A systematic error; that is, deviations from the true value that tend to be in one direction. Bias can occur in any type of forecasting method, but it is especially common in judgmental forecasting. Researchers have identified many biases in judgmental forecasting. Bias is sometimes a major source of error. For example, Tull (1967) and Tyebjee (1987) reported a strong optimistic bias for new product forecasting. Some procedures have been found to reduce biases (Fischhoff and MacGregor 1982). Perhaps the most important way to control for biases is to use structured judgment.