of this writing, August 2000, the overall economy is still in great shape and appears it will stay that way for a while.