Introduction In Chapter 4, we introduced measures of central tendency and dispersion that are used to describe data. In particular, we looked at the mean and standard deviation. In Chapter 6, we then introK duced the concept of a z-score and showed that this was really nothing more than the number of standard deviations some value differed from a mean. We then proceeded to show how z-scores were used to calculate the probability of random events under the normal curve. Then, in Chapter 8, we took our first look at inferential statistics and found that this allows us to make predictions about population using a sample from that population. In this chapter, we offer a different way of presenting the information provided by the sample mean and the error that might occur when we use it to estimate the mean of the entire population from which it was supposed to come. This will lead us into something called the confidence interval. After we have mastered the confidence interval, we conclude by discussing how sample sizes are determined in research.