# Meta-Analytic-Predictive (MAP) Priors with shrinkr and beastt

#### Jacob M. Maronge

## Overview

A meta-analytic-predictive (MAP) prior summarizes several historical control arms into a single prior for the control mean of a new trial (Neuenschwander et al., 2010). It is a random-effects meta-analysis: study-specific control means are treated as exchangeable draws from a common population, and the prior for the next trial is the posterior predictive distribution for a new, as-yet-unobserved study. To guard against prior-data conflict, the MAP is then robustified with a vague mixture component (Schmidli et al., 2014), so the historical data are automatically down-weighted when they disagree with the current trial.

`shrinkr` and `beastt` split this work cleanly:

- `shrinkr` runs the hierarchical meta-analysis across the historical studies and returns the MAP as a `distributional` object.
- `beastt` robustifies that MAP (`robustify_norm()`), combines it with the internal control arm to form the posterior (`calc_post_norm()`), and reports the effective sample size (ESS).

The hand-off between them is a single `dist_normal` object. We use a continuous outcome with known within-arm SD, which keeps every step conjugate.

    library(shrinkr)
    library(beastt)
    library(distributional)
    library(tibble)
    library(ggplot2)

    set.seed(1104)
    sigma_known <- 2   # known within-arm response SD (same in all arms)

## The historical evidence

A MAP prior starts from what the historical studies reported: a control-arm mean and its standard error. Here we have six prior studies whose control means cluster fairly tightly around 10. With a known SD, each standard error is just `sigma / sqrt(n)`.

    n_hist <- c(58, 43, 62, 51, 47, 55)
    hist <- tibble(
      study = paste0("H", 1:6),
      n     = n_hist,
      ybar  = c(9.9, 10.3, 9.7, 10.1, 10.4, 9.8),
      se    = sigma_known / sqrt(n_hist)
    )
    hist
    #> # A tibble: 6 × 4
    #>   study     n  ybar    se
    #>   <chr> <dbl> <dbl> <dbl>
    #> 1 H1       58   9.9 0.263
    #> 2 H2       43  10.3 0.305
    #> 3 H3       62   9.7 0.254
    #> 4 H4       51  10.1 0.280
    #> 5 H5       47  10.4 0.292
    #> 6 H6       55   9.8 0.270

    ggplot(hist, aes(study, ybar)) +
      geom_hline(yintercept = mean(hist$ybar), linetype = "dashed", color = "grey50") +
      geom_pointrange(aes(ymin = ybar - 1.96 * se, ymax = ybar + 1.96 * se),
                      color = "steelblue", linewidth = 0.8) +
      labs(x = NULL, y = "Control mean", title = "Historical control arms") +
      theme_minimal(base_size = 12)

The current trial contributes its own control arm of 70 patients:

    n_int    <- 70
    int_ctrl <- tibble(y = rnorm(n_int, mean = 10, sd = sigma_known))

## Hierarchical meta-analysis with `shrinkr`

With a known SD and a flat Stage-1 prior, each study’s posterior for its control mean is exactly `N(ybar, se^2)` — so the reported summaries are the Stage-1 result, and we can hand them straight to `shrink()`. The hierarchical model is

\[ \hat\theta_g \mid \theta_g \sim N(\theta_g,\, se_g^2), \qquad \theta_g \mid \mu, \tau \sim N(\mu, \tau^2), \]

with a vague prior on the population mean `mu` and a weakly-informative half-normal on the between-study SD `tau`.

    hierarchical_priors <- list(
      mu  = dist_normal(0, 100),
      tau = dist_truncated(dist_normal(0, sigma_known / 4), lower = 0)
    )

It is worth checking what that `tau` prior implies about differences between study means before fitting. `sample_prior_predictive()` draws study effects and `prior_pairwise_differences()` summarizes the implied `|theta_i - theta_j|` (the location `mu` cancels, so this isolates heterogeneity even though `mu` is vague).

    prior_pred <- sample_prior_predictive(hierarchical_priors,
                                          n_groups = nrow(hist), n_draws = 2000)
    plot(prior_pairwise_differences(prior_pred))

If that spread looks unreasonable on the clinical scale, adjust the `tau` prior now. Then fit, passing the study summaries through `shrink()`’s `mle` / `var_matrix` interface.

    fit_map <- shrink(
      mle                 = hist$ybar,
      var_matrix          = hist$se^2,
      hierarchical_priors = hierarchical_priors,
      chains = 4, iter = 4000, warmup = 1000,
      seed = 2026, refresh = 0, verbose = FALSE
    )

    summarize_mu_tau(fit_map)
    #> # A tibble: 3 × 9
    #>   parameter      mean     sd      q2.5     q50  q97.5  rhat ess_bulk ess_tail
    #>   <chr>         <dbl>  <dbl>     <dbl>   <dbl>  <dbl> <dbl>    <dbl>    <dbl>
    #> 1 mu          10.0    0.147  9.73      10.0    10.3    1.00    6275.    3440.
    #> 2 tau          0.195  0.151  0.00793    0.163   0.571  1.00    4048.    5659.
    #> 3 tau_squared  0.0606 0.0949 0.0000629  0.0265  0.326  1.00    4048.    5659.

## Building the MAP prior

The MAP is the posterior predictive distribution for a new study’s control mean. Marginalizing over `(mu, tau)`, the Normal approximation has mean `E[mu]` and variance `Var(mu) + E[tau^2]` — the second term is the predictive spread from heterogeneity, which is what keeps a MAP honestly wider than a simple pooled mean.

    make_map <- function(fit) {
      d <- extract_mu_tau(fit)
      dist_normal(mean(d$mu), sqrt(var(d$mu) + mean(d$tau_squared)))
    }

    map_prior <- make_map(fit_map)
    map_prior
    #> <distribution[1]>
    #> [1] N(10, 0.082)

A convenient way to read the MAP’s strength is its prior effective sample size: for a Normal prior on a mean with known SD, that is `sigma^2 / Var(prior)` — how many control patients the prior is “worth”.

    map_ess <- sigma_known^2 / variance(map_prior)
    map_ess
    #> [1] 48.65804

## Robustify and form the posterior with `beastt`

`robustify_norm()` mixes the MAP (“informative”) with a vague component so the data can overrule the prior under conflict. Passing the MAP’s prior ESS as `n` makes the vague component a unit-information prior (variance `sigma^2`); we put equal weight on the two components.

    rmp         <- robustify_norm(map_prior, n = map_ess, weights = c(0.5, 0.5))
    vague_prior <- dist_normal(mix_means(rmp)[["vague"]], mix_sigmas(rmp)[["vague"]])

    plot_dist("MAP (informative)" = map_prior,
              "Vague component"   = vague_prior,
              "Robust mixture"    = rmp)

Now combine the robust mixture with the internal control arm. With a known SD the posterior is again a mixture of normals, and `beastt` updates the mixture weights automatically — down-weighting the informative component if the internal data disagree with it. The no-borrowing reference simply uses the vague component alone.

    post_borrow <- calc_post_norm(int_ctrl, response = y,
                                  prior = rmp, internal_sd = sigma_known)
    post_nobrrw <- calc_post_norm(int_ctrl, response = y,
                                  prior = vague_prior, internal_sd = sigma_known)

    plot_dist("No borrowing"           = post_nobrrw,
              "Borrowing (robust MAP)" = post_borrow)

The effective sample size compares posterior variance with and without borrowing (Pennello & Thompson, 2008): a borrowed posterior with variance `Vb` is as informative as `n_int * V0 / Vb` patients.

    ess_post <- n_int * variance(post_nobrrw) / variance(post_borrow)

    tibble(
      quantity     = c("Posterior mean", "Posterior SD", "Effective sample size"),
      no_borrowing = round(c(mean(post_nobrrw), sqrt(variance(post_nobrrw)), n_int), 2),
      robust_map   = round(c(mean(post_borrow), sqrt(variance(post_borrow)), ess_post), 2)
    )
    #> # A tibble: 3 × 3
    #>   quantity              no_borrowing robust_map
    #>   <chr>                        <dbl>      <dbl>
    #> 1 Posterior mean                9.66       9.77
    #> 2 Posterior SD                  0.24       0.21
    #> 3 Effective sample size        70         93.2

Because the historical and internal data are compatible, the robust MAP sharpens the control posterior and lifts the effective sample size well above the 70 internal controls. Under a prior-data conflict the informative component would lose weight and that gain would shrink toward zero — the self-correcting behavior the robust mixture is there to provide.

## Summary

- `shrinkr` runs the hierarchical meta-analysis and builds the MAP as a `dist_normal`, taking study summaries through the `mle` / `var_matrix` interface.
- `beastt` robustifies it (`robustify_norm()`), forms the internal control posterior (`calc_post_norm()`), and reports the ESS.
- Check the heterogeneity prior with `sample_prior_predictive()` / `prior_pairwise_differences()` before fitting, and lean on the robust mixture so the data can overrule the historical evidence when they disagree.

## References

Neuenschwander, B., Capkun-Niggli, G., Branson, M., & Spiegelhalter, D. J. (2010). Summarizing historical information on controls in clinical trials. Clinical Trials, 7(1), 5–18.

Schmidli, H., Gsteiger, S., Roychoudhury, S., O’Hagan, A., Spiegelhalter, D., & Neuenschwander, B. (2014). Robust meta-analytic-predictive priors in clinical trials with historical control information. Biometrics, 70(4), 1023–1032.

Pennello, G., & Thompson, L. (2008). Experience with reviewing Bayesian medical device trials. Journal of Biopharmaceutical Statistics, 18(1), 81–115.