Observation and Variable

An observation is a single thing you measured — one row in a tidy table — and a variable is one property you recorded about it, living in a column. If you survey 100 people about age and income, each person is an observation (a row) and age and income are variables (columns).

Confusing the two is a frequent beginner error. The same data can be sliced as either “one row per person” or “one row per person-per-year,” and which you choose defines what counts as an observation; deciding this up front keeps your table consistent and your later group-by and join steps sane.