Statistical workflow and reproducibility

Overview

This vignette describes how the statistical components of a goldilocks design fit together. Its purpose is to support protocol development, simulation reports, and independent review: the assumptions used to generate trial data, the models used for interim prediction, the decision thresholds, and the final analysis should be distinguishable and prespecified.

The package supports three related analyses:

1. sim_trials() simulates repeated trials to estimate operating characteristics;
2. survival_adapt() simulates and evaluates one adaptive trial; and
3. evaluate_interim() applies the prespecified interim rule to an observed trial data cut.

The same posterior-predictive calculation and ordered stopping rule are used for simulated and observed interim analyses.

From assumptions to operating characteristics

Data-generating assumptions

For simulation, sim_comp_data() generates enrollment times, randomized treatment assignments, event times, and loss to follow-up. The first participant enrolls at time zero; subsequent enrollment follows a piecewise-constant Poisson process. Event times follow arm-specific piecewise-exponential distributions. Dropout times are exponential and independent of event times and enrollment within each arm, with rate -log1p(-prop_loss) / end_of_study. Observed follow-up ends at the earliest event, dropout, or per-subject administrative horizon. prop_loss specifies the dropout CDF at that horizon, not the fraction actually censored by dropout; events can occur first, and dropout counts vary across trials. The technical methods vignette gives the full censoring model and its implications for binary analyses.

The event-time distribution used to simulate trials need not equal the model used for interim prediction. generation_cutpoints defines the data-generating hazard intervals, whereas cutpoints defines the intervals used for posterior estimation, predictive imputation, and Bayesian survival analysis. This separation permits sensitivity analyses for model misspecification without changing the prespecified analysis model.

Interim prediction

At look \ell, let n_\ell be the number enrolled and let N_{\max} be the maximum sample size. The observed interim data update the Gamma priors for the piecewise-exponential hazards. Posterior-predictive simulation then estimates

P_{n_\ell} = \Pr(\text{final success after follow-up of the current cohort} \mid \mathcal{D}^{\mathrm{obs}}_\ell)

and

P_{\max,\ell} = \Pr(\text{final success after enrollment to } N_{\max} \mid \mathcal{D}^{\mathrm{obs}}_\ell).

Each predictive replicate completes pending outcomes under a posterior draw of the event-time hazards and applies the prespecified completed-data analysis. The proportions of successful replicates estimate P_{n_\ell} and P_{\max,\ell}.

The ordered decision rule is

d_\ell = \begin{cases} \text{declare immediate success}, & \widehat{P}_{n_\ell} > Q_\ell, \\ \text{stop accrual for expected success}, & S_\ell < \widehat{P}_{n_\ell} \le Q_\ell, \\ \text{declare binding futility}, & \widehat{P}_{n_\ell} \le S_\ell \text{ and } \widehat{P}_{\max,\ell} < F_\ell, \\ \text{continue enrollment}, & \text{otherwise.} \end{cases}

The package requires Q_\ell \ge S_\ell. With the default Q_\ell = 1, immediate success is disabled. All comparisons are strict, so equality with a boundary does not cross it.

N_impute controls the number of posterior-predictive replicates. For Bayesian completed-data analyses, N_mcmc controls the posterior draws within each replicate. The reported Monte Carlo standard errors and exact binomial bounds describe numerical uncertainty in the predictive probabilities; the interim decision itself uses the point estimate.

For fixed-horizon binary analyses, event counts and denominators by treatment arm are sufficient statistics. Carrying these sufficient statistics into the completed-data analysis gives the same risk-difference or beta-binomial analysis as participant-level endpoint records.

Bayesian survival calculation

Let d_{aj} and y_{aj} denote the observed event count and person-time in arm a and interval j. With the independent prior

\lambda_{aj} \sim \operatorname{Gamma}(\alpha_{0aj},\beta_{0aj}),

Gamma-exponential conjugacy gives

\lambda_{aj} \mid \mathcal{D}^{\mathrm{obs}}_\ell \sim \operatorname{Gamma}( \alpha_{0aj}+d_{aj}, \beta_{0aj}+y_{aj} ).

For analysis cut-points 0<c_1<\cdots<c_{J-1}<\tau, define interval widths

\mathbf{w} = (c_1,c_2-c_1,\ldots,\tau-c_{J-1}).

A posterior hazard draw implies cumulative hazard and event probability

H_a(\tau)=\sum_{j=1}^{J}\lambda_{aj}w_j, \qquad p_a(\tau)=1-\exp\{-H_a(\tau)\}.

These widths span the fixed endpoint horizon end_of_study; they are not shortened to the longest follow-up observed at an interim look. If no participant has contributed information to a later interval, empty_interval = "prior" leaves its posterior equal to its prior. This makes prior-predictive assessment particularly important when little late follow-up is expected at early looks.

Completed-data analysis

The completed-data method determines whether each predictive replicate, and when required the final trial data, meets the success criterion.

method	Estimand and analysis	With imputation at the final analysis	Without final imputation
bayes-surv	Posterior treatment-minus-control event probability, or the treatment event probability in a single-arm design	Average posterior summaries across completed imputations	Analyze observed right-censored follow-up
bayes-bin	Beta-binomial posterior for fixed-horizon event status	Average posterior summaries across completed imputations	Exclude participants without complete endpoint ascertainment
rmst	Treatment-minus-control RMST through fixed rmst_tau, using a Wald test	Pool differences and Greenwood variances using Rubin’s rules	Retain censoring; require support through the fixed horizon
cox	Log hazard ratio from a Cox model	Pool estimates and variances using Rubin’s rules	Analyze observed right-censored follow-up
riskdiff-wald	Treatment-minus-control event-risk difference using a Wald test	Pool estimates and variances using Rubin’s rules	Exclude participants without complete endpoint ascertainment
riskdiff-fm	Treatment-minus-control event-risk difference using a Farrington-Manning score test	Not available when outcomes are missing: no validated FM pooling rule is implemented	Exclude participants without complete endpoint ascertainment
logrank	Difference between survival distributions using a log-rank test	Not available because an imputation-pooling rule has not been specified	Analyze observed right-censored follow-up

For methods that accept imputed_final = TRUE, complete final outcomes use the selected test directly. FM simulations with this flag require zero dropout in both arms. Rubin pooling requires at least two imputations and positive total variance; zero total variance is non-estimable.

For a frequentist method, the success measure is 1-p; for a Bayesian method, it is the posterior probability of the prespecified alternative. In both cases, success requires the measure to be strictly greater than prob_ha.

The imputation model and completed-data analysis model are deliberately distinct. For example, method = "bayes-bin" uses the piecewise-exponential model to impute endpoint status for participants whose endpoint is pending, then applies a beta-binomial model to the completed binary outcomes. Both sets of assumptions should therefore be examined in sensitivity analyses.

Summaries for design evaluation

For one simulated trial or an observed data cut, the interim decision history shows the two predictive probabilities, their Monte Carlo uncertainty, the three decision thresholds, and the action at each completed look. For repeated simulations, summarise_sims() estimates power or type I error, probabilities of each stopping outcome, sample-size summaries, and Monte Carlo uncertainty. summarise_calendar_time() adds trial duration, accrual duration, analysis readiness, and follow-up burden.

The planning functions prop_to_haz() and ppwe() connect clinically interpretable event probabilities to piecewise-exponential hazards. plot_enrollment() displays the expected enrollment trajectory and planned interim milestones. These quantities are useful checks that the numerical design corresponds to the assumptions intended for the protocol.

Prespecification and reproducibility

Several conventions should be stated explicitly in a simulation report or statistical analysis plan:

• treatment = 0 denotes control and treatment = 1 denotes treatment;
• named arm-specific inputs use control, then treatment;
• method determines the effect scale for h0 and the direction of alternative; for RMST, benefit is a positive difference in time units;
• rmst_tau is fixed in advance for RMST and cannot exceed end_of_study; a shorter restriction time does not shorten the follow-up schedule;
• generation_cutpoints governs event-time generation, whereas cutpoints governs prediction and analysis;
• analysis intervals use the survival counting-process convention (\text{start},\text{stop}] when assigning observed events;
• Qn, Sn, Fn, prob_ha, N_impute, and N_mcmc are part of the prespecified decision algorithm; and
• a recorded simulation seed permits exact reproduction of the Monte Carlo study across the supported computing options.

Operating characteristics should be evaluated over clinically plausible null and alternative scenarios, including nuisance parameters that may affect the amount of information available at interim looks. Numerical Monte Carlo error should be reported alongside every estimated operating characteristic.

Implementation map

The internal R/analysis_*.R files group the completed-data methods and the analysis stages in one place. Method files use the method value in the name, with underscores replacing hyphens. The two risk-difference methods share one file because they use the same event counts, effect estimate, and supporting calculations.

Method or responsibility	Source file
logrank	R/analysis_logrank.R
cox	R/analysis_cox.R
rmst	R/analysis_rmst.R
bayes-surv	R/analysis_bayes_surv.R
bayes-bin	R/analysis_bayes_bin.R
riskdiff-wald, riskdiff-fm	R/analysis_riskdiff.R
Shared tests, pooling, and binary endpoint validation	R/analysis_common.R
Completed-data dispatch	R/analysis_completed.R
Predictively completed trial analysis	R/analysis_predictive.R
Interim decision rules	R/analysis_interim.R
Final analysis and imputation orchestration	R/analysis_final.R

Public entry points retain their function names: survival_adapt.R, sim_trials.R, and evaluate_interim.R. Hazard posterior calculations and imputation are shared across analysis methods, so their files retain names such as survival_posterior.R and predictive_imputation.R.