A data set can be drawn three ways on a number line: a dot plot (every value), a histogram (counts in bins) and a box plot (the five-number summary). Its center is the mean or the median, and its spread is the standard deviation or the interquartile range. The crew finds the standard deviation by dividing the squared distances by n. A symmetric set is well described by the mean and standard deviation; a skewed set is better described by the median and IQR. Comparing two groups means comparing both center and spread.