Module 04 Summary

What this module established

Tidy data means every variable is a column and every observation is a row. When a column name is a value — a month, a year, a category — the table is untidy, and every verb you want to use will fight you.

Carry forward

• group_by() with summarise() is the pair that changes the grain. Print the row count either side, and drop the grouping when done.
• case_when() takes the first TRUE, so order is meaning. Test missingness first and always supply .default.
• A join is the only common verb that can make a table bigger. Check the key with anyDuplicated() and compare row counts every time.

Before moving on

Move on when you can explain why a reversed pair of case_when() conditions produces a plausible, wrong column with no warning.

Mark complete