如何最好地计算 R 的同比差异

Question

Below is the sample code. The task at hand is to create a year over year difference (2021 q4 value - 2020 q4 value) for only the fourth quarter and percentage difference. Desired result is below. Usually I would do a pivot_wider and such. However, how does one do this and not take all quarters into account?

  year <- c(2020,2020,2020,2020,2021,2021,2021,2021,2020,2020,2020,2020,2021,2021,2021,2021)
  qtr <- c(1,2,3,4,1,2,3,4,1,2,3,4,1,2,3,4)
  area <- c(1012,1012,1012,1012,1012,1012,1012,1012,1402,1402,1402,1402,1402,1402,1402,1402)
  employment <- c(100,102,104,106,108,110,114,111,52,54,56,59,61,66,65,49)

  test1 <- data.frame (year,qtr,area,employment)

  area       difference     percentage
  1012           5              4.7%
  1402           -10            -16.9

Answer 1

You would use filter on quarter:

test1 |>
  filter(qtr == 4) |>
  group_by(area) |>
    mutate(employment_lag = lag(employment),
           diff = employment - employment_lag) |>
    na.omit() |>
  ungroup() |>
  mutate(percentage = diff/employment_lag)

Output:

# A tibble: 2 × 7
   year   qtr  area employment  diff employment_start percentage
  <dbl> <dbl> <dbl>      <dbl> <dbl>            <dbl>      <dbl>
1  2021     4  1012        111     5              106     0.0472
2  2021     4  1402         49   -10               59    -0.169 

Update: Adding correct percentage.