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Chapter 2 Structured Data

Rows, units, and time

Some data arrives already rectangular: rows, columns, a value in every cell. The work then is not to build the table but to understand what a row means — one person, one person-year, one month, one country. A surprising share of the mistakes in applied research can be traced back to a misunderstanding on exactly that point.

This chapter walks the three shapes a rectangle can take. A cross-section holds many units at one moment. A panel holds many units at many moments. A time series holds one unit across many moments. They look alike on screen and demand entirely different analysis.