ABSTRACT
This chapter discusses the role of P-values and the useful extra information that confidence intervals can supply. P-values tell whether there is statistical significance, and a 95% confidence interval is the interval which includes the true population mean value in 95% of cases. When looking at the difference between two mean values, however, neither the P-value nor the confidence interval tells whether the size of this difference is practically meaningful. Something that can be helpful with this is the effect size. Essentially, this is the size of the difference in mean values between two groups, relative to the standard deviation. One commonly used measure of effect size is Cohen’s d. A study evaluated two psychological therapies, with the aim of increasing mental well-being. The mental well-being of 60 patients treated with a course of a new therapy was compared with that of a control group of 60 further patients who had received a course of a standard therapy.
