Describing numerical data
Read numerical distributions, calculate center and spread, and understand both course percentile methods.
Study sections
Follow the order or choose a sectionNumerical frequency tables: values and intervals
A numerical variable records an amount, such as household size, height, or marks. Before finding an average, organize the numbers so yo…
02Histograms and stem-and-leaf plots
A graph helps you see where numerical observations cluster, how spread out they are, and whether there are unusual values. Two useful c…
03Mean: ordinary, frequency-weighted and grouped
The mean is the total shared equally across all observations. It is often called the average. A measure of center tells us a typical lo…
04Numerical median, mode and choosing a center
The median is the middle after sorting. The mode is the most frequent value. They answer different questions from the mean, and an unus…
05Spread: range, variance and standard deviation
Two groups can have the same mean but feel very different. Spread tells us whether values stay close together or vary widely.
06Percentiles: learn both course methods
A percentile describes a position in an ordered dataset. The 50th percentile is the median. A 90th percentile is a cutoff near the uppe…
07Quartiles, IQR, five-number summaries and boxplots
Quartiles divide ordered data into roughly four parts. Q1 is the 25th percentile, Q2 is the median, and Q3 is the 75th percentile. The…