Descriptive statistics: tell us what statistical tests we can choose, central tendency (usually around demographic data) Mean is higher = positive skew, mean is lower = negative skew. If mean is in front of median = positive. Not for widely dist. data but good against outliers. Dist. can have more than one mode (most freq. diagnosis, most freq. age of pts. on unit: dispersion help us know the variability of our data. Know how your variable was measured to know what the range means. Large = more variability in our data, small = little variability in our data. Means "sum of", x is a value in the data set, is the mean of the data set, and n is the number of data points in the population. Null hypothesis = the hypothesis that there is no significant difference between specified populations, any observed difference being due to sampling or experimental error.