# Survival Analysis with brms and shrinkr

#### Jacob M. Maronge

#### 2026-06-29

## Overview

This vignette demonstrates hierarchical shrinkage for survival analysis using the classic `veteran` lung cancer dataset. We explore a key clinical question: Does the treatment effect vary by lung cancer cell type?

Rather than treating cell type-specific treatment effects as fixed interaction terms, we model them as random effects drawn from a common distribution. This hierarchical structure allows us to:

- Borrow strength across cell types, especially for small subgroups
- Estimate the overall mean treatment effect (`mu`)
- Quantify heterogeneity in treatment effects (`tau`)
- Shrink extreme subgroup estimates toward the group mean

We compare three modeling approaches:

1. Two-stage (brms + shrinkr): fit a Cox model in `brms`, then apply hierarchical shrinkage in `shrinkr`
2. Full hierarchical (brms): fit the hierarchical Cox model in one step
3. Two-stage (frequentist + shrinkr): use Cox model estimates from `survival::coxph()`, then apply shrinkage

The two-stage `brms` workflow produces nearly identical results to the full hierarchical model, while making it easy to explore alternative hierarchical priors without repeatedly refitting the Stage 1 model.

Some model-fitting steps are computationally intensive and are not evaluated during routine package checks. All code needed to reproduce the analysis is shown below.

## Setup

    library(shrinkr)
    library(brms)
    library(tidybayes)
    library(distributional)
    library(tidyverse)
    library(survival)
    library(posterior)
    library(patchwork)

    theme_set(theme_minimal(base_size = 12))

    cell_types <- c("squamous", "smallcell", "adeno", "large")

    prior_specs <- list(
      very_strong = list(name = "Very Strong", scale = 0.1),
      strong = list(name = "Strong", scale = 0.25),
      moderate = list(name = "Moderate", scale = 0.5),
      weak = list(name = "Weak", scale = 1.0),
      very_weak = list(name = "Very Weak", scale = 2.0)
    )

## The Veteran Dataset

    data(veteran, package = "survival")

    head(veteran)
    #>   trt celltype time status karno diagtime age prior
    #> 1   1 squamous   72      1    60        7  69     0
    #> 2   1 squamous  411      1    70        5  64    10
    #> 3   1 squamous  228      1    60        3  38     0
    #> 4   1 squamous  126      1    60        9  63    10
    #> 5   1 squamous  118      1    70       11  65    10
    #> 6   1 squamous   10      1    20        5  49     0
    table(veteran$celltype, veteran$trt)
    #>
    #>              1  2
    #>   squamous  15 20
    #>   smallcell 30 18
    #>   adeno      9 18
    #>   large     15 12

    veteran %>%
      group_by(celltype, trt) %>%
      summarise(
        n = n(),
        deaths = sum(status),
        median_time = median(time),
        .groups = "drop"
      )
    #> # A tibble: 8 × 5
    #>   celltype    trt     n deaths median_time
    #>   <fct>     <dbl> <int>  <dbl>       <dbl>
    #> 1 squamous      1    15     13       100
    #> 2 squamous      2    20     18       156.
    #> 3 smallcell     1    30     28        53
    #> 4 smallcell     2    18     17        27
    #> 5 adeno         1     9      9        92
    #> 6 adeno         2    18     17        49.5
    #> 7 large         1    15     14       177
    #> 8 large         2    12     12        82

Variables:

- `time`: survival time (days)
- `status`: death indicator (`1 = died`)
- `trt`: treatment (`1 = standard`, `2 = test`)
- `celltype`: cancer type (`squamous`, `smallcell`, `adeno`, `large`)
- `karno`: Karnofsky score (performance status)
- `age`: age in years

The dataset contains 137 patients across four cell types, with varying sample sizes.

## Approach 1: Two-Stage (brms + shrinkr)

### Stage 1: Fit Cox Model

We begin by fitting a Cox proportional hazards model that allows the treatment effect to vary by cell type. At this stage we estimate subgroup-specific treatment effects without adding hierarchical shrinkage across cell types. That hierarchical regularization is introduced in Stage 2.

    brms_uninformative <- brm(
      time | cens(1 - status) ~ trt:celltype + karno + age,
      data = veteran,
      family = cox(),
      chains = 4,
      iter = 4000,
      warmup = 1000,
      seed = 123
    )

    brms_uninformative_summary <- capture.output(print(summary(brms_uninformative)))

What this model does:

- Estimates a separate log hazard ratio for the test treatment in each cell type
- Adjusts for baseline performance status (`karno`) and age
- Leaves pooling across cell types to the second-stage hierarchical model

Results:

    cat(brms_uninformative_summary, sep = "\n")
    #>  Family: cox
    #>   Links: mu = log
    #> Formula: time | cens(1 - status) ~ trt:celltype + karno + age
    #>    Data: veteran (Number of observations: 137)
    #>   Draws: 4 chains, each with iter = 4000; warmup = 1000; thin = 1;
    #>          total post-warmup draws = 12000
    #>
    #> Regression Coefficients:
    #>                       Estimate Est.Error l-95% CI
    #> Intercept                 4.16      0.74     2.70
    #> karno                    -0.03      0.01    -0.04
    #> age                      -0.01      0.01    -0.02
    #> trt:celltypesquamous     -0.12      0.21    -0.52
    #> trt:celltypesmallcell     0.51      0.23     0.05
    #> trt:celltypeadeno         0.54      0.20     0.13
    #> trt:celltypelarge         0.16      0.23    -0.31
    #>                       u-95% CI Rhat Bulk_ESS Tail_ESS
    #> Intercept                 5.61 1.00     9000     8957
    #> karno                    -0.02 1.00    11647     9539
    #> age                       0.01 1.00     9875     8676
    #> trt:celltypesquamous      0.29 1.00     5515     7429
    #> trt:celltypesmallcell     0.95 1.00     5310     7315
    #> trt:celltypeadeno         0.94 1.00     5398     7230
    #> trt:celltypelarge         0.61 1.00     5399     7077
    #>
    #> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
    #> and Tail_ESS are effective sample size measures, and Rhat is the potential
    #> scale reduction factor on split chains (at convergence, Rhat = 1).

### Stage 2: Apply Hierarchical Shrinkage

Now we extract the cell type-specific treatment effect posteriors and apply hierarchical shrinkage.

#### Step 1: Extract posterior samples

    brms_posteriors <- brms_uninformative %>%
      spread_draws(`b_trt:celltypesquamous`, `b_trt:celltypesmallcell`,
                   `b_trt:celltypeadeno`, `b_trt:celltypelarge`) %>%
      select(-c(.chain, .iteration, .draw)) %>%
      pivot_longer(everything(), names_to = "celltype", values_to = "value") %>%
      mutate(celltype = gsub("b_trt:celltype", "", celltype)) %>%
      group_by(celltype) %>%
      summarise(draws = list(matrix(value, ncol = 1)), .groups = "drop") %>%
      deframe()

`brms_posteriors` is a named list containing posterior draws for each cell type.

#### Step 2: Fit a Gaussian mixture approximation

The `fit_mixture()` function approximates each subgroup posterior with a mixture of Gaussian components. This creates a flexible representation of the Stage 1 posterior that can be passed to `shrink()`.

    mix_brms <- fit_mixture(brms_posteriors, K_max = 3, verbose = TRUE)

    print(mix_brms)
    plot(mix_brms, draws = brms_posteriors)

Understanding the mixture approximation:

- Each posterior is approximated as a weighted sum of Gaussian components
- The number of components is chosen separately for each cell type
- This allows the approximation to capture skewness or heavier tails when needed

#### Step 3: Apply a hierarchical prior

    priors_moderate <- list(
      mu = dist_normal(0, 1),
      tau = dist_truncated(dist_normal(0, 0.5), lower = 0)
    )

    fit_twostage_brms <- shrink(
      mixture = mix_brms,
      hierarchical_priors = priors_moderate,
      chains = 4,
      iter = 4000,
      warmup = 1000,
      seed = 456
    )

    moderate_brms_output <- capture.output(print(fit_twostage_brms))

Results:

    cat(moderate_brms_output, sep = "\n")
    #> # A tibble: 3 × 7
    #>   variable     mean    sd    q2.5   q50 q97.5  rhat
    #>   <chr>       <dbl> <dbl>   <dbl> <dbl> <dbl> <dbl>
    #> 1 mu          0.246 0.265 -0.272  0.245 0.757  1.00
    #> 2 tau         0.361 0.176  0.123  0.324 0.812  1.00
    #> 3 tau_squared 0.161 0.177  0.0150 0.105 0.659  1.00

Interpreting the shrinkage:

- Cell type-specific estimates are pulled toward the overall mean (`mu`)
- The amount of shrinkage depends on `tau`, the between-cell-type heterogeneity
- Smaller `tau` implies stronger pooling
- Larger `tau` implies weaker pooling

## Approach 2: Full Hierarchical (brms)

For comparison, we fit the corresponding one-stage hierarchical Cox model directly in `brms`.

    brms_hierarchical <- brm(
      time | cens(1 - status) ~ trt + (0 + trt | celltype) + karno + age,
      data = veteran,
      family = cox(),
      prior = c(
        prior(normal(0, 1), class = b, coef = "trt"),
        prior(normal(0, 0.5), class = sd, group = "celltype", lb = 0)
      ),
      chains = 4,
      iter = 4000,
      warmup = 1000,
      seed = 123
    )

    brms_hierarchical_summary <- capture.output(print(summary(brms_hierarchical)))

    brms_hier_effects <- brms_hierarchical %>%
      spread_draws(r_celltype[celltype, term], b_trt) %>%
      filter(term == "trt") %>%
      mutate(theta = b_trt + r_celltype) %>%
      group_by(celltype) %>%
      summarise(
        hr_mean = exp(mean(theta)),
        hr_lower = exp(quantile(theta, 0.025)),
        hr_upper = exp(quantile(theta, 0.975)),
        .groups = "drop"
      )

Results:

    cat(brms_hierarchical_summary, sep = "\n")
    #>  Family: cox
    #>   Links: mu = log
    #> Formula: time | cens(1 - status) ~ trt + (0 + trt | celltype) + karno + age
    #>    Data: veteran (Number of observations: 137)
    #>   Draws: 4 chains, each with iter = 4000; warmup = 1000; thin = 1;
    #>          total post-warmup draws = 12000
    #>
    #> Multilevel Hyperparameters:
    #> ~celltype (Number of levels: 4)
    #>         Estimate Est.Error l-95% CI u-95% CI Rhat
    #> sd(trt)     0.36      0.18     0.13     0.80 1.00
    #>         Bulk_ESS Tail_ESS
    #> sd(trt)     4188     6657
    #>
    #> Regression Coefficients:
    #>           Estimate Est.Error l-95% CI u-95% CI Rhat
    #> Intercept     4.11      0.74     2.66     5.55 1.00
    #> trt           0.26      0.26    -0.24     0.79 1.00
    #> karno        -0.03      0.01    -0.04    -0.02 1.00
    #> age          -0.01      0.01    -0.02     0.01 1.00
    #>           Bulk_ESS Tail_ESS
    #> Intercept    11705     9033
    #> trt           5386     6740
    #> karno        13722     9846
    #> age          11879     7469
    #>
    #> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
    #> and Tail_ESS are effective sample size measures, and Rhat is the potential
    #> scale reduction factor on split chains (at convergence, Rhat = 1).

## Approach 3: Two-Stage (Frequentist + shrinkr)

We can also apply the second-stage shrinkage model to standard Cox model estimates and their covariance matrix.

    cox_model <- coxph(
      Surv(time, status) ~ trt:celltype + karno + age,
      data = veteran
    )

    cox_summary <- summary(cox_model)

    trt_idx <- grep("^trt:celltype", names(coef(cox_model)))

    trt_effects <- coef(cox_model)[trt_idx]
    trt_vcov <- vcov(cox_model)[trt_idx, trt_idx, drop = FALSE]

    names(trt_effects) <- gsub("^trt:celltype", "", names(trt_effects))
    rownames(trt_vcov) <- colnames(trt_vcov) <- names(trt_effects)

    print(cox_summary)
    #> Call:
    #> coxph(formula = Surv(time, status) ~ trt:celltype + karno + age,
    #>     data = veteran)
    #>
    #>   n= 137, number of events= 128
    #>
    #>                            coef exp(coef)  se(coef)      z Pr(>|z|)
    #> karno                 -0.031511  0.968980  0.005412 -5.823  5.8e-09 ***
    #> age                   -0.009056  0.990985  0.009125 -0.992  0.32102
    #> trt:celltypesquamous  -0.058593  0.943091  0.210626 -0.278  0.78087
    #> trt:celltypesmallcell  0.585704  1.796254  0.238641  2.454  0.01411 *
    #> trt:celltypeadeno      0.626150  1.870396  0.210631  2.973  0.00295 **
    #> trt:celltypelarge      0.236620  1.266960  0.237554  0.996  0.31922
    #> ---
    #> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
    #>
    #>                       exp(coef) exp(-coef) lower .95 upper .95
    #> karno                    0.9690     1.0320    0.9588    0.9793
    #> age                      0.9910     1.0091    0.9734    1.0089
    #> trt:celltypesquamous     0.9431     1.0603    0.6241    1.4251
    #> trt:celltypesmallcell    1.7963     0.5567    1.1252    2.8675
    #> trt:celltypeadeno        1.8704     0.5346    1.2378    2.8263
    #> trt:celltypelarge        1.2670     0.7893    0.7953    2.0182
    #>
    #> Concordance= 0.733  (se = 0.021 )
    #> Likelihood ratio test= 63.27  on 6 df,   p=1e-11
    #> Wald test            = 62.9  on 6 df,   p=1e-11
    #> Score (logrank) test = 67.8  on 6 df,   p=1e-12

    print("Treatment effects (log HR):")
    #> [1] "Treatment effects (log HR):"
    print(trt_effects)
    #>   squamous  smallcell      adeno      large
    #> -0.0585926  0.5857036  0.6261501  0.2366201

    print("\nStandard errors:")
    #> [1] "\nStandard errors:"
    print(sqrt(diag(trt_vcov)))
    #>  squamous smallcell     adeno     large
    #> 0.2106261 0.2386409 0.2106312 0.2375536

    fit_twostage_freq <- shrink(
      mle = trt_effects,
      var_matrix = trt_vcov,
      hierarchical_priors = priors_moderate,
      chains = 4,
      iter = 4000,
      warmup = 1000,
      seed = 456
    )

    moderate_freq_output <- capture.output(print(fit_twostage_freq))

Results:

    cat(moderate_freq_output, sep = "\n")
    #> # A tibble: 3 × 7
    #>   variable     mean    sd    q2.5   q50 q97.5  rhat
    #>   <chr>       <dbl> <dbl>   <dbl> <dbl> <dbl> <dbl>
    #> 1 mu          0.313 0.266 -0.218  0.312 0.834  1.00
    #> 2 tau         0.369 0.178  0.127  0.334 0.822  1.00
    #> 3 tau_squared 0.168 0.182  0.0160 0.112 0.675  1.00

## Compare Three Approaches

### Numerical comparison

    theta_brms <- summary(fit_twostage_brms)$theta %>%
      transmute(
        celltype = group,
        twostage_brms = mean
      )

    theta_freq <- summary(fit_twostage_freq)$theta %>%
      transmute(
        celltype = group,
        twostage_freq = mean
      )

    comparison <- brms_hier_effects %>%
      transmute(
        celltype,
        full_hier_brms = log(hr_mean)
      ) %>%
      left_join(theta_brms, by = "celltype") %>%
      left_join(theta_freq, by = "celltype") %>%
      mutate(
        diff_two_stage_vs_full = twostage_brms - full_hier_brms
      )

    knitr::kable(
      comparison[, 1:4],
      digits = 3,
      caption = "Comparison of treatment effects (log HR scale)"
    )

Comparison of treatment effects (log HR scale)
celltype | brms_hierarchical | brms_shrinkr | freq_shrinkr
--- | --- | --- | ---
squamous | -0.072 | -0.067 | -0.016
smallcell | 0.450 | 0.454 | 0.522
adeno | 0.472 | 0.472 | 0.557
large | 0.164 | 0.167 | 0.233

Key observations:

- The two-stage `brms + shrinkr` and full hierarchical `brms` fits are nearly identical
- This supports the equivalence of the two formulations in this example
- The frequentist Stage 1 approach differs modestly because the first-stage estimates differ
- All approaches show shrinkage toward a common mean

### Visual comparison

    theta_brms_plot <- summary(fit_twostage_brms)$theta %>%
      mutate(
        approach = "Two-Stage (brms + shrinkr)",
        hr_mean = exp(mean),
        hr_lower = exp(q2.5),
        hr_upper = exp(q97.5),
        celltype = group
      ) %>%
      select(celltype, approach, hr_mean, hr_lower, hr_upper)

    theta_freq_plot <- summary(fit_twostage_freq)$theta %>%
      mutate(
        approach = "Two-Stage (Frequentist + shrinkr)",
        hr_mean = exp(mean),
        hr_lower = exp(q2.5),
        hr_upper = exp(q97.5),
        celltype = group
      ) %>%
      select(celltype, approach, hr_mean, hr_lower, hr_upper)

    all_approaches <- bind_rows(
      theta_brms_plot,
      brms_hier_effects %>% mutate(approach = "Full Hierarchical (brms)"),
      theta_freq_plot
    ) %>%
      mutate(
        approach = factor(approach, levels = c(
          "Two-Stage (brms + shrinkr)",
          "Full Hierarchical (brms)",
          "Two-Stage (Frequentist + shrinkr)"
        ))
      )

    ggplot(all_approaches, aes(x = celltype, y = hr_mean, color = approach)) +
      geom_hline(yintercept = 1, linetype = "dashed", alpha = 0.5) +
      geom_pointrange(
        aes(ymin = hr_lower, ymax = hr_upper),
        position = position_dodge(width = 0.5),
        size = 0.8
      ) +
      scale_y_log10() +
      scale_color_brewer(palette = "Set1") +
      labs(
        title = "Comparison of Three Modeling Approaches",
        subtitle = "Treatment effects by cell type (hazard ratios)",
        x = "Cell Type",
        y = "Hazard Ratio (log scale)",
        color = "Approach"
      ) +
      theme(
        legend.position = "bottom",
        panel.grid.minor = element_blank()
      )

## Sensitivity Analysis: Exploring Different Priors

A main advantage of the two-stage framework is that we can explore many hierarchical priors in Stage 2 without refitting the Stage 1 survival model.

    prior_summary <- tibble(
      Strength = c("Very Strong", "Strong", "Moderate", "Weak", "Very Weak"),
      Prior = c(
        "Half-Normal(0, 0.1)",
        "Half-Normal(0, 0.25)",
        "Half-Normal(0, 0.5)",
        "Half-Normal(0, 1.0)",
        "Half-Normal(0, 2.0)"
      ),
      Scale = c(0.1, 0.25, 0.5, 1.0, 2.0),
      Interpretation = c(
        "Very similar effects expected",
        "Similar effects expected",
        "Moderate heterogeneity allowed",
        "Substantial differences allowed",
        "Large differences allowed"
      )
    )

    knitr::kable(prior_summary)

Strength | Prior | Scale | Interpretation
--- | --- | --- | ---
Very Strong | Half-Normal(0, 0.1) | 0.10 | Very similar effects expected
Strong | Half-Normal(0, 0.25) | 0.25 | Similar effects expected
Moderate | Half-Normal(0, 0.5) | 0.50 | Moderate heterogeneity allowed
Weak | Half-Normal(0, 1.0) | 1.00 | Substantial differences allowed
Very Weak | Half-Normal(0, 2.0) | 2.00 | Large differences allowed

    all_priors <- list(
      very_strong = list(
        mu = dist_normal(0, 1),
        tau = dist_truncated(dist_normal(0, 0.1), lower = 0)
      ),
      strong = list(
        mu = dist_normal(0, 1),
        tau = dist_truncated(dist_normal(0, 0.25), lower = 0)
      ),
      moderate = list(
        mu = dist_normal(0, 1),
        tau = dist_truncated(dist_normal(0, 0.5), lower = 0)
      ),
      weak = list(
        mu = dist_normal(0, 1),
        tau = dist_truncated(dist_normal(0, 1.0), lower = 0)
      ),
      very_weak = list(
        mu = dist_normal(0, 1),
        tau = dist_truncated(dist_normal(0, 2.0), lower = 0)
      )
    )

    # --- brms fits ---
    sensitivity_fits_brms <- lapply(all_priors, function(prior) {
      shrink(
        mixture = mix_brms,
        hierarchical_priors = prior,
        chains = 4,
        iter = 4000,
        warmup = 1000
      )
    })

    # --- frequentist fits ---
    sensitivity_fits_freq <- lapply(all_priors, function(prior) {
      shrink(
        mle = trt_effects,
        var_matrix = trt_vcov,
        hierarchical_priors = prior,
        chains = 4,
        iter = 4000,
        warmup = 1000
      )
    })

    # --- summaries ---
    sensitivity_summaries <- c(
      purrr::imap(sensitivity_fits_brms, function(fit, nm) {
        summ <- summary(fit)
        list(
          theta_summary = summ$theta,
          mu_tau_summary = summ$mu_tau,
          print_output = capture.output(print(fit))
        )
      }),
      purrr::imap(sensitivity_fits_freq, function(fit, nm) {
        summ <- summary(fit)
        list(
          theta_summary = summ$theta,
          mu_tau_summary = summ$mu_tau,
          print_output = capture.output(print(fit))
        )
      })
    )

    # --- name them clearly ---
    names(sensitivity_summaries) <- c(
      paste0(names(all_priors), "_brms"),
      paste0(names(all_priors), "_freq")
    )

### Prior densities

    tau_seq <- seq(0, 3, length.out = 200)

    prior_densities <- lapply(names(prior_specs), function(spec_name) {
      spec <- prior_specs[[spec_name]]
      tibble(
        tau = tau_seq,
        density = dnorm(tau_seq, 0, spec$scale) * 2,
        prior_strength = spec$name,
        scale = spec$scale
      )
    }) %>%
      bind_rows() %>%
      mutate(
        prior_strength = factor(prior_strength, levels = c(
          "Very Strong", "Strong", "Moderate", "Weak", "Very Weak"
        ))
      )

    ggplot(prior_densities, aes(x = tau, y = density, color = prior_strength)) +
      geom_line(linewidth = 1.2) +
      scale_color_brewer(palette = "RdYlBu", direction = -1) +
      labs(
        title = "Prior Densities for the Heterogeneity Parameter (tau)",
        subtitle = "Half-Normal(0, sigma) priors with increasing scale",
        x = "tau",
        y = "Density",
        color = "Prior Strength"
      ) +
      theme(legend.position = "right")

### Heterogeneity estimates

    tau_results <- lapply(names(sensitivity_summaries), function(fit_name) {
      summary_obj <- sensitivity_summaries[[fit_name]]
      prior_name <- sub("_(brms|freq)$", "", fit_name)
      approach <- if (grepl("_brms$", fit_name)) "brms + shrinkr" else "Frequentist + shrinkr"

      summary_obj$mu_tau_summary %>%
        filter(parameter == "tau") %>%
        mutate(
          prior_strength = prior_specs[[prior_name]]$name,
          prior_scale = prior_specs[[prior_name]]$scale,
          approach = approach
        )
    }) %>%
      bind_rows() %>%
      mutate(
        prior_strength = factor(
          prior_strength,
          levels = c("Very Strong", "Strong", "Moderate", "Weak", "Very Weak")
        )
      )

    if (all(c("q2.5", "q97.5") %in% names(tau_results))) {
      tau_results <- tau_results %>%
        mutate(lower = `q2.5`, upper = `q97.5`)
    } else if (all(c("q5", "q95") %in% names(tau_results))) {
      tau_results <- tau_results %>%
        mutate(lower = q5, upper = q95)
    } else {
      stop(
        "Could not find interval columns in sensitivity_summaries$mu_tau_summary. ",
        "Available columns are: ",
        paste(names(tau_results), collapse = ", ")
      )
    }

    ggplot(tau_results, aes(x = prior_scale, y = mean, color = approach)) +
      geom_point(size = 3, position = position_dodge(width = 0.1)) +
      geom_errorbar(
        aes(ymin = lower, ymax = upper),
        width = 0.1,
        linewidth = 1,
        position = position_dodge(width = 0.1)
      ) +
      geom_line(aes(group = approach), position = position_dodge(width = 0.1)) +
      scale_x_log10(breaks = c(0.1, 0.25, 0.5, 1.0, 2.0)) +
      scale_color_brewer(palette = "Set2") +
      labs(
        title = "Sensitivity of the Heterogeneity Parameter (tau)",
        subtitle = "How prior scale affects the estimated between-cell-type variation",
        x = "Prior Scale (log scale)",
        y = "Posterior tau",
        color = "Stage 1 Approach"
      ) +
      theme(legend.position = "bottom")

Interpretation:

- Stronger priors constrain `tau` toward smaller values and produce more shrinkage
- Weaker priors allow more between-cell-type variation
- The posterior for `tau` stabilizes as the prior becomes less restrictive

### Impact on cell type estimates

    theta_sensitivity <- lapply(names(sensitivity_summaries), function(fit_name) {
      summary_obj <- sensitivity_summaries[[fit_name]]
      prior_name <- sub("_(brms|freq)$", "", fit_name)
      approach <- if (grepl("_brms$", fit_name)) "brms + shrinkr" else "Frequentist + shrinkr"

      summary_obj$theta_summary %>%
        mutate(
          prior_strength = prior_specs[[prior_name]]$name,
          prior_scale = prior_specs[[prior_name]]$scale,
          approach = approach,
          hr_mean = exp(mean),
          hr_lower = exp(q2.5),
          hr_upper = exp(q97.5)
        )
    }) %>%
      bind_rows() %>%
      mutate(
        prior_strength = factor(prior_strength, levels = c(
          "Very Strong", "Strong", "Moderate", "Weak", "Very Weak"
        ))
      )

    ggplot(theta_sensitivity, aes(x = prior_scale, y = hr_mean, color = approach)) +
      geom_hline(yintercept = 1, linetype = "dashed", alpha = 0.5) +
      geom_point(size = 2, position = position_dodge(width = 0.1)) +
      geom_errorbar(
        aes(ymin = hr_lower, ymax = hr_upper),
        width = 0.1,
        position = position_dodge(width = 0.1)
      ) +
      geom_line(aes(group = approach), position = position_dodge(width = 0.1)) +
      facet_wrap(~group, ncol = 2, scales = "free_y") +
      scale_x_log10(breaks = c(0.1, 0.25, 0.5, 1.0, 2.0)) +
      scale_y_log10() +
      scale_color_brewer(palette = "Set2") +
      labs(
        title = "Sensitivity Analysis: Cell Type-Specific Treatment Effects",
        subtitle = "How the prior scale affects hazard ratio estimates",
        x = "Prior Scale (log scale)",
        y = "Hazard Ratio (log scale)",
        color = "Stage 1 Approach"
      ) +
      theme(
        legend.position = "bottom",
        panel.grid.minor = element_blank()
      )

## Key Takeaways

1. The two-stage `brms + shrinkr` workflow closely matches the full hierarchical `brms` analysis in this example.
2. The two-stage approach is modular: fit the survival model once, then explore many hierarchical priors efficiently.
3. Sensitivity analysis becomes straightforward because Stage 2 can be rerun without refitting Stage 1.
4. `fit_mixture()` provides a flexible approximation to the subgroup posteriors, and `shrink()` adds hierarchical regularization on top of that approximation.

## Session Info

    sessionInfo()
    #> R version 4.4.2 (2024-10-31 ucrt)
    #> Platform: x86_64-w64-mingw32/x64
    #> Running under: Windows 10 x64 (build 19045)
    #>
    #> Matrix products: default
    #>
    #>
    #> locale:
    #> [1] LC_COLLATE=C
    #> [2] LC_CTYPE=English_United States.utf8
    #> [3] LC_MONETARY=English_United States.utf8
    #> [4] LC_NUMERIC=C
    #> [5] LC_TIME=English_United States.utf8
    #>
    #> time zone: America/Chicago
    #> tzcode source: internal
    #>
    #> attached base packages:
    #> [1] stats     graphics  grDevices utils     datasets  methods   base
    #>
    #> other attached packages:
    #>  [1] patchwork_1.3.2      posterior_1.7.0      survival_3.7-0
    #>  [4] lubridate_1.9.5      forcats_1.0.1        stringr_1.6.0
    #>  [7] dplyr_1.2.1          purrr_1.2.2          readr_2.2.0
    #> [10] tidyr_1.3.2          tibble_3.3.1         ggplot2_4.0.3
    #> [13] tidyverse_2.0.0      distributional_0.7.1 tidybayes_3.0.7
    #> [16] brms_2.23.0          Rcpp_1.1.1           shrinkr_0.4.5
    #>
    #> loaded via a namespace (and not attached):
    #>  [1] tidyselect_1.2.1      svUnit_1.0.8          farver_2.1.2
    #>  [4] loo_2.9.0             S7_0.2.2              fastmap_1.2.0
    #>  [7] tensorA_0.36.2.1      digest_0.6.39         estimability_1.5.1
    #> [10] timechange_0.4.0      lifecycle_1.0.5       StanHeaders_2.32.10
    #> [13] magrittr_2.0.5        compiler_4.4.2        rlang_1.2.0
    #> [16] sass_0.4.10           tools_4.4.2           utf8_1.2.6
    #> [19] yaml_2.3.12           knitr_1.51            labeling_0.4.3
    #> [22] bridgesampling_1.2-1  pkgbuild_1.4.8        mclust_6.1.2
    #> [25] curl_7.1.0            RColorBrewer_1.1-3    abind_1.4-8
    #> [28] withr_3.0.2           grid_4.4.2            stats4_4.4.2
    #> [31] xtable_1.8-8          inline_0.3.21         emmeans_2.0.3
    #> [34] scales_1.4.0          cli_3.6.6             mvtnorm_1.4-1
    #> [37] rmarkdown_2.31        generics_0.1.4        otel_0.2.0
    #> [40] RcppParallel_5.1.11-2 rstudioapi_0.19.0     tzdb_0.5.0
    #> [43] cachem_1.1.0          rstan_2.32.7          splines_4.4.2
    #> [46] bayesplot_1.15.0      parallel_4.4.2        matrixStats_1.5.0
    #> [49] vctrs_0.7.3           V8_8.2.0              Matrix_1.7-1
    #> [52] jsonlite_2.0.0        hms_1.1.4             arrayhelpers_1.1-0
    #> [55] ggdist_3.3.3          jquerylib_0.1.4       glue_1.8.1
    #> [58] codetools_0.2-20      stringi_1.8.7         gtable_0.3.6
    #> [61] QuickJSR_1.9.2        pillar_1.11.1         htmltools_0.5.9
    #> [64] Brobdingnag_1.2-9     R6_2.6.1              evaluate_1.0.5
    #> [67] lattice_0.22-6        backports_1.5.1       bslib_0.11.0
    #> [70] rstantools_2.6.0      coda_0.19-4.1         gridExtra_2.3
    #> [73] nlme_3.1-166          checkmate_2.3.4       xfun_0.57
    #> [76] pkgconfig_2.0.3