# Futility and harm bounds for overall survival monitoring

#### Keaven Anderson

## Introduction

When clinical trials include overall survival (OS) as a secondary or exploratory endpoint, regulators may recommend not only monitoring for early evidence of efficacy and futility, but also for potential harm — that is, evidence that the experimental treatment may be worsening survival relative to control. This article demonstrates how the gsDesign package supports group sequential designs with three boundaries: an efficacy (upper) bound, a futility (lower) bound, and a harm bound, using `test.type = 7` (binding) and `test.type = 8` (non-binding).

### Regulatory context: FDA guidance on OS monitoring in oncology

The FDA draft guidance Assessment of Overall Survival Evidence in Support of Accelerated Approval of Oncology Therapeutics (U.S. Food and Drug Administration 2024) describes expectations for monitoring OS in the context of trials that may receive accelerated approval based on surrogate endpoints. The guidance states that sponsors should specify pre-planned boundaries for interim OS monitoring, including criteria for stopping a trial early if there is evidence of a detrimental effect on OS. Key points include:

- Sponsors should include a pre-specified statistical analysis plan for interim OS analyses, including the timing and number of interim looks.
- At a minimum, the guidance expects monitoring for OS harm (i.e., a detrimental trend in overall survival) using pre-specified boundaries.
- Separate from the harm boundary, the sponsor should establish a futility boundary to stop the trial if the experimental treatment is unlikely to demonstrate an OS benefit.
- The statistical plan should describe the spending functions used for each boundary and how the overall Type I error and Type II error are controlled.

This motivates the design framework with `test.type = 7` (binding futility and harm bounds) and `test.type = 8` (non-binding futility and harm bounds), where three boundaries are simultaneously specified using spending functions.

The harm bound implemented in gsDesign is a new method that is easy to use — a principled, straightforward extension of the widely used group sequential spending function framework. While we believe this approach is understandable, useful, and flexible, other methods for monitoring potential harm may also be considered. However, there are limitations with this approach. The example presented here has higher mortality risk than many cases. With lower mortality risk, modifications of this approach or other approaches may be preferable.

### Design framework overview

In a standard two-sided asymmetric group sequential design (`test.type = 3` or `4`), there are two boundaries:

- Efficacy (upper) bound: Reject \(H_0\) if the test statistic exceeds this boundary (evidence of treatment benefit).
- Futility (lower) bound: Stop for futility if the test statistic falls below this boundary (insufficient evidence of treatment benefit).

The harm bound extension (`test.type = 7` or `8`) adds a third boundary:

- Harm bound: Signal that the experimental treatment may be harming patients (evidence of a detrimental effect).

The harm bound lies below the futility bound. At each analysis, there are four possible outcomes:

1. Cross the efficacy bound (above): Stop for efficacy.
2. Between the efficacy and futility bounds: Continue the trial.
3. Cross the futility bound but not the harm bound (between futility and harm): Stop for futility.
4. Cross the harm bound (below): Stop for harm.

The harm bound is intended so that if a small observed p-value favoring control is observed, the harm bound will be crossed. That is, the harm bound flags evidence that the experimental treatment may be worsening survival — a negative treatment effect on the log hazard ratio scale.

## Design with non-binding bounds (`test.type = 8`)

We demonstrate a survival design using `gsSurvCalendar()` with `test.type = 8` (non-binding futility and harm bounds). The scenario is based on a 1:1 randomized trial monitoring overall survival with:

- Median control survival: 3 years (36 months), i.e., \(\lambda_C = \log(2)/36\).
- Target hazard ratio: HR = 0.75 (25% reduction in hazard).
- Power: 90% (\(\beta = 0.1\)).
- One-sided \(\alpha\): 0.0125 (e.g., the OS component of a trial with multiplicity adjustment).
- Enrollment: Uniform enrollment over 18 months.
- Study duration: 5 years (60 months) with planned analyses at years 1, 2, 3, 4, and 5 from start of enrollment.

The `astar` parameter controls the total spending for the harm bound under \(H_0\). We set `astar = 0.1`, meaning the total probability of crossing the harm bound under \(H_0\) is 10%.

### Spending function specification

We specify:

- Efficacy bound: Lan-DeMets O’Brien-Fleming (`sfLDOF`) spending function (conservative, spending little \(\alpha\) at early analyses).
- Futility bound: Hwang-Shih-DeCani (HSD) spending function with \(\gamma = -2\) (moderate \(\beta\)-spending under \(H_1\)).
- Harm bound: Lan-DeMets Pocock (`sfLDPocock`) spending function (spending under \(H_0\) for detecting harm).

    x8 <- gsSurvCalendar(
      test.type = 8,
      alpha = 0.0125,
      beta = 0.1,
      astar = 0.1,
      calendarTime = c(12, 24, 36, 48, 60),
      sfu = sfLDOF,
      sfl = sfHSD, sflpar = -2,
      sfharm = sfLDPocock,
      lambdaC = log(2) / 36,
      hr = 0.75,
      R = 18,
      minfup = 42
    )

### Summary

The `summary()` method provides a concise description of the design:

    cat(strwrap(summary(x8), width = 65), sep = "\n")
    #> Asymmetric two-sided group sequential design with non-binding
    #> futility and harm bounds, 5 analyses, time-to-event outcome with
    #> sample size 1148 and 657 events required, 90 percent power, 1.25
    #> percent (1-sided) Type I error to detect a hazard ratio of 0.75.
    #> Enrollment and total study durations are assumed to be 18 and 60
    #> months, respectively. Efficacy bounds derived using a Lan-DeMets
    #> O'Brien-Fleming approximation spending function (no parameters).
    #> Futility bounds derived using a Hwang-Shih-DeCani spending
    #> function with gamma = -2. Harm bounds derived using a Lan-DeMets
    #> Pocock approximation spending function.

### Detailed boundary table

The `gsBoundSummary()` function produces a tabular summary with columns for each boundary. By default, `B-value`, `Spending`, `CP`, `CP H1`, and `PP` are excluded. We note that for the first interim analysis, the efficacy bound is so extreme it is effectively impossible to cross. However, the harm and futility bounds are more moderate, allowing for early stopping if there is evidence of harm or futility. The futility bound is an indicator of why bounds are often non-binding — the futility bound is not intended to be a strict stopping rule, but rather a signal that the trial may be unlikely to succeed if it continues. Crossing the harm bound is a stronger indication that the treatment may be harmful, and the trial should be at least paused with a recommendation to review the safety and other endpoint data.

    gsBoundSummary(x8)
    #> Method: LachinFoulkes
    #>     Analysis               Value    Harm Futility Efficacy
    #>    IA 1: 11%                   Z -2.1121  -1.4408       NA
    #>       N: 766         p (1-sided)  0.9827   0.9252       NA
    #>   Events: 73        ~HR at bound  1.6434   1.4034       NA
    #>    Month: 12    P(Cross) if HR=1  0.0173   0.0748       NA
    #>              P(Cross) if HR=0.75  0.0004   0.0039       NA
    #>    IA 2: 38%                   Z -1.7667   0.1212   3.8622
    #>      N: 1148         p (1-sided)  0.9614   0.4518   0.0001
    #>  Events: 253        ~HR at bound  1.2491   0.9849   0.6149
    #>    Month: 24    P(Cross) if HR=1  0.0507   0.5554   0.0001
    #>              P(Cross) if HR=0.75  0.0004   0.0181   0.0574
    #>    IA 3: 63%                   Z -1.7256   1.0566   2.9347
    #>      N: 1148         p (1-sided)  0.9578   0.1454   0.0017
    #>  Events: 416        ~HR at bound  1.1846   0.9015   0.7497
    #>    Month: 36    P(Cross) if HR=1  0.0736   0.8641   0.0017
    #>              P(Cross) if HR=0.75  0.0004   0.0398   0.4990
    #>    IA 4: 83%                   Z -1.7170   1.7357   2.5278
    #>      N: 1148         p (1-sided)  0.9570   0.0413   0.0057
    #>  Events: 548        ~HR at bound  1.1580   0.8622   0.8057
    #>    Month: 48    P(Cross) if HR=1  0.0890   0.9631   0.0062
    #>              P(Cross) if HR=0.75  0.0004   0.0675   0.7996
    #>        Final                   Z -1.7149   2.3072   2.3072
    #>      N: 1148         p (1-sided)  0.9568   0.0105   0.0105
    #>  Events: 657        ~HR at bound  1.1433   0.8352   0.8352
    #>    Month: 60    P(Cross) if HR=1  0.1000   0.9888   0.0112
    #>              P(Cross) if HR=0.75  0.0004   0.1000   0.9000

Conditional power (CP, CP H1) and predictive power (PP) can also be included in the summary. Below we show the full table with all statistics, including conditional and predictive power at each boundary:

    gsBoundSummary(x8, exclude = c())
    #> Method: LachinFoulkes
    #>     Analysis               Value    Harm Futility Efficacy
    #>    IA 1: 11%                   Z -2.1121  -1.4408       NA
    #>       N: 766         p (1-sided)  0.9827   0.9252       NA
    #>   Events: 73        ~HR at bound  1.6434   1.4034       NA
    #>    Month: 12            Spending  0.0173   0.0039       NA
    #>                          B-value -0.7011  -0.4782       NA
    #>                               CP  0.0000   0.0000       NA
    #>                            CP H1  0.4619   0.5942       NA
    #>                               PP  0.0011   0.0097       NA
    #>                 P(Cross) if HR=1  0.0173   0.0748       NA
    #>              P(Cross) if HR=0.75  0.0004   0.0039       NA
    #>    IA 2: 38%                   Z -1.7667   0.1212   3.8622
    #>      N: 1148         p (1-sided)  0.9614   0.4518   0.0001
    #>  Events: 253        ~HR at bound  1.2491   0.9849   0.6149
    #>    Month: 24            Spending  0.0334   0.0143   0.0001
    #>                          B-value -1.0954   0.0751   2.3947
    #>                               CP  0.0000   0.0024   1.0000
    #>                            CP H1  0.0097   0.4033   0.9994
    #>                               PP  0.0000   0.0358   0.9994
    #>                 P(Cross) if HR=1  0.0507   0.5554   0.0001
    #>              P(Cross) if HR=0.75  0.0004   0.0181   0.0574
    #>    IA 3: 63%                   Z -1.7256   1.0566   2.9347
    #>      N: 1148         p (1-sided)  0.9578   0.1454   0.0017
    #>  Events: 416        ~HR at bound  1.1846   0.9015   0.7497
    #>    Month: 36            Spending  0.0229   0.0217   0.0016
    #>                          B-value -1.3725   0.8404   2.3343
    #>                               CP  0.0000   0.0396   0.9928
    #>                            CP H1  0.0000   0.3449   0.9928
    #>                               PP  0.0000   0.0776   0.9759
    #>                 P(Cross) if HR=1  0.0736   0.8641   0.0017
    #>              P(Cross) if HR=0.75  0.0004   0.0398   0.4990
    #>    IA 4: 83%                   Z -1.7170   1.7357   2.5278
    #>      N: 1148         p (1-sided)  0.9570   0.0413   0.0057
    #>  Events: 548        ~HR at bound  1.1580   0.8622   0.8057
    #>    Month: 48            Spending  0.0154   0.0277   0.0046
    #>                          B-value -1.5689   1.5860   2.3098
    #>                               CP  0.0000   0.1578   0.8708
    #>                            CP H1  0.0000   0.3906   0.9337
    #>                               PP  0.0000   0.1793   0.8485
    #>                 P(Cross) if HR=1  0.0890   0.9631   0.0062
    #>              P(Cross) if HR=0.75  0.0004   0.0675   0.7996
    #>        Final                   Z -1.7149   2.3072   2.3072
    #>      N: 1148         p (1-sided)  0.9568   0.0105   0.0105
    #>  Events: 657        ~HR at bound  1.1433   0.8352   0.8352
    #>    Month: 60            Spending  0.0110   0.0325   0.0062
    #>                          B-value -1.7149   2.3072   2.3072
    #>                 P(Cross) if HR=1  0.1000   0.9888   0.0112
    #>              P(Cross) if HR=0.75  0.0004   0.1000   0.9000

### Interpreting the boundaries

The design has five analyses at calendar times of 12, 24, 36, 48, and 60 months. At each analysis, the test statistic (Z-value) is compared against three boundaries:

    bounds <- data.frame(
      Analysis = 1:x8$k,
      Month = x8$T,
      Events = ceiling(x8$n.I),
      Harm = round(x8$harm$bound, 2),
      Futility = round(x8$lower$bound, 2),
      Efficacy = round(x8$upper$bound, 2)
    )
    kable(bounds, caption = "Z-value boundaries at each analysis")

Z-value boundaries at each analysis
Analysis | Month | Events | Harm | Futility | Efficacy
--- | --- | --- | --- | --- | ---
1 | 12 | 73 | -2.11 | -1.44 | 7.43
2 | 24 | 253 | -1.77 | 0.12 | 3.86
3 | 36 | 416 | -1.73 | 1.06 | 2.93
4 | 48 | 548 | -1.72 | 1.74 | 2.53
5 | 60 | 657 | -1.71 | 2.31 | 2.31

Decision rules at each analysis:

- If \(Z >\) efficacy bound: Stop for efficacy (reject \(H_0\)).
- If futility bound \(< Z \leq\) efficacy bound: Continue the trial.
- If harm bound \(< Z \leq\) futility bound: Stop for futility.
- If \(Z \leq\) harm bound: Stop for harm.

Note that the harm bound is always at or below the futility bound. At early analyses, the harm and futility bounds may coincide when the harm spending function has not yet allocated sufficient spending to differentiate them.

### Boundary crossing probabilities

We examine the operating characteristics under two scenarios: no treatment effect (HR = 1, i.e., under \(H_0\)) and the design alternative (HR = 0.75).

    probs <- data.frame(
      Scenario = c(rep("Under H0 (HR=1)", x8$k), rep("Under H1 (HR=0.75)", x8$k)),
      Analysis = rep(1:x8$k, 2),
      Month = rep(x8$T, 2),
      `P(Efficacy)` = c(cumsum(x8$upper$prob[, 1]), cumsum(x8$upper$prob[, 2])),
      `P(Futility)` = c(cumsum(x8$lower$prob[, 1]), cumsum(x8$lower$prob[, 2])),
      `P(Harm)` = c(cumsum(x8$harm$prob[, 1]), cumsum(x8$harm$prob[, 2])),
      check.names = FALSE
    )
    kable(probs, digits = 4, caption = "Cumulative boundary crossing probabilities")

Cumulative boundary crossing probabilities
Scenario | Analysis | Month | P(Efficacy) | P(Futility) | P(Harm)
--- | --- | --- | --- | --- | ---
Under H0 (HR=1) | 1 | 12 | 0.0000 | 0.0748 | 0.0173
Under H0 (HR=1) | 2 | 24 | 0.0001 | 0.5554 | 0.0507
Under H0 (HR=1) | 3 | 36 | 0.0017 | 0.8641 | 0.0736
Under H0 (HR=1) | 4 | 48 | 0.0062 | 0.9631 | 0.0890
Under H0 (HR=1) | 5 | 60 | 0.0112 | 0.9888 | 0.1000
Under H1 (HR=0.75) | 1 | 12 | 0.0000 | 0.0039 | 0.0004
Under H1 (HR=0.75) | 2 | 24 | 0.0574 | 0.0181 | 0.0004
Under H1 (HR=0.75) | 3 | 36 | 0.4990 | 0.0398 | 0.0004
Under H1 (HR=0.75) | 4 | 48 | 0.7996 | 0.0675 | 0.0004
Under H1 (HR=0.75) | 5 | 60 | 0.9000 | 0.1000 | 0.0004

Under \(H_0\), the cumulative probability of crossing the harm bound across all analyses is approximately 0.1, reflecting the spending allocated to the harm boundary. Under \(H_1\) (HR = 0.75), crossing the harm bound is very unlikely (4^{-4}), since the treatment is beneficial.

### Visualization

All standard `plot()` types are supported for `test.type = 7` and `8` designs, with a third line (or set of lines) shown for the harm bound.

#### Z-value boundaries

The default plot shows Z-value boundaries at each analysis. Three boundaries are displayed: efficacy (upper), futility (lower), and harm (below futility).

    plot(x8)

Z-value boundaries for non-binding harm bound design

#### Boundary crossing probabilities

The power plot (`plottype = 2`) shows cumulative boundary crossing probabilities as a function of the treatment effect. Three sets of lines appear: upper bound (cumulative efficacy crossing probability), 1-Futility bound, and 1-Harm bound. The harm lines are above the futility lines because the probability of crossing the harm bound is less than or equal to the probability of crossing the futility bound. We note that when the underlying treatment effect favors control, the high probability of crossing the harm bound indicates that the harm bound is sensitive and serves its intended purpose

    plot(x8, plottype = 2)

Boundary crossing probabilities for non-binding harm bound design

#### Approximate treatment effect at boundaries

The effect size plot (`plottype = 3`) shows the approximate treatment effect at each boundary. For survival designs, this is expressed as the approximate hazard ratio at the boundary.

    plot(x8, plottype = 3)

Approximate treatment effect at boundaries

#### Conditional power at boundaries

Conditional power (`plottype = 4`) at each interim analysis is shown for all three boundaries. This is generally not a very useful plot.

    plot(x8, plottype = 4)

Conditional power at boundaries

#### Spending function plot

The spending function plot (`plottype = 5`) shows the three spending functions: \(\alpha\) (efficacy), \(\beta\) (futility), and harm.

    plot(x8, plottype = 5)

Spending functions for non-binding harm bound design

#### B-values at boundaries

B-values (`plottype = 7`) are Z-values scaled by \(\sqrt{t}\) where \(t\) is the information fraction. As discussed by Proschan, Lan, and Wittes (2006), the expected value of B-values increases linearly with the information fraction under the assumption of a constant treatment effect (proportional hazards). This linear relationship makes B-values useful for visual assessment of treatment effect trends across interim analyses: departures from linearity may suggest non-proportional hazards or other changes in treatment effect over time. Three boundary lines are shown: efficacy, futility, and harm.

    plot(x8, plottype = 7)

B-values at boundaries

## Design with binding bounds (`test.type = 7`)

For `test.type = 7`, both the futility and harm bounds are binding — meaning the computation of the efficacy bound assumes the trial will stop if either bound is crossed. This yields a slightly less conservative efficacy bound (easier to cross), but at the cost of inflated Type I error if the stopping rule is not strictly followed.

We first create a binding design with \(\alpha = 0.0125\) to compare with the non-binding design above:

    x7 <- gsSurvCalendar(
      test.type = 7,
      alpha = 0.0125,
      beta = 0.1,
      astar = 0.1,
      calendarTime = c(12, 24, 36, 48, 60),
      sfu = sfLDOF,
      sfl = sfHSD, sflpar = -2,
      sfharm = sfLDPocock,
      lambdaC = log(2) / 36,
      hr = 0.75,
      R = 18,
      minfup = 42
    )

### Comparing binding and non-binding

    comparison <- data.frame(
      Bound = c("Efficacy", "Futility", "Harm"),
      `Binding (type 7)` = c(
        paste(round(x7$upper$bound, 3), collapse = ", "),
        paste(round(x7$lower$bound, 3), collapse = ", "),
        paste(round(x7$harm$bound, 3), collapse = ", ")
      ),
      `Non-binding (type 8)` = c(
        paste(round(x8$upper$bound, 3), collapse = ", "),
        paste(round(x8$lower$bound, 3), collapse = ", "),
        paste(round(x8$harm$bound, 3), collapse = ", ")
      ),
      check.names = FALSE
    )
    kable(comparison, caption = "Comparison of binding vs. non-binding Z-value boundaries")

Comparison of binding vs. non-binding Z-value boundaries
Bound | Binding (type 7) | Non-binding (type 8)
--- | --- | ---
Efficacy | 7.434, 3.862, 2.934, 2.523, 2.248 | 7.434, 3.862, 2.935, 2.528, 2.307
Futility | -1.458, 0.09, 1.016, 1.689, 2.248 | -1.441, 0.121, 1.057, 1.736, 2.307
Harm | -2.112, -1.767, -1.726, -1.717, -1.715 | -2.112, -1.767, -1.726, -1.717, -1.715

Note that the efficacy bounds for `test.type = 7` (binding) are slightly lower (easier to cross) than for `test.type = 8` (non-binding). The maximum number of events for `test.type = 7` (639) is also slightly smaller than for `test.type = 8` (657), reflecting the assumption that the trial will stop at the lower bounds.

    gsBoundSummary(x7)
    #> Method: LachinFoulkes
    #>     Analysis               Value    Harm Futility Efficacy
    #>    IA 1: 11%                   Z -2.1121  -1.4578       NA
    #>       N: 746         p (1-sided)  0.9827   0.9275       NA
    #>   Events: 71        ~HR at bound  1.6550   1.4158       NA
    #>    Month: 12    P(Cross) if HR=1  0.0173   0.0725       NA
    #>              P(Cross) if HR=0.75  0.0005   0.0039       NA
    #>    IA 2: 38%                   Z -1.7667   0.0895   3.8622
    #>      N: 1118         p (1-sided)  0.9614   0.4643   0.0001
    #>  Events: 246        ~HR at bound  1.2531   0.9886   0.6107
    #>    Month: 24    P(Cross) if HR=1  0.0507   0.5430   0.0001
    #>              P(Cross) if HR=0.75  0.0005   0.0181   0.0539
    #>    IA 3: 63%                   Z -1.7256   1.0159   2.9344
    #>      N: 1118         p (1-sided)  0.9578   0.1548   0.0017
    #>  Events: 404        ~HR at bound  1.1874   0.9038   0.7467
    #>    Month: 36    P(Cross) if HR=1  0.0736   0.8551   0.0017
    #>              P(Cross) if HR=0.75  0.0005   0.0398   0.4829
    #>    IA 4: 83%                   Z -1.7170   1.6890   2.5229
    #>      N: 1118         p (1-sided)  0.9570   0.0456   0.0058
    #>  Events: 533        ~HR at bound  1.1604   0.8639   0.8037
    #>    Month: 48    P(Cross) if HR=1  0.0890   0.9592   0.0063
    #>              P(Cross) if HR=0.75  0.0005   0.0675   0.7881
    #>        Final                   Z -1.7149   2.2480   2.2480
    #>      N: 1118         p (1-sided)  0.9568   0.0123   0.0123
    #>  Events: 639        ~HR at bound  1.1454   0.8370   0.8370
    #>    Month: 60    P(Cross) if HR=1  0.1000   0.9875   0.0125
    #>              P(Cross) if HR=0.75  0.0005   0.1000   0.9000

### Efficacy bounds at alternate \(\alpha\) levels

The `gsBoundSummary()` function accepts an `alpha` argument to display efficacy bounds at one or more alternate \(\alpha\) levels alongside the original design. Here we show the non-binding design (`x8`) with efficacy bounds for both \(\alpha = 0.0125\) (the design level) and \(\alpha = 0.025\):

    gsBoundSummary(x8, alpha = 0.025)
    #>     Analysis               Value α=0.0125 α=0.025 Futility    Harm
    #>    IA 1: 11%                   Z       NA  6.6513  -1.4408 -2.1121
    #>       N: 766         p (1-sided)       NA  0.0000   0.9252  0.9827
    #>   Events: 73        ~HR at bound       NA  0.2092   1.4034  1.6434
    #>    Month: 12    P(Cross) if HR=1       NA  0.0000   0.0748  0.0173
    #>              P(Cross) if HR=0.75       NA  0.0000   0.0039  0.0004
    #>    IA 2: 38%                   Z   3.8622  3.4312   0.1212 -1.7667
    #>      N: 1148         p (1-sided)   0.0001  0.0003   0.4518  0.9614
    #>  Events: 253        ~HR at bound   0.6149  0.6492   0.9849  1.2491
    #>    Month: 24    P(Cross) if HR=1   0.0001  0.0003   0.5554  0.0507
    #>              P(Cross) if HR=0.75   0.0574  0.1259   0.0181  0.0004
    #>    IA 3: 63%                   Z   2.9347  2.5948   1.0566 -1.7256
    #>      N: 1148         p (1-sided)   0.0017  0.0047   0.1454  0.9578
    #>  Events: 416        ~HR at bound   0.7497  0.7751   0.9015  1.1846
    #>    Month: 36    P(Cross) if HR=1   0.0017  0.0048   0.8641  0.0736
    #>              P(Cross) if HR=0.75   0.4990  0.6323   0.0398  0.0004
    #>    IA 4: 83%                   Z   2.5278  2.2359   1.7357 -1.7170
    #>      N: 1148         p (1-sided)   0.0057  0.0127   0.0413  0.9570
    #>  Events: 548        ~HR at bound   0.8057  0.8261   0.8622  1.1580
    #>    Month: 48    P(Cross) if HR=1   0.0062  0.0138   0.9631  0.0890
    #>              P(Cross) if HR=0.75   0.7996  0.8684   0.0675  0.0004
    #>        Final                   Z   2.3072  2.0432   2.3072 -1.7149
    #>      N: 1148         p (1-sided)   0.0105  0.0205   0.0105  0.9568
    #>  Events: 657        ~HR at bound   0.8352  0.8526   0.8352  1.1433
    #>    Month: 60    P(Cross) if HR=1   0.0112  0.0201   0.9888  0.1000
    #>              P(Cross) if HR=0.75   0.9000  0.9218   0.1000  0.0004

## Practical considerations

### Choice of spending functions

The choice of spending functions for the three boundaries should reflect regulatory and scientific considerations:

- Efficacy: A conservative spending function such as Lan-DeMets O’Brien-Fleming (`sfLDOF`) is typical, spending very little \(\alpha\) at early interim analyses when limited information is available.
- Futility: Moderate spending (e.g., HSD with \(\gamma = -2\)) allows early stopping for futility when the treatment effect is clearly absent.
- Harm: The Lan-DeMets Pocock (`sfLDPocock`) spending function provides more aggressive spending at early analyses, which is appropriate for harm monitoring since detecting a detrimental effect early is critical for patient safety.

### Interpreting the harm bound

The harm bound is intended so that if a small observed p-value favoring control is observed, the harm bound will be crossed. In terms of the test statistic, a negative Z-value indicates that the hazard rate is higher in the experimental arm than the control arm — i.e., the experimental treatment appears to be worsening survival. When the Z-value falls below the harm bound, this constitutes a statistical signal that the treatment may be harmful, and the trial should be stopped with a recommendation to review the safety data.

The harm spending is computed under \(H_0\) (no treatment effect), reflecting the probability of observing an apparent harmful effect by chance when there is actually no true effect. This controls the probability of a false harm signal.

### Harm bound capping

In the implementation, the harm bound is automatically capped so it never exceeds the futility bound. This ensures the ordering: harm bound \(\leq\) futility bound \(\leq\) efficacy bound at every analysis.

### When to use `test.type = 7` vs. `test.type = 8`

- `test.type = 8` (non-binding) is most often preferred in practice. Regulators will generally expect non-binding bounds, which preserve Type I error control regardless of whether the stopping rules are strictly followed. Since Data Monitoring Committees (DMCs) typically retain discretion to continue or stop a trial based on the totality of the evidence, the non-binding approach ensures that the statistical validity of the efficacy analysis is maintained even if a futility or harm boundary is crossed but the trial continues.
- `test.type = 7` (binding) is appropriate when there is a firm commitment to stop the trial upon crossing any boundary. This provides a small efficiency gain (slightly easier efficacy bounds and fewer required events) but requires strict protocol adherence. If the trial does not stop after crossing a binding boundary, Type I error may be inflated.

In most regulatory settings, `test.type = 8` is the safer and more common choice.

### Why a separate “binding harm / non-binding futility” option is unnecessary

One might consider a design where the futility bound is non-binding but the harm bound is binding. In practice, such a distinction has no computational effect. The harm bound is computed after the efficacy and futility bounds are set and does not feed back into those computations. When the futility bound is non-binding (as in `test.type = 8`), the efficacy bound is computed ignoring all lower-bound stopping. Since the harm bound lies below the futility bound, making the harm bound “binding” while the futility bound remains non-binding would not change the efficacy boundary, the required number of events, the final Z-values, or the p-values — the results are identical.

The only difference would be in interpretation: whether crossing the harm bound is treated as a firm commitment to stop or as advisory information for the DMC. This interpretive distinction does not require a separate `test.type`; it can be addressed in the protocol language and the DMC charter. The `test.type = 8` framework already provides full flexibility for the DMC to treat the harm bound as either advisory or mandatory.

### Adjusting the boundaries

The boundaries are adjustable through several design parameters:

- Alternate `astar`: Controls the Type I error allocated to excess OS harm detection.
- Alternate spending functions: Different spending functions for efficacy, futility, and harm boundaries change the aggressiveness of each boundary across analyses.
- Alternate timing of analyses: Changing the calendar times of interim analyses shifts the information available at each look.

Regardless of the statistical design, bounds must be clinically, ethically, and statistically sound. As previously noted, this approach is one option to address the regulatory expectation for OS harm monitoring, but other approaches may also be considered.

## References

Proschan, Michael A., K. K. Gordon Lan, and Janet Turk Wittes. 2006. Statistical Monitoring of Clinical Trials: A Unified Approach. New York, NY: Springer.
U.S. Food and Drug Administration. 2024. “Assessment of Overall Survival Evidence in Support of Accelerated Approval of Oncology Therapeutics: Draft Guidance for Industry.” https://www.fda.gov/media/188274/download.