Explain pivot_wider() alongside pivot_longer() in current Chapter 12

[mpjashby]

Rationale

Current Chapter 12 explains the distinction between wide and long data and teaches pivot_longer(), but it does not teach the inverse operation, pivot_wider(). The downloadable report associated with current Chapter 14 uses pivot_wider(), while the full explanation currently appears only in current Chapter 15.

With current Chapter 12 moving before current Chapter 14, explaining both pivot directions together would make the data-shape discussion complete and remove a forward dependency from the report exercise.

Suggested change

• Extend the long-and-wide-data section of 12_messy_data/index.qmd to introduce pivot_wider() immediately after pivot_longer().
• Reuse the same small toy dataset so students can see one transformation reversed.
• Explain names_from, values_from and, where useful, values_fill in plain language.
• Emphasise that long data are usually convenient for analysis while wider forms can be useful for human-readable tables or analyses requiring separate variables.
• Add a quiz question that asks students to choose between pivot_longer() and pivot_wider().
• Revise current Chapter 15 so its first use of pivot_wider() refers back to current Chapter 12 rather than presenting the function as new.

Completion criteria

• Students can distinguish the two pivot directions before reaching current Chapter 14.
• The Chapter 14 report uses only arguments that have already been explained.
• Current Chapter 15 reinforces rather than duplicates the initial explanation.