Stage 1 · Lesson 5 of 7
Grouping and summarising
Grouping splits rows by one or more columns, computes a summary for each group, and combines the results into a new table: people call it split, apply, combine. It is how you get orders per region, average score per or revenue per month.
In it is groupby followed by agg; in R it is group_by followed by summarise. Name your output columns clearly and count rows in each group alongside every average, so a group of 3 is not mistaken for one of 3,000.
Common trapaveraging averages: the mean of per-region means is not the overall mean when regions have different numbers of rows, so compute overall figures from the raw rows or weight by group size.
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Resources for this stage
Start with
- BookFreeR for Data Science (2e)
Wickham, Çetinkaya-Rundel, Grolemund · Beginner
Free online: the Whole game and Transform chapters teach the tidyverse verbs in the order you need them