Summarize a data set using measures of center (mean, median, mode) and spread (range, IQR, standard deviation).
A single number can never capture a whole data set, but a few well-chosen numbers get remarkably close. Center is described by the mean (add everything, divide by how many), the median (the middle value once sorted), and the mode (the most frequent value). Spread is described by the range (max minus min), and more precisely by the interquartile range (IQR) - the width of the middle 50% of the data, found from a five-number summary (minimum, Q1, median, Q3, maximum) that a box plot displays visually.
Standard deviation measures spread differently: it is roughly the TYPICAL distance any data point sits from the mean. A small standard deviation means the data huddles close to the mean; a large one means it is spread wide. When the mean and median are close together, a data set is roughly symmetric; when they pull apart, the data is skewed toward whichever side has the more extreme values dragging the mean along with it.
Mastery looks like: They compute mean, median, mode, range, and IQR from a data set, and explain what standard deviation measures in plain language.
Common stumbles: Forgetting to sort the data before finding the median or the five-number summary; confusing range (a single number) with the full five-number summary.