ABSTRACT
This chapter is about the principles of significance testing. When data differ under two conditions the authors suspect they have detected a real effect. However, the data might differ simply because of random and/or sampling errors. Significance tests are used to help us decide between the null and the alternative hypotheses. The first of these says there is no effect in the population, while the other says there is and is usually what they would like to support with our evidence. A few years ago there was a news report about a private clinic that claimed to be able to assist couples in producing a baby of the sex they would prefer. Although the exercise presented does not involve an everyday decision, people do use the logic of significance testing many times over in their everyday lives, usually without realising it and often despite the fact that, as psychology students, they find the concept impenetrable.
