Methodology

The cut-point approach

For one study arm with mean change \(\mu\), standard deviation \(\sigma\) and a minimal important difference (MID) threshold \(m\), assume the patient-level change \(X\) is Normally distributed. A responder is a patient whose change crosses the threshold. The responder probability is

\[ p = \Pr(X > m) = \Phi\!\left(\frac{\mu - m}{\sigma}\right) \quad\text{(higher change is better),} \]

or \(p = \Phi\!\left(\frac{m - \mu}{\sigma}\right)\) when a lower change is better. This is the cut-point (“dichotomization”) method reviewed by Thorlund and colleagues (2011) and detailed by Anzures-Cabrera, Sarpatwari & Higgins (2011). The between-arm contrast is then a familiar binary effect measure.

respondeR evaluates the requested tail directly on the log scale (pnorm(..., log.p = TRUE)), so \(\log p\) and \(\log(1 - p)\) keep full precision even when \(p\) is far below machine precision. Risk ratios and odds ratios are computed from these log probabilities, never from a clamped \(p\).

Sampling variance of a responder probability

The arm mean \(\bar x\) and SD \(s\) are estimates, so \(p\) is too. By the delta method, with \(a = (\mu - m)/\sigma\) for the Normal model,

\[ \mathrm{Var}(\hat p) \approx \phi(a)^2\left[\frac{1}{n} + \frac{a^2}{2(n-1)}\right], \qquad \mathrm{Var}(\log \hat p) \approx \lambda(a)^2\left[\frac{1}{n} + \frac{a^2}{2(n-1)}\right], \]

where \(\lambda(a) = \phi(a)/\Phi(a)\) is the inverse Mills ratio, computed as \(\exp\{\log\phi(a) - \log\Phi(a)\}\) so it stays finite in the tails. The logit scale uses \(\lambda(a) + \lambda(-a)\). For the Student-\(t\) and lognormal models the gradients are numerical (central differences of \(\log p\)) and the joint sampling distribution of \((\bar x, s)\) uses the model’s own third and fourth central moments \(\mu_3, \mu_4\):

\[ \mathrm{Var}(s) \approx \frac{\mu_4 - \sigma^4 (n-3)/(n-1)}{4 n \sigma^2}, \qquad \mathrm{Cov}(\bar x, s) \approx \frac{\mu_3}{2 n \sigma}. \]

For the Normal model these reduce to \(\sigma^2 / (2(n-1))\) and 0. These are first-order (large-sample) formulas. In finite samples the sampling variance of an SD converges slowly under heavy tails or skew, so for the \(t\) (\(\nu > 4\)) and lognormal models the intervals tend to be somewhat too wide in the tails of the distribution. A \(t\) model with \(\nu \le 4\) degrees of freedom has no finite fourth moment; the Normal-theory value is then used, which makes the intervals much too narrow, and respondeR warns. Treat the non-Normal models as sensitivity analyses.

Each per-arm variance is floored at half a patient’s resolution, \(p_0(1 - p_0)/n\) with \(p_0 = 0.5/n\), so that a probability deep in a tail (whose model-based variance underflows) can never receive an infinite weight.

The pooling methods

Studies report per-arm summaries; the methods differ in how those are combined. Study \(i\) contributes \((\bar d_{e,i}, s_{e,i}, n_{e,i})\) and \((\bar d_{c,i}, s_{c,i}, n_{c,i})\).

responder_analysis(sample_responder_data, mid = 1, method = "individual")
#> Responder analysis (respondeR)
#>   MID = 1 (a higher change is a response); k = 3 studies; fixed
#>   effects; delta per-study variance
#>   Percentages; 95% confidence intervals.
#> 
#>      Method PE PC                  RD                  RR                  OR
#>  Individual  -  - 25.5 (20.0 to 31.0) 2.15 (1.79 to 2.58) 3.20 (2.45 to 4.18)
#>                    NNT
#>  NNTB 3.9 (3.2 to 5.0)
#> 
#> Use as.data.frame() for every column, summary() for details, plot() for a forest plot.

paradox <- data.frame(
  study = c("A", "B"), change_e = c(2.5, 0.5), sd_e = 1, n_e = c(400, 20),
  change_c = c(2.0, 0.0), sd_c = 1, n_c = c(20, 400)
)
responder_rd_individual(paradox, mid = 1)[, c("study", "rd")]
#>   study         rd
#> 1     A 0.09184805
#> 2     B 0.14988228
as.data.frame(responder_analysis(paradox, mid = 1,
  method = c("individual", "weighted")))[, c("method", "rd")]
#>       method        rd
#> 1 individual 0.1208652
#> 2   weighted 0.1600305

responder_analysis(sample_responder_data, mid = 1, control = "median")
#> Responder analysis (respondeR)
#>   MID = 1 (a higher change is a response); k = 3 studies; fixed
#>   effects; delta per-study variance
#>   Percentages; 95% confidence intervals.
#> 
#>           Method   PE   PC                  RD                  RR
#>       Individual    -    - 25.5 (20.0 to 31.0) 2.15 (1.79 to 2.58)
#>    Weighted mean 47.3 21.5                25.8                2.20
#>  Unweighted mean 47.7 21.5                26.2                2.22
#>           Median 48.7 21.5                27.2                2.26
#>                   OR                   NNT
#>  3.20 (2.45 to 4.18) NNTB 3.9 (3.2 to 5.0)
#>                 3.27              NNTB 3.9
#>                 3.32              NNTB 3.8
#>                 3.46              NNTB 3.7
#> 
#> Use as.data.frame() for every column, summary() for details, plot() for a forest plot.

Relative effect measures and the NNT

\[ \mathrm{RR} = \frac{p_e}{p_c}, \quad \mathrm{OR} = \frac{p_e/(1 - p_e)}{p_c/(1 - p_c)}, \quad \mathrm{NNT} = \frac{1}{\mathrm{RD}}. \]

Confidence intervals for RR and OR are formed on the log scale. Following Altman (1998), a positive risk difference is reported as a number needed to treat for one more responder (NNTB) and a negative one as a number needed to treat for one fewer (NNTH); when the risk-difference interval includes zero the NNT interval passes through infinity and is printed in that form.

The responder risk ratio deserves care. When responders are common in the control arms, \(\log p_c\) is close to 0 with a small variance, so the pooled risk ratio is compressed toward 1. The odds ratio is symmetric in responders and non-responders and, under the cut-point model, closer to constant across baseline risks; it is the default route for summary-of-findings tables.

Summary-of-findings rows

responder_sof() applies the pooled relative effect to an assumed control risk and reports, per 1000 patients, the control risk, the intervention risk and the difference with their intervals (Guyatt and colleagues, 2013). The default control risk is the median control-arm responder probability:

ind <- responder_analysis(sample_responder_data, mid = 1, method = "individual")
responder_sof(ind, control_risk = c(0.1, 0.3))
#> Summary of findings (responders)
#> 
#>      Method Control risk         Intervention risk
#>  individual 100 per 1000 262 per 1000 (214 to 317)
#>  individual 300 per 1000 578 per 1000 (512 to 642)
#>                                Difference        Relative effect
#>  162 more per 1000 (114 more to 217 more) OR 3.20 (2.45 to 4.18)
#>  278 more per 1000 (212 more to 342 more) OR 3.20 (2.45 to 4.18)

Common-language effect size (threshold-free)

The common-language effect size (CLES, the probabilistic index) is the probability that a randomly chosen treated patient has a better change than a randomly chosen control. Under a Normal model it is exact:

\[ \mathrm{CLES} = \Phi(\delta), \qquad \delta = \frac{\mu_e - \mu_c}{\sqrt{\sigma_e^2 + \sigma_c^2}}. \]

Per-study \(\delta_i\) are pooled by inverse variance and back-transformed. It is a population-level probability, not the chance that treatment helps a particular patient.

cles <- responder_cles(sample_responder_data)
c(cles = cles$cles, lb = cles$cles_lb, ub = cles$cles_ub)
#>      cles        lb        ub 
#> 0.6899041 0.6505162 0.7272252

The SMD bridge (method = "smd")

The second approach of Anzures-Cabrera et al. (2011) pools the standardized mean difference and maps it to an odds ratio. respondeR pools Hedges’ \(g\) and uses the logistic-distribution conversion \(\ln\mathrm{OR} = \frac{\pi}{\sqrt 3}\, g\) (Hasselblad and Hedges, 1995; Chinn, 2000). (The Cox conversion uses a factor of about 1.65 instead of \(\pi/\sqrt 3 \approx 1.81\).) The odds ratio interval is the pooled \(g\) interval transformed, so ci_method applies. Risks follow from the pooled control risk \(p_c\): \(p_e = \mathrm{logit}^{-1}(\mathrm{logit}\, p_c + \ln\mathrm{OR})\). Because \(p_c\) is itself estimated, the risk-difference and risk-ratio intervals use the delta method with \(\partial\mathrm{RD}/\partial \ln\mathrm{OR} = p_e(1 - p_e)\) and \(\partial\mathrm{RD}/\partial p_c = p_e(1-p_e)/\{p_c(1-p_c)\} - 1\), including the covariance between \(g\) and the pooled control mean. That covariance comes from the full Jacobian of the within-study sampling errors; under random effects the same correlation is assumed between the pooled \(g\) and the pooled control mean (an assumption with a visible effect on the risk-difference interval), and the control mean is pooled with a Wald variance while ci_method applies to \(g\). The prediction interval is transformed to the risk-difference scale at \(p_c\).

responder_analysis(sample_responder_data, mid = 1, method = "smd")
#> Responder analysis (respondeR)
#>   MID = 1 (a higher change is a response); k = 3 studies; fixed
#>   effects; delta per-study variance
#>   Percentages; 95% confidence intervals.
#> 
#>          Method   PE   PC                  RD                  RR
#>  SMD (logistic) 50.1 22.1 28.1 (22.1 to 34.1) 2.27 (1.89 to 2.73)
#>                   OR                   NNT
#>  3.55 (2.69 to 4.69) NNTB 3.6 (2.9 to 4.5)
#> 
#> Use as.data.frame() for every column, summary() for details, plot() for a forest plot.

Converting a pooled effect fitted elsewhere

responder_from_effect() applies the same bridge to a single pooled effect, for example from a published meta-analysis, given a baseline control responder risk; responder_analysis() calls it for you when you pass a metafor rma object, and reads the arms directly when you pass a meta::metacont object. The effect is taken as experimental minus control and oriented by direction, so a pain reduction with direction = "lower" raises the responder risk. For a mean difference the experimental proportion is the control probability shifted along the Normal cut-point,

\[ p_e = \Phi\!\left(\Phi^{-1}(p_c) + \frac{\mathrm{MD}}{\sigma}\right), \]

which needs a reference within-arm SD \(\sigma\) (sd_ref). With a fixed baseline probability the intervals are conditional on it, as in a GRADE summary-of-findings table; with an estimated baseline (a mean, SD and \(n\)) its uncertainty is propagated jointly with that of the effect. See vignette("meta-analysis-objects") for worked examples.

Random effects and heterogeneity

The individual, weighted and SMD methods can use random effects (pooling = "random") with DerSimonian-Laird or REML (tau_method = "REML", via metafor) between-study variance. Cochran’s \(Q\), \(I^2\) and \(\tau^2\) are inverse-variance based and refer to the scale that is pooled: the risk difference for individual, Hedges’ \(g\) for smd.

Three interval methods are available for the pooled estimate:

• "wald": Normal critical value; can under-cover with few studies.
• "hksj": Hartung-Knapp-Sidik-Jonkman, a \(t_{k-1}\) interval scaled by the observed dispersion. When the studies agree more closely than their within-study variances predict, it can be much narrower than Wald.
• "hksj_adhoc": HKSJ with the ad hoc floor \(\max(1, q)\) of Rover, Knapp and Friede (2015), never narrower than Wald. Recommended when there are few studies.

near_identical <- data.frame(
  study = 1:4, change_e = c(1, 1.01, 0.99, 1), sd_e = 1, n_e = 100,
  change_c = 0, sd_c = 1, n_c = 100
)
sapply(c("wald", "hksj", "hksj_adhoc"), function(ci) {
  r <- responder_analysis(near_identical, mid = 0.5, method = "individual",
                          pooling = "random", ci_method = ci)
  r$rd_ub - r$rd_lb
})
#>       wald       hksj hksj_adhoc 
#> 0.10356253 0.00914819 0.16815726

The prediction interval is \(\hat\mu \pm t_{k-2}\sqrt{\hat\tau^2 + V}\), where \(V\) is the variance that built the confidence interval, for every tau_method (Higgins, Thompson and Spiegelhalter, 2009). It is not reported for fewer than three studies and is unstable for small \(k\).

Uncertainty in the MID

If the threshold is itself estimated, mid_sd treats it as uncertain. The MID is one threshold shared by every study and both arms, so its uncertainty is fully correlated across studies. respondeR pools without it and then adds it once, through the derivative of the whole pooled estimator with respect to the MID:

\[ \mathrm{Var}_{\text{total}}(\hat\theta) = \mathrm{Var}_{\text{sampling}}(\hat\theta) + \left(\frac{\partial \hat\theta}{\partial m}\right)^2 \mathrm{SD}(m)^2 . \]

It therefore does not change the weights, \(\tau^2\) or the point estimate, and it does not shrink as studies are added. A complementary approach is to report the results across a range of thresholds with responder_curve():

plot(responder_curve(sample_responder_data, mids = seq(0, 2, by = 0.25)), at = 1)

The bands are pointwise.

Multi-arm trials

A trial with two active arms and one control contributes two comparisons that reuse the same control patients. Entering them as independent studies counts those patients twice and overstates the precision. Mark such comparisons with a trial column: when comparisons of one trial share an identical control arm, respondeR divides the control sample size equally among them (Cochrane Handbook, section 23.3.4). The bundled vas_pain data use this for two three-arm trials.

Final values and absolute thresholds

A responder can also be defined by the outcome reaching an absolute state, for example a pain score of 3 or less at follow-up, rather than by a change. The same cut-point model applies to the final-value distribution: put the final means and SDs in the change_e/change_c columns and the absolute threshold in mid. The interpretation differs (the proportion in the target state, not the proportion who improved by the MID), so label the analysis accordingly.

final <- data.frame(
  study = c("A", "B"), change_e = c(2.6, 3.1), sd_e = c(1.9, 2.1), n_e = c(60, 45),
  change_c = c(3.8, 4.2), sd_c = c(2.0, 2.2), n_c = c(58, 47)
)
# Proportion with a final pain score of 3 or less (lower is better).
responder_analysis(final, mid = 3, direction = "lower", method = "individual")
#> Responder analysis (respondeR)
#>   MID = 3 (a lower change is a response); k = 2 studies; fixed effects;
#>   delta per-study variance
#>   Percentages; 95% confidence intervals.
#> 
#>      Method PE PC                  RD                  RR                  OR
#>  Individual  -  - 21.7 (11.2 to 32.1) 1.67 (1.29 to 2.18) 2.47 (1.57 to 3.89)
#>                    NNT
#>  NNTB 4.6 (3.1 to 8.9)
#> 
#> Use as.data.frame() for every column, summary() for details, plot() for a forest plot.

Interval options

• ci_type = "logit" builds the weighted method’s risk-difference interval by the MOVER method from logit-scale arm intervals, allowing for their correlation, so it stays within \([-1, 1]\). It also applies to responder_rd_individual(). Pooled individual and smd intervals are formed on the scale that is pooled. A nonzero mid_sd correlates the arms in a way MOVER cannot carry, so Wald intervals are used, with a warning.
• dist = "lognormal" or "t" changes the change-score model as a sensitivity analysis for skewed or heavy-tailed data.

Assumptions and limitations

• Change-score distribution. The probabilities assume a parametric model for the patient-level change within each arm. Means and SDs alone cannot establish that the model fits; skewed outcomes can bias the responder proportions. Use dist as a sensitivity analysis.
• Summary-statistic input. Only means, SDs and sample sizes are used.
• Choice of MID. Results depend on the threshold. Report the MID, show the curve across plausible thresholds, and consider the threshold-free CLES.
• Normal-approximation intervals. Wald intervals can misbehave for extreme proportions or tiny samples, and the delta method is a first-order approximation.

Choosing a method

Changes from version 0.1.0

Version 0.2.0 changes several results. The per-study variance defaults to the delta method; fixed-effect risk differences use sample-size weights; ratio measures use exact log probabilities; MID uncertainty is added after pooling; the weighted method anchors on the within-study difference; the SMD intervals keep the chosen interval method and the control-risk uncertainty; prediction intervals follow one rule; and shared control arms of multi-arm trials are split. se_method = "binomial", rd_weights = "inverse_variance" reproduces the version 0.1.0 individual point estimate.

Relation to the thesis

The thesis behind this package (Sofi-Mahmudi, 2024) evaluated the median, unweighted, weighted and individual strategies by simulation, pooling means and SDs by inverse variance and converting a pooled risk ratio to a risk difference with a representative baseline risk. The estimators here follow its framework but differ in detail (the pooled SD, the anchored weighted method, the variance models and the interval options), so the thesis simulations do not validate every option in this package.

References

Sofi-Mahmudi, A. (2024). Identifying an optimal strategy for converting pain as a continuous outcome to a responder analysis [Master’s thesis, McMaster University]. MacSphere. https://hdl.handle.net/11375/30210

Thorlund, K., Walter, S. D., Johnston, B. C., Furukawa, T. A., & Guyatt, G. H. (2011). Pooling health-related quality of life outcomes in meta-analysis: a tutorial and review of methods for enhancing interpretability. Research Synthesis Methods, 2(3), 188 to 203. doi:10.1002/jrsm.46

Anzures-Cabrera, J., Sarpatwari, A., & Higgins, J. P. T. (2011). Expressing findings from meta-analyses of continuous outcomes in terms of risks. Statistics in Medicine, 30(25), 2967 to 2985. doi:10.1002/sim.4298

Altman, D. G. (1998). Confidence intervals for the number needed to treat. BMJ, 317(7168), 1309 to 1312.

Altman, D. G., & Deeks, J. J. (2002). Meta-analysis, Simpson’s paradox, and the number needed to treat. BMC Medical Research Methodology, 2, 3.

Chinn, S. (2000). A simple method for converting an odds ratio to effect size for use in meta-analysis. Statistics in Medicine, 19(22), 3127 to 3131.

Guyatt, G. H., Oxman, A. D., Santesso, N., et al. (2013). GRADE guidelines: 12. Preparing summary of findings tables: binary outcomes. Journal of Clinical Epidemiology, 66(2), 158 to 172.

Hasselblad, V., & Hedges, L. V. (1995). Meta-analysis of screening and diagnostic tests. Psychological Bulletin, 117(1), 167 to 178.

Higgins, J. P. T., Thompson, S. G., & Spiegelhalter, D. J. (2009). A re-evaluation of random-effects meta-analysis. Journal of the Royal Statistical Society: Series A, 172(1), 137 to 159.

McGraw, K. O., & Wong, S. P. (1992). A common language effect size statistic. Psychological Bulletin, 111(2), 361 to 365.

Rover, C., Knapp, G., & Friede, T. (2015). Hartung-Knapp-Sidik-Jonkman approach and its modification for random-effects meta-analysis with few studies. BMC Medical Research Methodology, 15, 99. doi:10.1186/s12874-015-0091-1