# Binomial two arm trial design and analysis

## Overview and notation

This vignette provides an overview of the binomial two arm trial design and analysis. We consider designs for superiority, non-inferiority, and super-superiority trials. These can reflect binary endpoints measuring successful outcomes (e.g., response) or unsuccessful outcomes (e.g., failure). We focus here on risk-difference and fixed (not group sequential) designs to limit the scope. Both risk ratio and odds ratio are also available in the package as outlined in Miettinen and Nurminen (1985).

The basic method for computing the fixed sample size that is the basis for group sequential design sizes for superiority was developed by Fleiss, Tytun, and Ury (1980), but is applied here without the continuity correction as recommended by Gordon and Watson (1996). This method was extended to non-inferiority and super-superiority trials by Farrington and Manning (1990).

We will see that while asymptotic formulations are generally good approximations, fast simulation methods can provide more accurate results both for Type I error and power.

The R packages we use are:

    library(gsDesign)
    library(ggplot2)
    library(tidyr)
    library(gt)
    library(dplyr)

## Sample size

The rate arguments in `nBinomial()` are `p1` and `p2`. `p1` is the rate in group 1 and `p2` is the rate in group 2. For a simple example, we can compute the sample size for a superiority design with a 2:1 sample size ratio where the experimental and control groups have assumed success rates of 0.2 and 0.1, respectively. One-sided Type I error is 0.025 and Type II error is 0.15 (power is 85%). This gives the same result if the roles of `p1` and `p2` are changed and `ratio` is inverted.

    nBinomial(p1 = 0.2, p2 = 0.1, ratio = 2, alpha = 0.025, beta = 0.15) |> ceiling()
    #> [1] 496
    nBinomial(p1 = 0.1, p2 = 0.2, ratio = 0.5, alpha = 0.025, beta = 0.15) |> ceiling()
    #> [1] 496

The above results apply as well to a failure endpoint, but now 0.2 would be the control group rate and 0.1 would be the experimental group rate. Results can also be computed using the risk-ratio and odds-ratio methods of Miettinen and Nurminen (1985). For 2:1 randomization (Experimental:Control) we set `ratio = 0.5` since group 1 represents the control group in the following. We see the results are the same for the risk-ratio method as the risk-difference method from above (default `scale = "Difference"`). For the odds-ratio method, we see the sample size is larger in this case.

    scale <- c("Difference", "RR", "OR")
    tibble(scale, "Sample size" = c(
      nBinomial(p1 = 0.2, p2 = 0.1, ratio = 0.5, alpha = 0.025, beta = 0.15, scale = scale[1]) |> ceiling(),
      nBinomial(p1 = 0.2, p2 = 0.1, ratio = 0.5, alpha = 0.025, beta = 0.15, scale = scale[2]) |> ceiling(),
      nBinomial(p1 = 0.2, p2 = 0.1, ratio = 0.5, alpha = 0.025, beta = 0.15, scale = scale[3]) |> ceiling()
    )) |>
      gt() |>
      tab_header("Sample size by scale for a superiority design",
        subtitle = "alpha = 0.025, beta = 0.15, pE = 0.2, pC = 0.1"
      )

Sample size by scale for a superiority design
alpha = 0.025, beta = 0.15, pE = 0.2, pC = 0.1
scale | Sample size
--- | ---
Difference | 525
RR | 525
OR | 489

## Testing and confidence intervals

Next we assume we have results from a trial with 20 / 30 and 10 / 30 successes in the two groups. The `testBinomial()` function computes the Z-value for a binomial test. We see that the scale chosen (default is `"Difference"`) does not matter for the Z-value.

    testBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30)
    #> [1] 2.581989
    testBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30, scale = "RR")
    #> [1] 2.581989
    testBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30, scale = "OR")
    #> [1] 2.581989

As opposed to `nBinomial()`, it makes a difference how treatment groups are assigned. For example, if we have 20 / 30 and 10 / 30 successes in the two groups, we can compute the Z-value for a binomial test of the null hypothesis that the difference between the two groups is equal to 0.

    testBinomial(x1 = 10, n1 = 30, x2 = 20, n2 = 30)
    #> [1] -2.581989

We compute a one-sided asymptotic \(p\)-value using the default method of Farrington and Manning (1990). With `adj = 1`, we use the Miettinen and Nurminen (1985) method which multiplies the variance estimate of the rate difference by `n / (n - 1)` (continuity correction). Generally, not using the continuity correction is preferred (Gordon and Watson (1996)). The `adj` argument is available in `testBinomial()`, `ciBinomial()` and `simBinomial()`, but is not used in `nBinomial()`. Since the Chi-square test is often used, we show that it gives a 2-sided \(p\)-value that is twice the one-sided \(p\)-value.

    testBinomial(x1 = 10, n1 = 30, x2 = 20, n2 = 30) |> pnorm(lower.tail = TRUE)
    #> [1] 0.004911637
    testBinomial(x1 = 10, n1 = 30, x2 = 20, n2 = 30, adj = 1) |> pnorm(lower.tail = TRUE)
    #> [1] 0.005227859
    testBinomial(x1 = 10, n1 = 30, x2 = 20, n2 = 30, chisq = 1) |>
      pchisq(df = 1, lower.tail = FALSE) / 2
    #> [1] 0.004911637

We can compute a confidence interval for the rate difference using the `ciBinomial()` function.

    p1 <- 20 / 30
    p2 <- 10 / 30
    rd <- p1 - p2
    rr <- p1 / p2
    orr <- (p1 * (1 - p2)) / (p2 * (1 - p1))

    rbind(
      ciBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30),
      ciBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30, scale = "RR"),
      ciBinomial(x1 = 20, n1 = 30, x2 = 10, n2 = 30, scale = "OR")
    ) |>
      mutate(
        scale = c("Risk difference", "Risk-ratio", "Odds-ratio"),
        Effect = c(rd, rr, orr)
      ) |>
      gt() |>
      tab_header("Confidence intervals for a binomial effect size",
        subtitle = "x1 = 20, n1 = 30, x2 = 10, n2 = 30"
      ) |>
      fmt_number(columns = c(lower, upper, Effect), n_sigfig = 3)

Confidence intervals for a binomial effect size
x1 = 20, n1 = 30, x2 = 10, n2 = 30
lower | upper | scale | Effect
--- | --- | --- | ---
0.0812 | 0.545 | Risk difference | 0.333
1.17 | 3.62 | Risk-ratio | 2.00
1.38 | 11.6 | Odds-ratio | 4.00

Again, how treatment groups are assigned makes a difference.

    ciBinomial(x1 = 10, n1 = 30, x2 = 20, n2 = 30)
    #>        lower       upper
    #> 1 -0.5454184 -0.08115662

## Non-inferiority and super-superiority

By setting a non-inferiority margin, we can loosen the stringency on what is required compared to a superiority study. The trial need only show the experimental treatment is close to as good as control treatment. By setting a super-superiority margin, we can show that the experimental treatment is better than the control treatment by at least a specified amount. This increases the stringency of the design compared to a superiority study. The method of Farrington and Manning (1990) is used for both non-inferiority and super-superiority designs. The parameter `delta0` sets the margin with `delta0 = 0` for superiority, `delta0 < 0` for non-inferiority, and `delta0 > 0` for super-superiority. We see that this has a substantial impact on the sample size requirement to achieve desired power.

    tibble(
      Design = c("Superiority", "Non-inferiority", "Super-superiority"),
      `p1 (pE)` = c(0.2, 0.2, 0.2),
      `p2 (pC)` = c(0.1, 0.1, 0.1),
      `delta0` = c(0, -0.02, 0.02),
      `Sample size` = c(
        ceiling(nBinomial(p1 = 0.2, p2 = 0.1, alpha = 0.025, beta = 0.15, ratio = 0.5)),
        ceiling(nBinomial(p1 = 0.2, p2 = 0.1, alpha = 0.025, beta = 0.15, ratio = 0.5, delta0 = -0.02)),
        ceiling(nBinomial(p1 = 0.2, p2 = 0.1, alpha = 0.025, beta = 0.15, ratio = 0.5, delta0 = 0.02))
      )
    ) |>
      gt() |>
      tab_header("Sample size for binomial two arm trial design",
        subtitle = "alpha = 0.025, beta = 0.15"
      ) |>
      fmt_number(columns = c(`p1 (pE)`, `p2 (pC)`), decimals = 2) |>
      cols_label(
        Design = "Design",
        `p1 (pE)` = "Experimental group rate",
        `p2 (pC)` = "Control group rate",
        delta0 = "Null hypothesis value of rate difference (delta0)",
        `Sample size` = "Sample size"
      ) |>
      tab_footnote("Randomization ratio is 2:1 (Experimental:Control) with assumed control failure rate p1 = 0.2 and experimental rate 0.1.")

Sample size for binomial two arm trial design
alpha = 0.025, beta = 0.15
Design | Experimental group rate | Control group rate | Null hypothesis value of rate difference (delta0) | Sample size
--- | --- | --- | --- | ---
Superiority | 0.20 | 0.10 | 0.00 | 525
Non-inferiority | 0.20 | 0.10 | -0.02 | 375
Super-superiority | 0.20 | 0.10 | 0.02 | 796
Randomization ratio is 2:1 (Experimental:Control) with assumed control failure rate p1 = 0.2 and experimental rate 0.1.

Testing for non-inferiority and super-superiority is equivalent to whether or not the confidence intervals contain the margin `delta0`. Since both the superiority and non-inferiority Z-tests exceed the critical value of 1.96, both tests are significant. The third test fails to establish super-superiority since the Z-value is less than 1.96. Since the confidence interval contains 0.02 but neither 0 nor -0.02, it comes to the same conclusion as all 3 of these tests.

    testBinomial(x1 = 18, n1 = 30, x2 = 10, n2 = 30, delta0 = 0) # superiority
    #> [1] 2.070197
    testBinomial(x1 = 18, n1 = 30, x2 = 10, n2 = 30, delta0 = -0.02) # non-inferiority
    #> [1] 2.225796
    testBinomial(x1 = 18, n1 = 30, x2 = 10, n2 = 30, delta0 = 0.02) # super-superiority
    #> [1] 1.915403
    ciBinomial(x1 = 18, n1 = 30, x2 = 10, n2 = 30) # CI
    #>        lower     upper
    #> 1 0.01426069 0.4872467

## Simulation

The `simBinomial()` function simulates the binomial distribution to compute the Z-value. It does not simulate individual observations, but rather simulates the binomial distribution. `simBinomial()` has the same arguments as `testBinomial()` but also includes the number of simulated trials in the argument `nsim`. A vector of Z-values is returned.

    simBinomial(p1 = 0.2, p2 = 0.1, n1 = 30, n2 = 30, nsim = 10)
    #>  [1] -1.5191091  1.8077538  3.2539569  2.0869968
    #>  [5]  2.0784610  0.4021506  1.5191091  0.4021506
    #>  [9]  1.3856406  2.0107528

To see if the asymptotic method controls Type I error at the desired level, we can compute the Type I error rate from the simulated Z-values.

    z <- simBinomial(p1 = 0.15, p2 = 0.15, n1 = 30, n2 = 30, nsim = 1000000)
    mean(z > qnorm(0.975)) # Type I error rate
    #> [1] 0.026665

We see that the Type I error rate is slightly inflated. To get an exact Type I error rate, we can compute the appropriate quantile of the simulated Z-values. We say exact, but the degree of exactness depends on the number of simulations which is 1 million in this case. Since the `simBinomial()` function is fast, we can use a large number of simulations. In any case, this produces a slightly conservative Type I error rate.

    zcut <- quantile(z, 0.975)
    tibble("Z cutoff" = zcut, "p cutoff" = pnorm(zcut, lower.tail = FALSE)) |>
      gt() |>
      fmt_number(columns = c("Z cutoff", "p cutoff"), n_sigfig = 3) |>
      tab_header("Exact cutoff for Type I error rate",
        subtitle = "Based on 1 million simulations"
      ) |>
      tab_footnote("The Z cutoff is the quantile of the simulated Z-values at 0.975 using p1 = p2 = 0.15.")

Exact cutoff for Type I error rate
Based on 1 million simulations
Z cutoff | p cutoff
--- | ---
2.01 | 0.0222
The Z cutoff is the quantile of the simulated Z-values at 0.975 using p1 = p2 = 0.15.

Now we examine power with the asymptotic and exact cutoffs.

    z <- simBinomial(p1 = 0.2, p2 = 0.1, n1 = 30, n2 = 30, nsim = 1000000)
    cat("Power with asymptotic cutoff ", mean(z > qnorm(0.975)))
    #> Power with asymptotic cutoff  0.192457
    cat("\nPower with exact cutoff", mean(z > zcut))
    #>
    #> Power with exact cutoff 0.166568

Finally, we compute power based on simulation for the sample size calculation above. We see in this case that the sample size of 489 based on the odds-ratio formulation more accurately approximates the targeted 85% power.

    ptab <- tibble(
      Scale = c("Risk-difference", "Odds-ratio"),
      n = c(525, 489),
      Power = c(
        mean(simBinomial(p1 = 0.2, p2 = 0.1, n1 = 525 / 3, n2 = 525 * 2 / 3, nsim = 100000) > qnorm(0.975)),
        mean(simBinomial(p1 = 0.2, p2 = 0.1, n1 = 489 / 3, n2 = 489 * 2 / 3, nsim = 100000) > qnorm(0.975))
      )
    )
    ptab |>
      gt() |>
      tab_header("Simulation power for sample size based on risk-difference and odds-ratio",
        subtitle = "pE = 0.2, pC = 0.1, alpha = 0.025, beta = 0.15"
      ) |>
      fmt_number(columns = c(n, Power), n_sigfig = 3) |>
      cols_label(Scale = "Scale", n = "Sample size", Power = "Power") |>
      tab_footnote("Power based on 100,000 simulated trials and nominal alpha = 0.025 test; 2 x simulation error = 0.002") |>
      tab_footnote("Power based on Z-test for risk-difference with no continuity correction.", location = cells_column_labels("Power"))

Simulation power for sample size based on risk-difference and odds-ratio
pE = 0.2, pC = 0.1, alpha = 0.025, beta = 0.15
Scale | Sample size | Power1
--- | --- | ---
Risk-difference | 525 | 0.876
Odds-ratio | 489 | 0.852
Power based on 100,000 simulated trials and nominal alpha = 0.025 test; 2 x simulation error = 0.002
1 Power based on Z-test for risk-difference with no continuity correction.

## Power table

The `binomialPowerTable()` function computes power for a given sample size and treatment effect across a range of underlying control event rates (`pC`) and effect sizes (`delta`). Both the asymptotic and simulation methods are available. The simulation method with a large simulation size is an accurate exact method. Thus, `binomialPowerTable()` provides a shortcut for the exact method power calculation. We see that the Type I error achieved depends using the asymptotic cutoff varies by the underlying control rate, reaching almost 0.03 when the underlying failure rate is 0.10.

    binomialPowerTable(
      pC = seq(0.1, 0.2, 0.02), delta = 0, delta0 = 0, n = 70, failureEndpoint = TRUE,
      ratio = 1, alpha = 0.025, simulation = TRUE, nsim = 1e6, adj = 0
    ) |>
      rename("Type I error" = "Power") |>
      gt() |>
      fmt_number(columns = "Type I error", n_sigfig = 3) |>
      tab_header("Type I error is not controlled with nominal p = 0.025 cutoff")

Type I error is not controlled with nominal p = 0.025 cutoff
pC | delta | pE | Type I error
--- | --- | --- | ---
0.10 | 0 | 0.10 | 0.0288
0.12 | 0 | 0.12 | 0.0283
0.14 | 0 | 0.14 | 0.0269
0.16 | 0 | 0.16 | 0.0258
0.18 | 0 | 0.18 | 0.0252
0.20 | 0 | 0.20 | 0.0243

Adding the continuity correction (`adj = 1`) helps a small amount at better controlling Type I error in this case. However, changing the nominal cutoff to `alpha = 0.023` without continuity correction is more successful at controlling Type I error consistently.

    binomialPowerTable(
      pC = seq(0.1, 0.2, 0.02), delta = 0, delta0 = 0, n = 70, failureEndpoint = TRUE,
      ratio = 1, alpha = 0.023, simulation = TRUE, nsim = 1e6, adj = 0
    ) |>
      rename("Type I error" = "Power") |>
      gt() |>
      fmt_number(columns = "Type I error", n_sigfig = 3) |>
      tab_header("Type I error is controlled at 0.025 with nominal p = 0.023 cutoff")

Type I error is controlled at 0.025 with nominal p = 0.023 cutoff
pC | delta | pE | Type I error
--- | --- | --- | ---
0.10 | 0 | 0.10 | 0.0216
0.12 | 0 | 0.12 | 0.0220
0.14 | 0 | 0.14 | 0.0226
0.16 | 0 | 0.16 | 0.0235
0.18 | 0 | 0.18 | 0.0235
0.20 | 0 | 0.20 | 0.0234

Now we look at power for a range of control rates and treatment effects. `binomialPowerTable()` uses all combinations of the input control group rate (`pC`) and treatment effect (`delta`). We use a nominal `alpha = 0.023` based on the simulations above to control Type I error. Initially we base power on the asymptotic approximation.

    power_table_asymptotic <- binomialPowerTable(
      pC = seq(0.1, 0.2, 0.025),
      delta = seq(0.15, 0.25, 0.02),
      n = 70,
      ratio = 1,
      alpha = 0.023
    )

Now we produce the same table based on 1 million simulations.

    power_table_simulation <- binomialPowerTable(
      pC = seq(0.1, 0.2, 0.025),
      delta = seq(0.15, 0.25, 0.02),
      n = 70,
      ratio = 1,
      alpha = 0.023,
      simulation = TRUE,
      nsim = 1000000
    )

We summarize these results in a plot and see that simulations suggest the asymptotic approximation slightly underestimates power. However, for all the `pC` and `delta` combinations evaluated, power is above 90% for all cases.

    rbind(
      power_table_asymptotic |> mutate(Method = "Asymptotic"),
      power_table_simulation |> mutate(Method = "Simulation")
    ) |>
      ggplot(aes(x = delta, y = Power, color = factor(pC), lty = Method)) +
      geom_line() +
      labs(x = "Treatment effect (delta)", y = "Power", color = "Control rate") +
      scale_color_brewer(palette = "Set1") +
      theme_minimal() +
      theme(legend.position = "bottom") +
      # Grid points on the x-axis at 0.05 intervals
      scale_x_continuous(breaks = seq(0.15, 0.25, by = 0.05)) +
      # Put y-axis scale on percent scale
      scale_y_continuous(
        labels = scales::percent_format(accuracy = 1),
        breaks = seq(0.2, 0.8, by = 0.2)
      ) +
      coord_cartesian(ylim = c(0.2, 0.8)) +
      ggtitle("Power for Binomial Two Arm Trial Design with n = 70")

Following is a table of the simulation results from above in a wide format.

    # Transform table with values from Power to a wide format with
    # Put "Control group rate" (pC) in rows and Treatment effect (delta) in columns
    # Put a spanner label over columns after first column with label "Treatment effect (delta)"
    power_table_simulation |>
      select(-pE) |>
      tidyr::pivot_wider(
        names_from = delta,
        values_from = Power
      ) |>
      dplyr::rename(`Control group rate` = pC) |>
      gt::gt() |>
      gt::tab_spanner(
        label = "Treatment effect (delta)",
        columns = 2:7
      ) |>
      gt::fmt_percent(decimals = 1) |>
      gt::tab_header("Power by Control Group Rate and Treatment Effect")

Power by Control Group Rate and Treatment Effect
Control group rate | Treatment effect (delta)
--- | ---
0.15 | 0.17 | 0.19 | 0.21 | 0.23 | 0.25
10.0% | 37.4% | 44.8% | 52.3% | 59.6% | 66.6% | 72.6%
12.5% | 34.1% | 41.2% | 48.2% | 55.5% | 62.4% | 68.7%
15.0% | 31.6% | 38.3% | 45.2% | 52.2% | 59.2% | 65.7%
17.5% | 29.6% | 36.1% | 42.9% | 49.8% | 56.7% | 63.3%
20.0% | 28.3% | 34.5% | 41.1% | 47.8% | 54.7% | 61.3%

## Summary

We have demonstrated sample size and power calculations for 2-arm binomial trial designs. Both asymptotic approximations and exact methods based on large simulations have been used. We focus on the Miettinen and Nurminen (1985) risk-difference asymptotic method for testing of differences, but without continuity correction as recommended by Gordon and Watson (1996). This method generalizes to non-inferiority and super-superiority trials as proposed by Farrington and Manning (1990). The risk-ratio and odds-ratio methods are also available. We have also shown how to compute Type I error and power using simulation methods. Based on the high-speed of simulations, there may be some value in using simulation to evaluate Type I error across a range of event rates and then use appropriate nominal cutoffs to evaluate power. These adjusted cutoffs would then also be used for testing when the trial is completed. This is different than Fisher’s exact test which which conditions on the total number of events observed. Results may vary under different circumstances, so that while the odds-ratio sample size method was more accurate for the given example, this result has not been generalized.

## References

Farrington, Conor P, and Godfrey Manning. 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 (12): 1447–54.
Fleiss, Joseph L, Alex Tytun, and Hans K Ury. 1980. “A Simple Approximation for Calculating Sample Sizes for Comparing Independent Proportions.” Biometrics 36 (2): 343–46.
Gordon, Ian, and Ray Watson. 1996. “The Myth of Continuity-Corrected Sample Size Formulae.” Biometrics 52 (1): 71–76.
Miettinen, Olli, and Markku Nurminen. 1985. “Comparative Analysis of Two Rates.” Statistics in Medicine 4 (2): 213–26.