Applying predictive decisions to an observed interim data cut

survival_adapt() simulates a complete adaptive trial. evaluate_interim() instead applies the same predictive decision calculation to one observed interim data cut. It does not generate event, censoring, or enrollment times, and it does not retain a participant-level copy of the input in its result.

Preparing the data cut

The input has one row for every enrolled participant and uses a common time unit throughout. Calendar-time enrollment is measured from first participant randomization at time zero. Subject-level time is measured from that participant’s randomization.

The status values make the reason for incomplete follow-up explicit:

`status`	Interpretation
`event`	The endpoint event has been observed.
`complete`	Event-free follow-up reached `end_of_study`.
`pending`	The participant remains under follow-up.
`censored`	Follow-up ended early without an event.

A blinded data-management team can prepare enrollment, follow-up, event, and status fields without access to treatment assignments:

blinded_cut <- data.frame(
  id = 1:8,
  enrollment = 0:7,
  time = c(8, 7, 6, 5, 4, 3, 2, 1),
  event = c(1, 0, 0, 1, 0, 0, 0, 0),
  status = c(
    "event", "pending", "censored", "event",
    "pending", "pending", "pending", "pending"
  )
)

blinded_cut
#>   id enrollment time event   status
#> 1  1          0    8     1    event
#> 2  2          1    7     0  pending
#> 3  3          2    6     0 censored
#> 4  4          3    5     1    event
#> 5  5          4    4     0  pending
#> 6  6          5    3     0  pending
#> 7  7          6    2     0  pending
#> 8  8          7    1     0  pending

The calculation itself is arm-specific. An independent unblinded statistician or service therefore joins the treatment assignment before running it. The participant-level input need not be distributed with the aggregate result.

randomization_export <- data.frame(
  id = 1:8,
  treatment = c(0, 1, 0, 1, 0, 1, 0, 1)
)

interim_cut <- blinded_cut
interim_cut$treatment <- randomization_export$treatment[
  match(interim_cut$id, randomization_export$id)
]
interim_cut <- interim_cut[
  c("id", "treatment", "enrollment", "time", "event", "status")
]

Evaluating the look

Suppose this is the second planned look of a trial with maximum sample size 12, equal randomization, and ten time units of follow-up per participant:

interim_result <- evaluate_interim(
  data = interim_cut,
  data_cut = 8,
  look = 2,
  N_total = 12,
  end_of_study = 10,
  rand_ratio = c(control = 1, treatment = 1),
  method = "logrank",
  alternative = "less",
  Fn = 0.05,
  Sn = 0.90,
  Qn = 1,
  prob_ha = 0.95,
  N_impute = 20,
  seed = 20260831
)

interim_result
#> Goldilocks interim evaluation
#> Look: 2 (N = 8)
#> Decision: continue
#> Predictive success if accrual stops now: 0
#> Predictive success at maximum sample size: 0.2
interim_result$decision
#>   look planned_N calendar_time decision                 decision_reason
#> 1    2         8             8 continue continue_thresholds_not_crossed
#>   ppp_stop_now success_threshold immediate_success_threshold
#> 1            0               0.9                           1
#>   immediate_success_crossed expected_success_crossed ppp_success_at_max
#> 1                     FALSE                    FALSE                0.2
#>   futility_threshold futility_crossed
#> 1               0.05            FALSE
interim_result$monte_carlo
#>              estimand successes draws estimate       mcse      lower     upper
#> 1 success_if_stop_now         0    20      0.0 0.00000000 0.00000000 0.1391083
#> 2  success_at_maximum         4    20      0.2 0.08944272 0.07135388 0.4010281
#>   threshold direction point_crossed bound_crossed
#> 1      0.90   greater         FALSE         FALSE
#> 2      0.05      less         FALSE         FALSE

Here Qn = 1 disables an immediate success claim. Crossing Sn stops accrual for expected success while enrolled participants continue follow-up. The method, alternative, and thresholds must match the prespecified trial design.

At the maximum sample size, equal randomization implies six participants per arm. Four have already accrued in each arm, so the predictive calculation includes two potential future participants per arm:

interim_result$diagnostics$target_allocation
#>   control treatment 
#>         6         6
interim_result$diagnostics$current_allocation
#>   control treatment 
#>         4         4
interim_result$diagnostics$potential_accruals
#>   control treatment 
#>         2         2

No block size or future randomization sequence is needed. The design must have an exactly allocable maximum sample size: for example, a 1:2 ratio requires N_total to be divisible by three. Observed enrollment in either arm cannot already exceed its implied maximum.

Using separate predictive and analysis priors

Suppose a sponsor wants external evidence to inform the prediction of future events, while the Bayesian survival success test uses a diffuse prior. Supply both prior_surv and prior_surv_final. The latter is used now, inside each interim predictive replicate, as well as at the actual final analysis.

Step in one interim predictive replicate	Prior used
Update observed events and exposure, draw hazards, and generate outstanding outcomes	`prior_surv`
Analyze that hypothetical completed trial and compare its posterior probability with `prob_ha`	`prior_surv_final`

Repeat these steps and calculate the fraction of completed trials that pass. The same pair of priors is used for current-sample and maximum-sample predictions. Each completed-data analysis starts from the specified analysis prior, rather than using the prediction posterior as a new prior.

For this illustration, interpret the data’s time units as months. The predictive Gamma priors below have mean hazards of 0.05 and 0.03 per month for control and treatment. Their rate parameters use patient-months. These values are illustrative; the actual predictive prior needs justification from the external evidence and a prior-predictive assessment.

predictive_prior <- list(
  control = c(shape = 10, rate = 200),
  treatment = c(shape = 6, rate = 200)
)
analysis_prior <- c(shape = 0.1, rate = 0.1)

bayes_prior_result <- evaluate_interim(
  data = interim_cut,
  data_cut = 8,
  look = 2,
  N_total = 12,
  end_of_study = 10,
  method = "bayes-surv",
  alternative = "less",
  h0 = 0,
  prior_surv = predictive_prior,    # Generates outstanding outcomes
  prior_surv_final = analysis_prior, # Tests each hypothetical completed trial
  Fn = 0.05,
  Sn = 0.90,
  Qn = 1,
  prob_ha = 0.975,
  N_impute = 100,
  N_mcmc = 1000,
  seed = 20260909
)

knitr::kable(
  bayes_prior_result$diagnostics$prior[
    c("stage", "arm", "shape", "rate", "mean_hazard")
  ],
  digits = 3,
  caption = "Gamma priors used in the two parts of this interim calculation."
)

Gamma priors used in the two parts of this interim calculation.
stage	arm	shape	rate	mean_hazard
interim	control	10.0	200.0	0.05
interim	treatment	6.0	200.0	0.03
final	control	0.1	0.1	1.00
final	treatment	0.1	0.1	1.00

bayes_prior_result$probabilities
#>              estimand probability threshold direction threshold_crossed
#> 1 success_if_stop_now        0.00      0.90   greater             FALSE
#> 2  success_at_maximum        0.03      0.05      less              TRUE

Here the rows labelled final describe the analysis prior already used to test the hypothetical completed datasets. diagnostics$posterior describes the predictive posterior based on the observed interim data.

Omitting prior_surv_final would also use predictive_prior in the success tests. The package does not automatically replace an informative predictive prior with a weak analysis prior. The two priors need not be distinct; the default intentionally uses one prior for both roles.

Use these same two arguments in survival_adapt() or sim_trials() when calibrating the design. The small Monte Carlo sizes here demonstrate the API; precision and operating characteristics must be assessed for the deployed design. At the actual final analysis, prior_surv_final is also used for any optional imputation of missing outcomes. Even with a separate analysis prior, predictive borrowing can affect the stopping decision and sample size.

This example concerns method = "bayes-surv". With method = "bayes-bin", the completed-data success tests use prior_bin at both interim and final; prior_surv_final controls only optional final imputation.

Using a prespecified RMST endpoint

For a design that prespecifies RMST as its analysis, supply method = "rmst" and the fixed restriction time. The same illustrative data cut can demonstrate this separate design, targeting treatment-minus-control RMST through six time units while follow-up continues through ten:

rmst_result <- evaluate_interim(
  data = interim_cut,
  data_cut = 8,
  look = 2,
  N_total = 12,
  end_of_study = 10,
  rand_ratio = c(control = 1, treatment = 1),
  method = "rmst",
  rmst_tau = 6,
  alternative = "greater",
  h0 = 0,
  Fn = 0.05,
  Sn = 0.90,
  Qn = 1,
  prob_ha = 0.95,
  N_impute = 20,
  seed = 20260908
)

rmst_result$decision
#>   look planned_N calendar_time      decision                   decision_reason
#> 1    2         8             8 stop_futility futility_estimate_below_threshold
#>   ppp_stop_now success_threshold immediate_success_threshold
#> 1            0               0.9                           1
#>   immediate_success_crossed expected_success_crossed ppp_success_at_max
#> 1                     FALSE                    FALSE                  0
#>   futility_threshold futility_crossed
#> 1               0.05             TRUE
rmst_result$monte_carlo
#>              estimand successes draws estimate mcse lower     upper threshold
#> 1 success_if_stop_now         0    20        0    0     0 0.1391083      0.90
#> 2  success_at_maximum         0    20        0    0     0 0.1391083      0.05
#>   direction point_crossed bound_crossed
#> 1   greater         FALSE         FALSE
#> 2      less          TRUE         FALSE
rmst_result$metadata$design[c("method", "rmst_tau", "alternative", "h0")]
#> $method
#> [1] "rmst"
#> 
#> $rmst_tau
#> [1] 6
#> 
#> $alternative
#> [1] "greater"
#> 
#> $h0
#> [1] 0

Positive effects mean longer event-free time on treatment, so benefit uses alternative = "greater". Both h0 and rmst_tau use the data’s time units. The horizon remains fixed across looks and predictive imputations. Input status = "complete" still requires event-free follow-up to end_of_study, even when a participant has already reached rmst_tau. The RMST vignette explains the final analysis and its support and variance requirements. These small Monte Carlo examples demonstrate the API; the deployed design needs its own calibration and precision assessment.

Reviewing and retaining the audit trail

The decision uses the Monte Carlo point estimates. The standard errors and exact one-sided bounds describe finite-draw uncertainty but do not change the decision. The interim record contains the same quantities retained for a simulated adaptive trial and can be summarized or plotted in the same way:

summarise_trial_trace(interim_result)
#>   interim_looks_completed last_look last_decision final_N final_post_prob_ha
#> 1                       1         2      continue      NA                 NA
#>   ppp_stop_now ppp_success_at_max warning_count
#> 1            0                0.2             0
interim_result$trace
#>   look planned_N calendar_time active_followup N_enrolled N_treatment N_control
#> 1    2         8             8               5          8           4         4
#>   events_treatment events_control N_pending N_not_enrolled ppp_stop_now
#> 1                1              1         6              4            0
#>   ppp_stop_now_mcse ppp_stop_now_lower ppp_stop_now_upper ppp_stop_now_draws
#> 1                 0                  0          0.1391083                 20
#>   success_threshold immediate_success_threshold immediate_success_crossed
#> 1               0.9                           1                     FALSE
#>   expected_success_crossed ppp_success_at_max ppp_success_at_max_mcse
#> 1                    FALSE                0.2              0.08944272
#>   ppp_success_at_max_lower ppp_success_at_max_upper ppp_success_at_max_draws
#> 1               0.07135388                0.4010281                       20
#>   futility_threshold futility_crossed inner_mc_uncertain_stop_now
#> 1               0.05            FALSE                           0
#>   inner_mc_uncertain_success_at_max decision                 decision_reason
#> 1                                 0 continue continue_thresholds_not_crossed
#>   empty_interval_fallback_count empty_interval_fallbacks warning_count
#> 1                             0                                      0
#>   warning_messages
#> 1

The saved audit information records the evaluated design, package version, data cut, time-origin convention, and simulation seed. Supplying seed makes the predictive calculation exactly reproducible.