Normal curve: can be used to construct precise descriptive statements about empirical distributions. It is also central to the theory that underlies inferential statistics. Is a theoretical model, a kind of frequency polygon that is unimodal (i. e. it has a single mode or peak), perfectly smooth, and symmetrical (unskewed) so that its mean, median, and mode are all exactly the same value. Z scores always have the same values for their mean (0) and standard deviation (1) Finding z scores: subtract the value of the mean (x-bar) from the value of the score (xi, divide the quantity found in step 1 b the value of the standard deviation (s). The result is the z-score equivalent for this raw score.