Prediction of price is not hundred percent correct. traders use technical analysis to predict based on historical trends. they expect the market to act in the same as previous.but that is not correct 100 %.similarly, with the experience and expertise, they can increase the probability of the success.
Not all people are able for it because it is fluctuating nature and all the time price of bitcoin use to increase or decrease. If today people will hold for long time then it will get high and at time of increasing demand price of bitcoin will increase automatically, at time when people start showing interest for bitcoin and that time price of bitcoin will go high and time for selling it will be but at time of low demand and no extra investing then price will get down so it is not very tough but only expert users can predict it easily.