# 9. Covariates

# 1 Introduction

Covariates can be used to extend standard bage models in two ways:

1. incorporating information beyond what is contained in classifying variables, such as age, sex, or region; and
2. dealing with subsets of the data with unusual behavior.

This vignette gives a brief description of covariates in bage and then illustrates their use with a case study of births in South Korea.

# 2 Mathematical details

A model in bage typically has a structure that is something like: \[\begin{align} y_{ast} & \sim \text{Poisson}(\gamma_{ast} w_{ast}) \\ \log \gamma_{ast} & \sim \text{Gamma}(\xi^{-1}, (\mu_{ast} \xi)^{-1}) \\ \log \mu_{ast} & = \beta^{(0)} + \beta_{as}^{\text{age:sex}} + \beta_t^{\text{time}} \tag{2.1} \end{align}\] where

- \(y_{ast}\) is the outcome for people in age group \(a\) and sex \(s\) during period \(t\);
- \(w_{ast}\) is exposure;
- \(\gamma_{ast}\) is a rate;
- \(\xi\) governs overall dispersion;
- \(\beta^{(0)}\) is an intercept;
- \(\pmb{\beta}^{\text{age:sex}}\) and \(\pmb{\beta}^{\text{time}}\) are terms formed the classifying variables age, sex, and time;
- \(\beta_{as}^{\text{age:sex}}\) and \(\pmb{\beta}_t^{\text{time}}\) are elements of these terms; and
- the intercept and the terms formed from the classifying variables all have prior distributions.

In a model with covariates, (2.1) changes to \[\begin{equation} \log \mu_{ast} = \beta^{(0)} + \beta_{as}^{\text{age:sex}} + \beta_t^{\text{time}} + (\pmb{Z} \pmb{\zeta})_{ast} \end{equation}\]

where

- \(\pmb{Z}\) is a \(N \times P\) covariate matrix;
- \(\pmb{\zeta}\) is a \(P\)-element vector of coefficients; and
- \((\pmb{Z} \pmb{\zeta})_{ast}\) is an element from the vector obtained by multiplying \(\pmb{Z}\) and \(\pmb{\zeta}\).

Change (2.2) to \[\begin{equation} \mu_i = \sum_{m=0}^M \beta_{j_i^m}^{(m)} + (\pmb{Z} \pmb{\eta})_i \tag{2.2} \end{equation}\]

where - \(\pmb{Z}\) is an \(I \times P\) matrix of covariates; and - \(\pmb{\eta}\) is a vector of coefficients.

The covariate matrix \(\pmb{Z}\) is derived from the raw covariate data by scaling any numeric variables to have mean 0 and standard deviation 1, and by converting any categorical variables to sets of indicator variables. The conversion to indicator variables follows the rules that R uses for “treatment” contrasts. If the categorical has \(C\) categories, then \(C-1\) indicator variabls are constructed, with the first category being omitted.

Each element of \(\pmb{\eta}\) has prior \[\begin{equation} \eta_p \sim \text{N}(0, 1) \end{equation}\]

# 3 Example: Births in South Korea

To illustrate the use of covariates, we will analyse data on births in South Korea.

## 3.1 Preliminaries

Besides bage itself, we use dplyr and vctrs for data manipulation, and ggplot2 for plotting results.

    suppressPackageStartupMessages({
      library(bage)
      library(dplyr)
      library(ggplot2)
    })

Our data is the data frame `kor_births`, which is part of bage. The data frame gives numbers of births disaggregated by age of mother, region, and year. It also contains a numeric variable called `gdp_pc_2023` that gives GDP per capita (in thousands of US dollars) in 2023, and a categorical variable (with levels `"Low"`, `"Medium"`, and `"High"`) describing population density in 2020.

    kor_births
    #> # A tibble: 1,872 × 7
    #>    age   region  time births  popn gdp_pc_2023 dens_2020
    #>    <chr> <fct>  <int>  <int> <int>       <dbl> <chr>
    #>  1 10-14 Busan   2011      1 89822        25.7 High
    #>  2 10-14 Busan   2012      0 83884        25.7 High
    #>  3 10-14 Busan   2013      0 79061        25.7 High
    #>  4 10-14 Busan   2014      1 74741        25.7 High
    #>  5 10-14 Busan   2015      0 68783        25.7 High
    #>  6 10-14 Busan   2016      1 64905        25.7 High
    #>  7 10-14 Busan   2017      1 64251        25.7 High
    #>  8 10-14 Busan   2018      0 63249        25.7 High
    #>  9 10-14 Busan   2019      0 62154        25.7 High
    #> 10 10-14 Busan   2020      0 63498        25.7 High
    #> # ℹ 1,862 more rows

## 3.2 Covariates that bring in extra information

The variables `gdp_pc_2023` and `dens_2020` are both examples of covariates that contribute extra information to the model, beyond what is contained in the outcome, exposure, and classifying variables.

We use function `set_covariates()` to instruct `mod_pois()` to treat these variables as covariates.

    mod_gdp_dens <- mod_pois(births ~ (age + region + time)^2,
                             data = kor_births,
                             exposure = popn) |>
      set_covariates(~ gdp_pc_2023 + dens_2020) |>
      fit()
    #> Building log-posterior function...
    #> Finding maximum...
    #> Drawing values for hyper-parameters...
    mod_gdp_dens
    #>
    #>     ------ Fitted Poisson model ------
    #>
    #>    births ~ (age + region + time)^2
    #>
    #>                  exposure: popn
    #>
    #>         term  prior along n_par n_par_free std_dev
    #>  (Intercept) NFix()     -     1          1       -
    #>          age   RW()   age     9          9    2.64
    #>       region    N()     -    16         16    0.03
    #>         time   RW()  time    13         13    0.20
    #>   age:region   RW()   age   144        144    0.20
    #>     age:time   RW()  time   117        117    1.20
    #>  region:time   RW()  time   208        208    0.19
    #>
    #>  covariates: ~gdp_pc_2023 + dens_2020
    #>
    #>  disp: mean = 1
    #>
    #>  n_draw var_time var_age optimizer
    #>    1000     time     age    nlminb
    #>
    #>  time_total time_max time_draw iter converged                    message
    #>        1.88     1.32      0.51   36      TRUE   relative convergence (4)

To obtain estimates of the coefficients (ie estimates of the \(\zeta_p\)) we call function `components()` and filter out rows for the `"covariates"` term.

    mod_gdp_dens |>
      components() |>
      filter(term == "covariates")
    #> # A tibble: 3 × 4
    #>   term       component level                       .fitted
    #>   <chr>      <chr>     <chr>                  <rdbl<1000>>
    #> 1 covariates coef      gdp_pc_2023     0.005 (-0.68, 0.66)
    #> 2 covariates coef      dens_2020Low       -0.5 (-1.8, 0.8)
    #> 3 covariates coef      dens_2020Medium  -0.75 (-2.1, 0.65)

## 3.3 Covariates that allow for unusual subsets

In East Asia, years with the Dragon zodiac sign sometimes have larger-than-usual numbers of births. To allow for this possibility, we create a dragon-year covariate, and incorporate it into a new model.

    births <- kor_births |>
      mutate(is_dragon_year = time == 2012)
    mod_dragon <- mod_pois(births ~ (age + region + time)^2,
                          data = births,
                          exposure = popn) |>
      set_covariates(~ is_dragon_year) |>
      fit()
    #> Building log-posterior function...
    #> Finding maximum...
    #> Drawing values for hyper-parameters...

    mod_dragon |>
      components() |>
      filter(term == "covariates")
    #> # A tibble: 1 × 4
    #>   term       component level                           .fitted
    #>   <chr>      <chr>     <chr>                      <rdbl<1000>>
    #> 1 covariates coef      is_dragon_yearTRUE 0.059 (-0.047, 0.18)

There is some evidence for extra births, though there is substantial uncertainty about the size of the effect.

Next we expand the model to allow the dragon-year effect to differ across age groups. We create a variable that takes the values `"baseline"` in all years, except in dragon years, when it takes the name of the age group. We turn this variable into a factor with `"baseline"` as its first level.

    births <- births |>
      mutate(is_dragon_year_age = if_else(time == 2012, age, "baseline"),
             is_dragon_year_age = factor(is_dragon_year_age,
                                         levels = c("baseline", unique(age))))
    births |>
      filter(time %in% 2011:2013)
    #> # A tibble: 432 × 9
    #>    age   region           time births  popn gdp_pc_2023 dens_2020 is_dragon_year
    #>    <chr> <fct>           <int>  <int> <int>       <dbl> <chr>     <lgl>
    #>  1 10-14 Busan            2011      1 89822        25.7 High      FALSE
    #>  2 10-14 Busan            2012      0 83884        25.7 High      TRUE
    #>  3 10-14 Busan            2013      0 79061        25.7 High      FALSE
    #>  4 10-14 Chungcheongbuk…  2011      3 47811        40.3 Low       FALSE
    #>  5 10-14 Chungcheongbuk…  2012      1 44864        40.3 Low       TRUE
    #>  6 10-14 Chungcheongbuk…  2013      0 42259        40.3 Low       FALSE
    #>  7 10-14 Chungcheongnam…  2011      0 62308        50.4 Low       FALSE
    #>  8 10-14 Chungcheongnam…  2012      0 56746        50.4 Low       TRUE
    #>  9 10-14 Chungcheongnam…  2013      0 54415        50.4 Low       FALSE
    #> 10 10-14 Daegu            2011      3 75776        22.3 Medium    FALSE
    #> # ℹ 422 more rows
    #> # ℹ 1 more variable: is_dragon_year_age <fct>

We create a new model with the age-sepcific dragon-year indicator.

    mod_dragon_age <- mod_pois(births ~ (age + region + time)^2,
                             data = births,
                             exposure = popn) |>
      set_covariates(~ is_dragon_year_age) |>
      fit()
    #> Building log-posterior function...
    #> Finding maximum...
    #> Drawing values for hyper-parameters...
    mod_dragon_age
    #>
    #>     ------ Fitted Poisson model ------
    #>
    #>    births ~ (age + region + time)^2
    #>
    #>                  exposure: popn
    #>
    #>         term  prior along n_par n_par_free std_dev
    #>  (Intercept) NFix()     -     1          1       -
    #>          age   RW()   age     9          9    2.64
    #>       region    N()     -    16         16    0.02
    #>         time   RW()  time    13         13    0.18
    #>   age:region   RW()   age   144        144    0.17
    #>     age:time   RW()  time   117        117    1.22
    #>  region:time   RW()  time   208        208    0.14
    #>
    #>  covariates: ~is_dragon_year_age
    #>
    #>  disp: mean = 1
    #>
    #>  n_draw var_time var_age optimizer
    #>    1000     time     age    nlminb
    #>
    #>  time_total time_max time_draw iter converged                    message
    #>        1.81     1.27      0.49   41      TRUE   relative convergence (4)

Rather than a single dragon-year coefficient, we have a coefficient for each age group. We extract them and tidy up the labels.

    mod_dragon_age |>
      components() |>
      filter(term == "covariates") |>
      mutate(age = sub("is_dragon_year_age", "", level)) |>
      select(age, .fitted)
    #> # A tibble: 9 × 2
    #>   age               .fitted
    #>   <chr>        <rdbl<1000>>
    #> 1 10-14   0.31 (-0.11, 0.8)
    #> 2 15-19 0.003 (-0.19, 0.19)
    #> 3 20-24 0.042 (-0.13, 0.23)
    #> 4 25-29 0.076 (-0.11, 0.26)
    #> 5 30-34 0.075 (-0.11, 0.25)
    #> 6 35-39 0.033 (-0.15, 0.22)
    #> 7 40-44 0.057 (-0.12, 0.24)
    #> 8 45-49   0.1 (-0.13, 0.32)
    #> 9 50-54    0.2 (-0.2, 0.59)

## 3.4 Forecasting

If all the covariates in a model are fixed, then the model can be forecasted as normal.

    mod_gdp_dens |>
      forecast(labels = 2024:2025)
    #> `components()` for past values...
    #> `components()` for future values...
    #> `augment()` for future values...
    #> # A tibble: 288 × 10
    #>    age   region             time births  popn gdp_pc_2023 dens_2020 .observed
    #>    <chr> <fct>             <int>  <dbl> <int>       <dbl> <chr>         <dbl>
    #>  1 10-14 Busan              2024     NA    NA        25.7 High             NA
    #>  2 10-14 Busan              2025     NA    NA        25.7 High             NA
    #>  3 10-14 Chungcheongbuk-do  2024     NA    NA        40.3 Low              NA
    #>  4 10-14 Chungcheongbuk-do  2025     NA    NA        40.3 Low              NA
    #>  5 10-14 Chungcheongnam-do  2024     NA    NA        50.4 Low              NA
    #>  6 10-14 Chungcheongnam-do  2025     NA    NA        50.4 Low              NA
    #>  7 10-14 Daegu              2024     NA    NA        22.3 Medium           NA
    #>  8 10-14 Daegu              2025     NA    NA        22.3 Medium           NA
    #>  9 10-14 Daejeon            2024     NA    NA        27.6 Medium           NA
    #> 10 10-14 Daejeon            2025     NA    NA        27.6 Medium           NA
    #> # ℹ 278 more rows
    #> # ℹ 2 more variables: .fitted <rdbl<1000>>, .expected <rdbl<1000>>

If, however, a covariate varies over time, forecasting only works if values for future periods are provided. The following code will result in an error:

    mod_dragon |>
      forecast(labels = 2024:2025)

Instead we need to create a `newdata` data frame…

    newdata <- expand.grid(age = unique(kor_births$age),
                           region = unique(kor_births$region),
                           time = 2024:2025) |>
      mutate(is_dragon_year = FALSE)
    head(newdata)
    #>     age region time is_dragon_year
    #> 1 10-14  Busan 2024          FALSE
    #> 2 15-19  Busan 2024          FALSE
    #> 3 20-24  Busan 2024          FALSE
    #> 4 25-29  Busan 2024          FALSE
    #> 5 30-34  Busan 2024          FALSE
    #> 6 35-39  Busan 2024          FALSE

…and supply it to `forecast()`.

    mod_dragon |>
      forecast(newdata = newdata)
    #> `components()` for past values...
    #> `components()` for future values...
    #> `augment()` for future values...
    #> # A tibble: 288 × 11
    #>    age   region           time births  popn gdp_pc_2023 dens_2020 is_dragon_year
    #>    <chr> <fct>           <int>  <dbl> <int>       <dbl> <chr>     <lgl>
    #>  1 10-14 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  2 15-19 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  3 20-24 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  4 25-29 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  5 30-34 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  6 35-39 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  7 40-44 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  8 45-49 Busan            2024     NA    NA          NA <NA>      FALSE
    #>  9 50-54 Busan            2024     NA    NA          NA <NA>      FALSE
    #> 10 10-14 Chungcheongbuk…  2024     NA    NA          NA <NA>      FALSE
    #> # ℹ 278 more rows
    #> # ℹ 3 more variables: .observed <dbl>, .fitted <rdbl<1000>>,
    #> #   .expected <rdbl<1000>>