Types of data

What kind of number you are holding.

Categorical and numeric

Categorical values are labels from a fixed set, such as country or status. Numeric values sit on a scale and support arithmetic. The type decides which summary and which chart make sense.

Ordinal categories have an order without even spacing, such as small, medium and large.
An identifier made of digits, such as a postcode, is categorical however numeric it looks.

Structure

Tidy data puts one observation per row and one variable per column. Most tooling assumes it, and getting there is often the real first task.