Stage 2 · Lesson 1 of 8
Describing data: centre and spread
Before any test or model, describe each column. The is the sum divided by the count; the median is the middle value when sorted; the standard deviation is the typical distance from the mean; the interquartile range, or IQR, is the spread of the middle half of the data.
For skewed data such as incomes, order values or time on site, a few huge values drag the mean and standard deviation, so the median and IQR describe the typical case better. Report counts alongside every summary.
Common trapquoting an average salary or average order value without saying the data is skewed, which describes almost nobody and leads to bad decisions such as pricing for a customer who does not exist.
A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock
Resources for this stage
Start with· follow one of these
- VideoFreeStatQuest with Josh Starmer
StatQuest · Beginner
Free: short, clear videos on each concept, from the normal distribution to p-values and power
- CourseFreeKhan Academy Statistics and Probability
Khan Academy · Beginner
Free: the probability, sampling distributions, confidence intervals and significance tests units, with practice
Practise
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Interactive visuals for probability, distributions, inference and regression: play with each one