Chapter 7 — Practice Demo Lab: Reference Solution

Lab: Produce a summary report together

Note

Reference solution for the Chapter 7 Practice Demo Lab (Tasks 1–5 of Lab: Produce a summary report together). Assumes air_clean from Chapter 6. The graded Canvas lab has its own private answer key.

Task 1 — Agree on the question

Worked example: “Which day in January 2000 was the worst air-quality day, on average across all stations?” (measured by mean hourly O₃). Any of the three suggested questions works.

Task 2 — Build the summary

worst_days <- air_clean |>
  filter(!is.na(O3)) |>
  group_by(date) |>
  summarise(mean_o3 = round(mean(O3), 1),
            n_readings = n(),
            .groups = "drop") |>
  arrange(desc(mean_o3))
kable(head(worst_days, 5),
      caption = "Days with the highest mean hourly O3 (ppb) across all stations, Jan 2000.")

Days with the highest mean hourly O3 (ppb) across all stations, Jan 2000.

date	mean_o3	n_readings
2000-01-09	25.5	190
2000-01-08	25.2	189
2000-01-04	21.9	188
2000-01-01	18.0	190
2000-01-02	17.6	189

Task 3 — Reshape if useful

This summary has one grouping variable (date), so it is already a readable ranked table — no pivot_wider() needed. (If you had grouped by two variables — say date × location — you would pivot_wider() so each station is a column.)

Task 4 — Caption it

January 09, 2000 was the worst average-ozone day of the month, at 25.5 ppb mean across stations.

Task 5 — Submit

Submit the dplyr pipeline (Task 2), the rendered table, the one-sentence caption, and all group members’ names. (Graded lab: follow Canvas.)