A day trader often closes all trades before the end of the trading day, so not to hold open positions overnight. A day traders' effectiveness may be limited by the bid-ask spread, trading commissions, as well as expenses for real-time news feeds and analytics software. Successful day trading requires extensive knowledge and experience. Day traders employ a variety of methods to make trading decisions. Some traders employ computer trading models that use technical analysis to calculate favorable probabilities, while some trade on their instinct.