Tidying and joining data

Session 6

2024-11-20

Categories of functions in {tidyr}:

1. Pivoting
2. Rectangling
3. Nesting and unnesting
4. Splitting and combining character columns
5. Handling missing values

From https://tidyr.tidyverse.org/#getting-started

“Pivoting” which converts between long and wide forms. tidyr 1.0.0 introduces pivot_longer() and pivot_wider(), replacing the older spread() and gather() functions. See vignette(“pivot”) for more details.

“Rectangling”, which turns deeply nested lists (as from JSON) into tidy tibbles. See unnest_longer(), unnest_wider(), hoist(), and vignette(“rectangle”) for more details.

Nesting converts grouped data to a form where each group becomes a single row containing a nested data frame, and unnesting does the opposite. See nest(), unnest(), and vignette(“nest”) for more details.

Splitting and combining character columns. Use separate() and extract() to pull a single character column into multiple columns; use unite() to combine multiple columns into a single character column.

Make implicit missing values explicit with complete(); make explicit missing values implicit with drop_na(); replace missing values with next/previous value with fill(), or a known value with replace_na().

1 What is involved in tidying messy datasets?

• Column headers are values, not variable names
• Multiple variables stored in one column
• Variables are stored in both rows and columns
• Multiple types in one table
• One type in multiple tables

from Tidy data vignette https://tidyr.tidyverse.org/articles/tidy-data.html

2 Pivoting

3 How would this look if we turn it into tidy data?

City	2000	2010	2020
Baltimore city, Maryland	300000	290000	285000
Washington, DC	300000	315000	330000
Richmond, Virginia	200000	202000	210000

City	Year	Value
Baltimore city, Maryland	2000	300000
Baltimore city, Maryland	2010	290000
Baltimore city, Maryland	2020	285000

…

pivot_longer(
  cols = !any_of("City"),
  names_to = "Year",
  values_to = "Value"
)

4 Tidying messy datasets

• Column headers are values, not variable names
• Multiple variables stored in one column
• Variables are stored in both rows and columns
• Multiple types in one table
• One type in multiple tables

5 How would this look if we turn it into tidy data?

City name	2000	2010	2020
Baltimore city, Maryland	300000	290000	285000
Washington, DC	300000	315000	330000
Richmond, Virginia	200000	202000	210000

City	State	Year	Value
Baltimore city	Maryland	2000	300000
Washington	DC	2000	300000
Richmond	Virginia	2000	200000

…

6 Data organization in spreadsheets

What are the benefits of careful data organization in spreadsheets?

• reduce errors
• ease later analyses

What are the practical approaches for data organization in spreadsheets?

1. be consistent,
2. write dates like YYYY-MM-DD,
3. do not leave any cells empty,
4. put just one thing in a cell,
5. organize the data as a single rectangle (with subjects as rows and variables as columns, and with a single header row),
6. create a data dictionary,
7. do not include calculations in the raw data files,
8. do not use font color or highlighting as data,
9. choose good names for things,
10. make backups,
11. use data validation to avoid data entry errors,
12. and save the data in plain text files.