Significance testing – was it a real effect?

When data differ under two conditions we suspect we 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 we would like to support with our evidence. This chapter is about the principles of significance testing.