unitdid

The unitdid package provides a set of functions for the analysis of the unit-level difference-in-differences (Arkhangelsky, Yanagimoto, and Zohar 2024).

Installation

You can install the development version of unitdid from R-universe with:

install.packages('unitdid',
                 repos = 'https://kazuyanagimoto.r-universe.dev')

Example

This is a basic example with the simulated base_heterocp data set:

library(unitdid)
library(dplyr)
library(ggplot2)

base_heterocp |>
  head()
#> # A tibble: 6 × 5
#>      id  year byear cyear      y
#>   <int> <int> <int> <int>  <dbl>
#> 1     1  1999  1955  1985 -0.848
#> 2     1  2000  1955  1985  0.759
#> 3     1  2001  1955  1985 -1.03 
#> 4     1  2002  1955  1985  0.858
#> 5     1  2003  1955  1985 -0.866
#> 6     1  2004  1955  1985 -0.651

Individual-level child penalties are estimated by unitdid():

mdl_base <- base_heterocp |>
  unitdid(yname = "y",
        iname = "id",
        tname = "year",
        ename = "cyear",
        bname = "byear")

# Estimated individual-level child penalties (y_tilde)
get_unitdid(mdl_base)
#> # A tibble: 32,257 × 6
#>       id  year byear cyear      y    y_tilde
#>    <int> <int> <int> <int>  <dbl>      <dbl>
#>  1   705  2000  1957  2000 0.138  -0.0287   
#>  2   997  2000  1958  2000 0.138   0.0849   
#>  3   998  2000  1958  2000 0.119  -0.104    
#>  4  1013  2000  1958  2000 0.115  -0.0000709
#>  5  1082  2000  1958  2000 0.0362  0.00549  
#>  6  1127  2000  1958  2000 0.386   0.125    
#>  7  1225  2001  1959  2001 0.158  -0.118    
#>  8  1228  2000  1959  2000 0.241  -0.0937   
#>  9  1228  2001  1959  2000 0.443   0.0226   
#> 10  1230  2000  1959  2000 0.143  -0.0266   
#> # ℹ 32,247 more rows

Aggregation of Individual-level Child Penalties

They can be aggregated to the full, event (year at event (treatment). Mainly for staggered DiD design), event_age (age at event. Mainly for child penalties) levels:

summary(mdl_base) # default agg = "full"
#> # A tibble: 6 × 3
#>      mean rel_time     n
#>     <dbl>    <int> <dbl>
#> 1 -0.0653        0  4357
#> 2 -0.193         1  4357
#> 3 -0.307         2  4357
#> 4 -0.310         3  4357
#> 5 -0.350         4  4357
#> 6 -0.349         5  4357

sum_eage <- summary(mdl_base, agg = "event_age")

sum_eage |>
  filter(rel_time == 0) |>
  mutate(rel_time = -1,
         mean = 0) |>
  bind_rows(sum_eage) |>
  filter(between(event_age, 25, 34)) |>
  mutate(lbl_facet = paste0("Age ", event_age)) |>
  ggplot(aes(x = rel_time, y = mean)) +
  geom_point() +
  geom_line() +
  geom_vline(xintercept = -1, linetype = "dashed") +
  geom_hline(yintercept = 0) +
  facet_wrap(~lbl_facet, ncol = 5) +
  labs(x = "Time to First Childbirth",
       y = "Child Penalties on y") +
  theme_minimal() +
  theme(panel.grid.major.x = element_blank(),
        panel.grid.minor = element_blank())

Variance of Individual-level Child Penalties

Since the individual-level child penalties are estimated with measurement errors, the variance of the y_tilde is not equal to the variance of the individual-level child penalties.

The compute_varcov = "var" option of the unitdid estimates the variance of the measurement errors and the variance of the individual-level child penalties by subtracting the variance of the measurement errors from the variance of y_tilde

mdl_base <- base_heterocp |>
  unitdid(yname = "y",
        iname = "id",
        tname = "year",
        ename = "cyear",
        bname = "byear",
        compute_varcov = "var")

sum_eage <- summary(mdl_base, agg = "event_age")

sum_eage |>
  filter(rel_time == 0) |>
  mutate(rel_time = -1,
         var = 0) |>
  bind_rows(sum_eage) |>
  filter(between(event_age, 25, 34)) |>
  mutate(lbl_facet = paste0("Age ", event_age)) |>
  ggplot(aes(x = rel_time, y = sqrt(var))) +
  geom_point() +
  geom_line() +
  geom_vline(xintercept = -1, linetype = "dashed") +
  geom_hline(yintercept = 0) +
  facet_wrap(~lbl_facet, ncol = 5) +
  labs(x = "Time to First Childbirth",
       y = "S.D. of Child Penalties") +
  theme_minimal() +
  theme(panel.grid.major.x = element_blank(),
        panel.grid.minor = element_blank())

References

Arkhangelsky, Dmitry, Kazuharu Yanagimoto, and Tom Zohar. 2024. “Flexible Analysis of Individual Heterogeneity in Event Studies: Application to the Child Penalty.” arXiv. https://arxiv.org/abs/2403.19563.