In ordinary usage a jackknife is a large pocket knife with a multitude of pull-out tools, such as screwdrivers and scissors. The value of this device is that it is easy to carry around and it can be used for a large variety of tasks, although it is seldom ideal for any one. By analogy, the description jackknife was proposed by Tukey (1958) for use in statistics to describe a general approach for testing hypotheses and calculating confidence intervals in situations where no better methods are easily used. This method is not really computer-intensive by present-day standards. However, it is appropriate to give it some consideration because it is sometimes used as an alternative to the other methods that are discussed in this book.