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Non-inferiority and Equivalence Trials

Gosuke Homma

2026-10-28

Source: vignettes/bbssr-non-inferiority.Rmd
bbssr-non-inferiority.Rmd

Hypotheses and margin

Let group 1 receive the experimental treatment and group 2 the control. A non-inferiority trial with a margin δ>0\delta > 0 on the scale of the risk difference tests H0:p1−p2≤−δagainstH1:p1−p2>−δ, H_{0}: p_{1} - p_{2} \le -\delta \quad \text{against} \quad H_{1}: p_{1} - p_{2} > -\delta , which corresponds to alternative = 'greater' and margin = delta. With alternative = 'less' the null hypothesis is p1−p2≥δp_{1} - p_{2} \ge \delta, which suits an endpoint for which a lower response probability is better. A negative margin tests superiority by more than its absolute value. A margin other than 0 requires one of the two tests below and a one-sided alternative. The last section covers a margin on the scale of the risk ratio, margin.scale = 'RR'.

The two tests

Both tests refer Z=p̂1−p̂2+δSÊ Z = \frac{\hat{p}_{1} - \hat{p}_{2} + \delta}{\widehat{\mathrm{SE}}} to the standard normal distribution and reject H0H_{0} when Z>z1−αZ > z_{1 - \alpha}. The test of Blackwelder (1982), Test = 'Blackwelder', uses the standard error at the observed proportions, SÊ=p̂1(1−p̂1)N1+p̂2(1−p̂2)N2. \widehat{\mathrm{SE}} = \sqrt{\frac{\hat{p}_{1}(1 - \hat{p}_{1})}{N_{1}} + \frac{\hat{p}_{2}(1 - \hat{p}_{2})}{N_{2}}} . The test of Farrington and Manning (1990), Test = 'Farrington-Manning', uses the same expression at the maximum likelihood estimates p̃1\tilde{p}_{1} and p̃2\tilde{p}_{2} under the restriction p1−p2=−δp_{1} - p_{2} = -\delta. With δ=0\delta = 0 the Farrington-Manning test is the chi-squared test and the Blackwelder test is the Wald test.

The two tests are asymptotic, but the package computes their rejection regions, power and sample sizes exactly, as for the other tests.

rr.bw <- BinaryRR(N1 = 60, N2 = 60, alpha = 0.025, Test = 'Blackwelder', margin = 0.1)
rr.fm <- BinaryRR(N1 = 60, N2 = 60, alpha = 0.025, Test = 'Farrington-Manning',
                  margin = 0.1)
c(Blackwelder = sum(rr.bw), Farrington.Manning = sum(rr.fm))
#>        Blackwelder Farrington.Manning 
#>               1714               1698
plot(rr.fm)

The size of a test of non-inferiority is its largest rejection probability on the boundary p1=p2−δp_{1} = p_{2} - \delta of the null hypothesis. The function below evaluates it over a fine grid of p2p_{2}.

boundary_size <- function(RR, margin) {
  N1 <- attr(RR, 'N1')
  N2 <- attr(RR, 'N2')
  m <- matrix(as.vector(RR), N1 + 1L, N2 + 1L)
  p2 <- seq(margin + 0.001, 0.999, by = 0.001)
  size <- vapply(p2, function(q) {
    sum(outer(dbinom(0:N1, N1, q - margin), dbinom(0:N2, N2, q)) * m)
  }, numeric(1))
  data.frame(p2 = p2[which.max(size)], size = max(size))
}
sizes <- cbind(Test = c('Blackwelder', 'Farrington-Manning'),
               rbind(boundary_size(rr.bw, 0.1), boundary_size(rr.fm, 0.1)))
sizes
#>                 Test    p2       size
#> 1        Blackwelder 0.993 0.06067597
#> 2 Farrington-Manning 0.550 0.02730085

The nominal level is 0.025. With 60 patients per group the size is 0.0607 for the Blackwelder test, attained at p2=p_{2} = 0.993, and 0.0273 for the Farrington-Manning test. The size of an asymptotic test at a given sample size can lie on either side of the nominal level, so it is worth computing for the sample sizes of a design.

Sample size

With a margin, BinarySampleSize() replaces the difference in the normal approximation by its distance p1−p2+δp_{1} - p_{2} + \delta from the boundary of the null hypothesis. method = 'standard' evaluates the variance in the term of the significance level at the large sample values of the restricted estimates, which gives formula (4) of Farrington and Manning (1990). method = 'alternative.variance' uses the variance under the alternative in both terms, which gives the formula of Blackwelder (1982). method = 'exact' steps from the formula of Farrington and Manning, one patient at a time, to the sample size at which the exact power of the test first attains the target. Below, both groups have a response probability of 0.7 and the margin is 0.15.

ni.ss <- expand.grid(method = c('exact', 'standard', 'alternative.variance'),
                     Test = c('Blackwelder', 'Farrington-Manning'),
                     stringsAsFactors = FALSE)
ni.ss$N <- NA
ni.ss$Power <- NA
for (i in seq_len(nrow(ni.ss))) {
  ss <- BinarySampleSize(p1 = 0.7, p2 = 0.7, r = 1, alpha = 0.025, tar.power = 0.8,
                         Test = ni.ss$Test[i], method = ni.ss$method[i], margin = 0.15)
  ni.ss$N[i] <- ss$N
  ni.ss$Power[i] <- round(ss$Power, 4)
}
ni.ss
#>                 method               Test   N  Power
#> 1                exact        Blackwelder 292 0.8009
#> 2             standard        Blackwelder 292 0.8009
#> 3 alternative.variance        Blackwelder 294 0.8024
#> 4                exact Farrington-Manning 290 0.8005
#> 5             standard Farrington-Manning 292 0.8020
#> 6 alternative.variance Farrington-Manning 294 0.8062

Power is the exact power of the test at the sample size of each row, so the rows of the two formulas show how close the normal approximation comes to the target for each test.

Blinded sample size re-estimation

Friede, Mitchell and Mueller-Velten (2007) re-estimate the sample size of a non-inferiority trial from the blinded overall response rate, which is the pooled response probability of the package. The assumed difference is usually 0, so both recovered probabilities equal the blinded pooled response probability, and the sample size is re-estimated by the formula of Farrington and Manning (ss.method = 'standard') or of Blackwelder (ss.method = 'alternative.variance'). The design below is planned for a pooled response probability of 0.7 and re-estimates the sample size after half of the patients.

plan <- BinarySampleSize(p1 = 0.7, p2 = 0.7, r = 1, alpha = 0.025, tar.power = 0.8,
                         Test = 'Farrington-Manning', method = 'standard', margin = 0.15)
ni <- BinaryPowerBSSR(
  p = seq(0.4, 0.8, by = 0.1), Delta.A = 0, Delta.T = 0,
  N1 = plan$N1, N2 = plan$N2, omega = 0.5, r = 1, alpha = 0.025, tar.power = 0.8,
  Test = 'Farrington-Manning', ss.method = 'standard', margin = 0.15
)
ni
#> Blinded sample size re-estimation for a binary endpoint
#> 
#>   Test            : Farrington-Manning
#>   Alternative     : greater
#>   Design rule     : unrestricted
#>   Initial size    : N1 = 146, N2 = 146
#>   Interim fraction: 0.5, giving n1 = 73 and n2 = 73
#>   Treatment effect: assumed 0, true 0 (risk difference)
#>   Margin          : 0.15
#>   Re-estimation   : normal approximation (standard), group rounding at level 0.025
#>   Alpha           : 0.025, target power 0.8
#> 
#>    p  p1  p2 power.BSSR power.TRAD   E.N
#>  0.4 0.4 0.4     0.7978     0.7473 329.0
#>  0.5 0.5 0.5     0.7912     0.7401 342.1
#>  0.6 0.6 0.6     0.7978     0.7473 329.0
#>  0.7 0.7 0.7     0.8015     0.8020 290.8
#>  0.8 0.8 0.8     0.8069     0.8925 228.5
plot(ni)

summary(ni)
#>    p1  p2   p      E.N      SD.N N.q25 N.q50 N.q75      P.N.max
#> 1 0.4 0.4 0.4 328.9911 10.868511   322   330   336 4.408925e-02
#> 2 0.5 0.5 0.5 342.0553  3.210567   342   344   344 5.435178e-01
#> 3 0.6 0.6 0.6 328.9911 10.868511   322   330   336 4.408925e-02
#> 4 0.7 0.7 0.7 290.7938 19.451117   278   294   304 8.652446e-06
#> 5 0.8 0.8 0.8 228.4956 24.167543   212   228   244 1.323903e-13

The planned total sample size is 292. The variance of a binary endpoint is largest at one half, so the fixed-sample design loses power when the true pooled response probability lies closer to one half than the planning value, and the re-estimation enlarges the trial in that case. At a pooled response probability of 0.4 the power is 0.798 with re-estimation and 0.747 without.

Type I error rate on the boundary

With a margin, BinaryTypeIErrorBSSR() evaluates the type I error rate on the boundary of the null hypothesis. The boundary is parametrized by the pooled response probability theta, and the values at which a response probability falls outside the unit interval are dropped.

ni.tie <- BinaryTypeIErrorBSSR(
  Delta.A = 0, N1 = plan$N1, N2 = plan$N2, omega = 0.5, r = 1, alpha = 0.025,
  tar.power = 0.8, Test = 'Farrington-Manning', ss.method = 'standard', margin = 0.15,
  theta = seq(0.1, 0.9, by = 0.01)
)
ni.tie
#> Type I error rate of blinded sample size re-estimation for a binary endpoint
#> 
#>   Test            : Farrington-Manning
#>   Alternative     : greater
#>   Initial size    : N1 = 146, N2 = 146
#>   Interim size    : n1 = 73, n2 = 73
#>   Assumed effect  : 0 (RD)
#>   Margin          : 0.15
#>   Nominal level   : 0.025
#>   Grid            : 81 values of theta in [0.1, 0.9]
#>   Maximum         : certified over theta in [0.1, 0.9]
#> 
#> Largest type I error rate
#>        Design    theta     TIE
#>          BSSR 0.261652 0.02579
#>  Fixed sample 0.500000 0.02650
plot(ni.tie)

The largest type I error rate is certified over the interval spanned by theta, here from 0.1 to 0.9. With the default theta = seq(0, 1, by = 0.005) the interval is the whole part of the boundary on which both response probabilities lie in the unit interval, which with an allocation ratio of 1 is theta from δ/2\delta / 2 to 1−δ/21 - \delta / 2.

The same rejection probabilities follow from BinaryPowerBSSR() with the true difference on the boundary, Delta.T = -margin.

on.boundary <- BinaryPowerBSSR(
  p = c(0.5, 0.7), Delta.A = 0, Delta.T = -0.15,
  N1 = plan$N1, N2 = plan$N2, omega = 0.5, r = 1, alpha = 0.025, tar.power = 0.8,
  Test = 'Farrington-Manning', ss.method = 'standard', margin = 0.15
)
rows <- vapply(c(0.5, 0.7), function(t) which.min(abs(ni.tie$theta - t)), integer(1))
max(abs(on.boundary$power.BSSR - ni.tie$TIE.BSSR[rows]))
#> [1] 0

BinaryAlphaAdjBSSR() accepts the margin in the same way and adjusts the nominal level on the boundary.

Margin on the scale of the risk ratio

With margin.scale = 'RR' the margin is a ratio R0>0R_{0} > 0, and alternative = 'greater' tests H0:p1/p2≤R0againstH1:p1/p2>R0. H_{0}: p_{1} / p_{2} \le R_{0} \quad \text{against} \quad H_{1}: p_{1} / p_{2} > R_{0} . With alternative = 'less' the null hypothesis is p1/p2≥R0p_{1} / p_{2} \ge R_{0}, as in the second example of Farrington and Manning (1990), where the relative risk of pertussis among recipients of an acellular vaccine compared with those of a whole cell vaccine is to be shown to be below 1.5. A margin on this scale also requires one of the two tests and a one-sided alternative, and in the functions with re-estimation the assumed and the true effects are then risk ratios (effect = 'RR').

Both tests refer statistic (5) of Farrington and Manning (1990), Z=p̂1−R0p̂2SÊ,SÊ2=p̃1(1−p̃1)N1+R02p̃2(1−p̃2)N2, Z = \frac{\hat{p}_{1} - R_{0} \hat{p}_{2}}{\widehat{\mathrm{SE}}} , \qquad \widehat{\mathrm{SE}}^{2} = \frac{\tilde{p}_{1}(1 - \tilde{p}_{1})}{N_{1}} + R_{0}^{2} \frac{\tilde{p}_{2}(1 - \tilde{p}_{2})}{N_{2}} , to the standard normal distribution. The Blackwelder test takes p̃1=p̂1\tilde{p}_{1} = \hat{p}_{1} and p̃2=p̂2\tilde{p}_{2} = \hat{p}_{2}, which is Method 1 of the article. The Farrington-Manning test takes the maximum likelihood estimates under the restriction p1=R0p2p_{1} = R_{0} p_{2}. With θ=N2/N1\theta = N_{2} / N_{1}, the estimate p̃1\tilde{p}_{1} is the smaller root of (1+θ)x2−{R0(1+θp̂2)+θ+p̂1}x+R0(p̂1+θp̂2)=0, (1 + \theta) x^{2} - \{R_{0}(1 + \theta \hat{p}_{2}) + \theta + \hat{p}_{1}\} x + R_{0}(\hat{p}_{1} + \theta \hat{p}_{2}) = 0 , formula (13) of the article, and p̃2=p̃1/R0\tilde{p}_{2} = \tilde{p}_{1} / R_{0}. With R0=1R_{0} = 1 the restricted estimates are the pooled proportion, so the Farrington-Manning test is the chi-squared test.

rr.ratio <- BinaryRR(N1 = 60, N2 = 60, alpha = 0.025, Test = 'Farrington-Manning',
                     margin = 0.8, margin.scale = 'RR')
rr.one <- BinaryRR(N1 = 60, N2 = 60, alpha = 0.025, Test = 'Farrington-Manning',
                   margin = 1, margin.scale = 'RR')
rr.chisq <- BinaryRR(N1 = 60, N2 = 60, alpha = 0.025, Test = 'Chisq')
c(ratio.0.8 = sum(rr.ratio), ratio.1 = sum(rr.one), chisq = sum(rr.chisq))
#> ratio.0.8   ratio.1     chisq 
#>      1727      1355      1355
identical(as.vector(rr.one), as.vector(rr.chisq))
#> [1] TRUE

BinarySampleSize() replaces the difference in the normal approximation by the distance p1−R0p2p_{1} - R_{0} p_{2} from the boundary, or R0p2−p1R_{0} p_{2} - p_{1} for alternative = 'less'. With the allocation ratio r=N1/N2r = N_{1} / N_{2} the size of group 2 is N2=(z1−αv0+z1−βv1)2(p1−R0p2)2,v1=p1(1−p1)r+R02p2(1−p2), N_{2} = \frac{\left(z_{1 - \alpha} \sqrt{v_{0}} + z_{1 - \beta} \sqrt{v_{1}}\right)^{2}}{(p_{1} - R_{0} p_{2})^{2}} , \qquad v_{1} = \frac{p_{1}(1 - p_{1})}{r} + R_{0}^{2} p_{2}(1 - p_{2}) , where 1−β1 - \beta is the target power and v0v_{0} is the same expression at the large sample values of the restricted estimates. method = 'standard' gives formula (8) of Farrington and Manning (1990), and method = 'alternative.variance', which sets v0=v1v_{0} = v_{1}, gives the formula of their Method 1. The table below repeats the comparison of the sample sizes for equal response probabilities of 0.7 and the margin 0.8.

rr.ss <- expand.grid(method = c('exact', 'standard', 'alternative.variance'),
                     Test = c('Blackwelder', 'Farrington-Manning'),
                     stringsAsFactors = FALSE)
rr.ss$N <- NA
rr.ss$Power <- NA
for (i in seq_len(nrow(rr.ss))) {
  ss <- BinarySampleSize(p1 = 0.7, p2 = 0.7, r = 1, alpha = 0.025, tar.power = 0.8,
                         Test = rr.ss$Test[i], method = rr.ss$method[i], margin = 0.8,
                         margin.scale = 'RR')
  rr.ss$N[i] <- ss$N
  rr.ss$Power[i] <- round(ss$Power, 4)
}
rr.ss
#>                 method               Test   N  Power
#> 1                exact        Blackwelder 278 0.8022
#> 2             standard        Blackwelder 284 0.8085
#> 3 alternative.variance        Blackwelder 276 0.7993
#> 4                exact Farrington-Manning 282 0.8036
#> 5             standard Farrington-Manning 284 0.8047
#> 6 alternative.variance Farrington-Manning 276 0.7946

The second example of Farrington and Manning (1990), with p1=p2=0.01p_{1} = p_{2} = 0.01, R0=1.5R_{0} = 1.5, alternative = 'less', the one-sided level 0.025 and the power 0.9, needs 12,890 patients per group by formula (8), and its true power is 90.4 per cent. The rejection region at this sample size has more than 1.6×1081.6 \times 10^{8} outcomes, so inst/reproduce/reproduce-published.R sums the exact power over the outcomes that carry the probability, and reproduces both values together with Tables I and II of the article.

The re-estimation of Friede, Mitchell and Mueller-Velten (2007) carries over with the assumed ratio Delta.A = 1, which recovers the pooled proportion for both groups. The design below is planned for a pooled response probability of 0.7, re-estimates the sample size by formula (8) after half of the patients, and allows at most twice the planned sample size.

plan.rr <- BinarySampleSize(p1 = 0.7, p2 = 0.7, r = 1, alpha = 0.025, tar.power = 0.8,
                            Test = 'Farrington-Manning', method = 'standard',
                            margin = 0.8, margin.scale = 'RR')
ni.rr <- BinaryPowerBSSR(
  p = seq(0.4, 0.8, by = 0.1), Delta.A = 1, Delta.T = 1,
  N1 = plan.rr$N1, N2 = plan.rr$N2, omega = 0.5, r = 1, alpha = 0.025, tar.power = 0.8,
  Test = 'Farrington-Manning', effect = 'RR', ss.method = 'standard',
  N.max = 2 * plan.rr$N, margin = 0.8, margin.scale = 'RR'
)
ni.rr
#> Blinded sample size re-estimation for a binary endpoint
#> 
#>   Test            : Farrington-Manning
#>   Alternative     : greater
#>   Design rule     : unrestricted
#>   Initial size    : N1 = 142, N2 = 142
#>   Interim fraction: 0.5, giving n1 = 71 and n2 = 71
#>   Treatment effect: assumed 1, true 1 (risk ratio)
#>   Margin          : 0.8 (risk ratio)
#>   Re-estimation   : normal approximation (standard), group rounding at level 0.025, N.max = 568
#>   Alpha           : 0.025, target power 0.8
#> 
#>    p  p1  p2 power.BSSR power.TRAD   E.N
#>  0.4 0.4 0.4     0.5812     0.3375 568.0
#>  0.5 0.5 0.5     0.7441     0.4653 555.9
#>  0.6 0.6 0.6     0.8021     0.6240 433.6
#>  0.7 0.7 0.7     0.8081     0.8047 286.5
#>  0.8 0.8 0.8     0.8212     0.9497 179.5

At equal response probabilities pp the distance from the boundary is (1−R0)p(1 - R_{0}) p, so it shrinks with pp, and a lower pooled response probability calls for more patients. The planned total sample size is 284. At a pooled response probability of 0.4 the expected total sample size of the re-estimation design is 568.0, close to the upper bound of 568, and the power is 0.581 with re-estimation and 0.337 without.

On the boundary p1=R0p2p_{1} = R_{0} p_{2} the pooled response probability is θ=(1+rR0)p2/(1+r)\theta = (1 + r R_{0}) p_{2} / (1 + r), and both response probabilities are linear in θ\theta, so BinaryTypeIErrorBSSR() certifies the largest type I error rate as on the scale of the risk difference. With the default theta the interval runs from 0 to min⁡(1,1/R0)(1+rR0)/(1+r)\min(1, 1 / R_{0}) (1 + r R_{0}) / (1 + r), at which the larger response probability reaches 1.

tie.rr <- BinaryTypeIErrorBSSR(
  Delta.A = 1, N1 = plan.rr$N1, N2 = plan.rr$N2, omega = 0.5, r = 1, alpha = 0.025,
  tar.power = 0.8, Test = 'Farrington-Manning', effect = 'RR', ss.method = 'standard',
  N.max = 2 * plan.rr$N, margin = 0.8, margin.scale = 'RR'
)
attr(tie.rr, 'interval')
#> [1] 0.0 0.9
attr(tie.rr, 'max')
#>   Design      theta        TIE      bound
#> 1   BSSR 0.04517183 0.02704639 0.02704639
#> 2   TRAD 0.14697132 0.03055082 0.03055082
plot(tie.rr)

The largest type I error rate is 0.0270 for the re-estimation design, at θ=\theta = 0.045, and 0.0306 for the fixed-sample design, at θ=\theta = 0.147. Both exceed the nominal level of 0.025, and the fixed-sample design exceeds it by more, so the excess comes from the asymptotic test rather than from the re-estimation. BinaryAlphaAdjBSSR() accepts margin.scale in the same way and lowers the nominal level until the rate is controlled.

Tests of equivalence

With alternative = 'equivalence' the null hypothesis p1−p2≤−δLp_{1} - p_{2} \le -\delta_{L} or p1−p2≥δUp_{1} - p_{2} \ge \delta_{U} is tested against −δL<p1−p2<δU-\delta_{L} < p_{1} - p_{2} < \delta_{U} by two one-sided tests, the test against 'greater' with the margin δL\delta_{L} and the test against 'less' with the margin δU\delta_{U}, each at the level α\alpha. The margins are given as margin = c(lower, upper), or as a single value for both. The null hypothesis is rejected when both tests reject, so the rejection region is the intersection of the two one-sided regions.

eq <- BinaryRR(N1 = 80, N2 = 80, alpha = 0.05, Test = 'Farrington-Manning',
               alternative = 'equivalence', margin = 0.2)
lo <- BinaryRR(N1 = 80, N2 = 80, alpha = 0.05, Test = 'Farrington-Manning',
               alternative = 'greater', margin = 0.2)
up <- BinaryRR(N1 = 80, N2 = 80, alpha = 0.05, Test = 'Farrington-Manning',
               alternative = 'less', margin = 0.2)
identical(as.vector(eq), as.vector(lo) & as.vector(up))
#> [1] TRUE

The normal approximation to the sample size adds the approximate powers of the two one-sided tests and subtracts 1, which gives a lower bound for the approximate probability that both reject, and solves for the size at which this bound reaches the target power. The exact search of BinarySampleSize() starts from that size.

plan.eq <- BinarySampleSize(p1 = 0.4, p2 = 0.4, r = 1, alpha = 0.05, tar.power = 0.8,
                            Test = 'Farrington-Manning', alternative = 'equivalence',
                            margin = 0.2)
plan.eq
#> Sample size for a two-arm trial with a binary endpoint
#> 
#>   Test             : Farrington-Manning
#>   Alternative      : equivalence
#>   Response rates   : p1 = 0.4, p2 = 0.4
#>   Allocation ratio : 1 to 1
#>   Margin           : -0.2 to 0.2 (risk difference)
#>   Alpha            : 0.05
#>   Target power     : 0.8
#>   Exact search     : crossing
#> 
#>   Required sample size: N1 = 101, N2 = 101, total N = 202
#>   Attained power      : 0.8005

In a re-estimation design the assumed difference Delta.A must lie between the margins, and is usually 0. The type I error rate is evaluated on both boundaries of the null hypothesis, p1−p2=−δLp_{1} - p_{2} = -\delta_{L} and p1−p2=δUp_{1} - p_{2} = \delta_{U}, and its largest value is certified on each. For normally distributed outcomes, Glimm, Yau and Woehling (2021) found that blinded re-estimation can inflate the type I error rate of a test of equivalence more than that of a test of superiority, because on a boundary away from no difference the blinded estimate of the variance depends on the true difference. For binary outcomes the rate can be computed exactly.

tie.eq <- BinaryTypeIErrorBSSR(
  Delta.A = 0, N1 = plan.eq$N1, N2 = plan.eq$N2, omega = 0.3, r = 1, alpha = 0.05,
  tar.power = 0.8, Test = 'Farrington-Manning', alternative = 'equivalence',
  ss.method = 'standard', margin = 0.2
)
attr(tie.eq, 'max')
#>   Design     theta        TIE      bound boundary
#> 1   BSSR 0.5000000 0.05400202 0.05400202    lower
#> 2   TRAD 0.6342773 0.05352131 0.05352131    lower
plot(tie.eq)

The largest type I error rate is 0.0540 for the re-estimation design and 0.0535 for the fixed-sample design of 202 patients. The re-estimation design exceeds the fixed-sample design, and both rates are computed for the same test of Farrington and Manning, whose fixed-sample rate already exceeds the level of 0.05. With equal margins and equal allocation the two boundaries are mirror images and give the same rates. BinaryAlphaAdjBSSR() lowers the level of both one-sided tests until the rate is controlled on both boundaries.

adj.eq <- BinaryAlphaAdjBSSR(
  Delta.A = 0, N1 = plan.eq$N1, N2 = plan.eq$N2, omega = 0.3, r = 1, alpha = 0.05,
  tar.power = 0.8, Test = 'Farrington-Manning', alternative = 'equivalence',
  ss.method = 'standard', margin = 0.2
)
adj.eq
#> Adjusted significance level for blinded sample size re-estimation
#> 
#>   Test            : Farrington-Manning
#>   Alternative     : equivalence
#>   Adjusted part   : final analysis only
#>   Margin          : -0.2 to 0.2 (risk difference)
#>   Maximum         : certified over theta in [0.1, 0.9] on the lower boundary and [0.1, 0.9] on the upper boundary
#>   Target level    : 0.05
#> 
#>        Design   max.TIE alpha.adj max.TIE.adj Boundary
#>          BSSR 0.0540020 0.0467474   0.0496199    lower
#>  Fixed sample 0.0535213 0.0474232   0.0492922    lower

The adjusted level of each one-sided test is 0.0467474 for the re-estimation design and 0.0474232 for the fixed-sample design.

References

Blackwelder, W. C. (1982). “Proving the null hypothesis” in clinical trials. Controlled Clinical Trials, 3, 345-353.

Farrington, C. P. and Manning, G. (1990). Test statistics and sample size formulae for comparative binomial trials with null hypothesis of non-zero risk difference or non-unity relative risk. Statistics in Medicine, 9, 1447-1454.

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