Before any model: what the numbers look like, and how to describe and show them honestly.
A histogram you can re-bin, mean against median, spread, quartiles and a box plot, and what one typo does.
mean · median · IQR
A correlation coefficient you can watch change, four data sets with the same numbers, and a hidden cause.
r · Anscombe
The right chart for the question, and the same data made to mislead.
bar · line · axes
A messy table made usable step by step: duplicates, formats, holes and typos.
duplicates · missing