Some investors believe that they can predict what the stock market will do on a day-to-day basis. As a result, they try to time the market - which means they enter the market when they feel the market will rise and exit the market when they feel the market will fall. Unfortunately, most people aren't basing their timing decisions on solid analysis - rather, they go with gut instincts. In fact, some would argue that there are no indicators to suggest which direction stock prices will go in the short runs and that markets are entirely unpredictable. Let's look at two scenarios - successful and unsuccessful market timing.Let's suppose an investor, through market timing activity, manages to cash out on the worst 10 days of the stock market - those days that experienced the greatest falls. The investor manages to make approximately 10 percentage points more than if he had merely remained invested. Another investor tries to time the market but fails in all his attempts and misses the 10 best days in the stock market. He loses almost 9 percentage points from trying to time the market.Now, we've seen both the upside and the downside and they're roughly similar, in terms of percentage points. In fact, if an investor kept trying to time the market, on a long enough timeline, his successes would roughly equal his failures - and the net effect would be roughly similar to a buy-and-hold strategy. Stock price movements in the short run are random and the chances of a move upwards are roughly similar to the chances of a move downward on any given day.