(The word tails refers to the tail ends of the statistical distribution such as the familiar bell-shaped normal curve that is used to test a hypothesis. The prediction that patients with attempted suicides will have a different rate of tranquilizer use - either higher or lower than control patients - is a two-tailed hypothesis. A two-tailed hypothesis states only that an association exists it does not specify the direction. The prediction that patients of attempted suicides will have a higher rate of use of tranquilizers than control patients is a one-tailed hypothesis. One- and two-tailed alternative hypothesesĪ one-tailed (or one-sided) hypothesis specifies the direction of the association between the predictor and outcome variables. Popper states, “… the belief that we can start with pure observation alone, without anything in the nature of a theory, is absurd: As may be illustrated by the story of the man who dedicated his life to natural science, wrote down everything he could observe, and bequeathed his ‘priceless’ collection of observations to the Royal Society to be used as inductive (empirical) evidence. Popper makes the very important point that empirical scientists (those who stress on observations only as the starting point of research) put the cart in front of the horse when they claim that science proceeds from observation to theory, since there is no such thing as a pure observation which does not depend on theory. The popularity of Popper’s philosophy is due partly to the fact that it has been well explained in simple terms by, among others, the Nobel Prize winner Peter Medawar (Medawar, 1969). Many scientists, even those who do not usually read books on philosophy, are acquainted with the basic principles of his views on science. Karl Popper is probably the most influential philosopher of science in the 20 thcentury (Wulff et al., 1986).
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