Chapter 2 of 7 · 4 practices · 3 min · 4-question quiz
Read the data correctly
Summary numbers hide most problems. These practices get you looking at what a row means, how values are distributed and how your tools read types.
Start: State what one row represents before you query a table4 practices in this chapter
- 1State what one row represents before you query a tableThe grain of a table is what one row means: one order, one order line, one user per day. Most join and aggregation mistakes come from guessing it wrong.
- 2Look at distributions and real rows, not only averagesGoogle's guide warns that means and medians hide things like multimodal behaviour, and recommends histograms, cumulative distribution plots and Q-Q plots. It also says to look at examples: sample…
- 3Check data types when you load data, especially strings and datesType surprises change results silently: a numeric column read as text sorts "100" before "20", and dates parsed in the wrong unit shift every calculation.
- 4Decide on one time zone and convert timestamps explicitly"Orders on 1 October" means different rows in UTC and in local time. If one table stores UTC and another stores local time, a daily join can shift a day's worth of rows.
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