Bio Clinical Biostatistics Effect Measures

Version Compatibility

Reference examples tested with: statsmodels 0.14+, numpy 1.26+, pandas 2.1+, matplotlib 3.8+, marginaleffects (Python) 0.0.13+ / (R) 0.20+. R packages cited: ratesci, exact2x2, marginaleffects, riskCommunicator, RobinCar.

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures
• R: packageVersion('<pkg>') then ?function_name

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Treatment Effect Measures for Clinical Trials

"Compute treatment effect sizes" -> Estimate the population-level treatment contrast (OR, RR, RD, HR, NNT) with a confidence interval calibrated to sample size and a clear declaration of whether the estimand is marginal or conditional under ICH E9(R1).

Algorithmic Taxonomy

Postdoc reading: Permutt 2020 Stat Biopharm Res 12:45 ("Do covariates change the estimand?") established that the conditional OR from a multivariable logistic regression is a different parameter than the marginal OR -- and the FDA May 2023 Final Guidance "Adjusting for Covariates in RCTs" requires the marginal estimand for primary reporting (see clinical-biostatistics/logistic-regression for g-computation/standardisation machinery).

Decision Tree by Scenario

Crude Effect Measures from 2x2 Tables

Goal: Compute unadjusted OR, RR, RD from a contingency table with calibrated CIs.

Approach: Use Table2x2 for OR/RR with Wald CIs, but switch to score-based methods (Miettinen-Nurminen, Newcombe-Wilson) for regulatory contexts.

from statsmodels.stats.contingency_tables import Table2x2
from statsmodels.stats.proportion import confint_proportions_2indep
import numpy as np

# Table layout: [[treated_event, treated_no_event], [control_event, control_no_event]]
table = np.array([[a, b], [c, d]])
t = Table2x2(table)
print('OR:', t.oddsratio, t.oddsratio_confint())        # Wald log-OR CI
print('RR:', t.riskratio, t.riskratio_confint())         # Wald log-RR CI

# Modern CI for RD (regulatory preferred):
ci_rd = confint_proportions_2indep(a, a+b, c, c+d, method='newcomb', alpha=0.05)
# 'newcomb' = Newcombe-Wilson hybrid; 'agresti-caffo' is the +1+1 +1+1 adjustment
# For Miettinen-Nurminen stratified CIs, use R `ratesci::scoreci`

Table orientation is critical: Table2x2 interprets the first column as "outcome present." pd.crosstab orders columns alphabetically; if outcome is coded 0/1, the table will have 0 first and the OR will be the reciprocal of intended. Always reorder: cross = cross[[1, 0]] or cross = cross[['Yes', 'No']]. This is a silent direction-reversing error.

For the underlying significance test of a 2x2: Pearson chi-square (no Yates) at n>=40 with adequate expected counts; otherwise Boschloo's exact (uniformly more powerful than Fisher's exact at the same Type-I per Mehta-Senchaudhuri 2003; Lydersen-Fagerland-Laake 2009). See clinical-biostatistics/categorical-tests for the algorithmic taxonomy.

Modern Confidence Intervals -- the Postdoc Toolkit

Wald CIs are the textbook default but have well-documented coverage failures (Brown-Cai-DasGupta 2001 Stat Sci 16:101 -- "chaotic" coverage near 0 and 1 even with moderate n; Newcombe 1998a Stat Med 17:873 documents 11 alternatives for the difference of two proportions).

Single proportion

from statsmodels.stats.proportion import proportion_confint
ci = proportion_confint(45, 60, alpha=0.05, method='wilson')
# Other methods: 'jeffreys', 'agresti_coull', 'beta' (Clopper-Pearson), 'normal' (Wald)

Difference of two proportions (RD)

Ratio of two proportions (RR)

Odds ratio

Hauck-Donner effect (1977 JASA 72:851; revived by Yee 2022 JASA 117:1763): the Wald test statistic is non-monotonic in the parameter estimate near the boundary -- a large OR can produce a tiny Wald chi-square so the test fails to reject when it should. Use LR or profile-likelihood inference. VGAM::hdeff() detects this in fitted models.

Number Needed to Treat (NNT) -- the Bender 2001 CCT 22:102 Way

Goal: Convert RD to clinically intuitive NNT with a CI that handles the singularity at RD = 0.

Approach: Compute RD CI first, then transform; when RD CI crosses zero, report NNTB(lower) -> infinity -> NNTH(upper).

import numpy as np

def nnt_with_ci(treated_events, treated_n, control_events, control_n, alpha=0.05):
    """Bender 2001 *CCT* 22:102 method; transforms RD CI to NNT CI handling singularity."""
    p_t = treated_events / treated_n
    p_c = control_events / control_n
    rd = p_c - p_t   # positive = treatment helps
    se_rd = np.sqrt(p_t * (1 - p_t) / treated_n + p_c * (1 - p_c) / control_n)
    z = 1.96 if alpha == 0.05 else None
    rd_ci = (rd - z * se_rd, rd + z * se_rd)

    nnt = 1 / rd if rd != 0 else float('inf')
    if rd_ci[0] > 0 and rd_ci[1] > 0:
        return f'NNTB {1/rd_ci[1]:.0f} to {1/rd_ci[0]:.0f}'
    elif rd_ci[0] < 0 and rd_ci[1] < 0:
        return f'NNTH {1/-rd_ci[1]:.0f} to {1/-rd_ci[0]:.0f}'
    else:
        # CI crosses zero -- Bender convention
        if rd > 0:
            return f'NNTB {nnt:.0f} (NNTB {1/rd_ci[1]:.0f} to inf to NNTH {1/-rd_ci[0]:.0f})'
        return f'NNTH {-nnt:.0f} (similar split)'

Bender 2001 Controlled Clinical Trials 22:102-110 resolves the discontinuity at RD = 0: NNT has a singularity there, so a CI that crosses zero produces a disjoint NNT CI ("NNTB(some) -> infinity -> NNTH(some)"). The Cochrane/BMJ convention is to report exactly this -- the infinity in the middle signals non-significance and is more honest than truncating to one side.

NNT from OR + baseline risk (when only the OR is published):

def nnt_from_or(odds_ratio, baseline_risk):
    baseline_odds = baseline_risk / (1 - baseline_risk)
    treatment_odds = baseline_odds * odds_ratio
    treatment_risk = treatment_odds / (1 + treatment_odds)
    arr = abs(baseline_risk - treatment_risk)
    return 1 / arr if arr > 0 else float('inf')

Always report baseline risk alongside NNT -- the same OR produces dramatically different NNTs.

Marginal RD via G-Computation -- The FDA 2023 Recipe

Goal: Compute the marginal RD (FDA 2023 primary estimand for binary endpoints) from a fitted logistic regression -- without requiring the analyst to switch model class.

Approach: Standardise over the observed covariate distribution. Predict per-subject probability under treatment=1 AND under treatment=0; take the mean of each; the difference is the marginal RD. SE via influence function or bootstrap.

import statsmodels.formula.api as smf
import numpy as np

# Step 1: Fit logistic with covariates
fit = smf.logit('y ~ z + x1 + x2 + x3', data=df).fit()

# Step 2: Predict under each treatment regime (counterfactual prediction)
df_z1 = df.assign(z=1)
df_z0 = df.assign(z=0)
p_z1 = fit.predict(df_z1)
p_z0 = fit.predict(df_z0)

# Step 3: Marginal estimates
marg_p1 = p_z1.mean()
marg_p0 = p_z0.mean()
marg_rd = marg_p1 - marg_p0          # FDA 2023 primary
marg_rr = marg_p1 / marg_p0
marg_or = (marg_p1 / (1 - marg_p1)) / (marg_p0 / (1 - marg_p0))

# Step 4: SE via bootstrap (or analytical influence function in R `marginaleffects`)
n_boot = 1000
boot_rds = np.zeros(n_boot)
rng = np.random.default_rng(42)
for b in range(n_boot):
    idx = rng.choice(len(df), size=len(df), replace=True)
    fit_b = smf.logit('y ~ z + x1 + x2 + x3', data=df.iloc[idx]).fit(disp=0)
    p1_b = fit_b.predict(df_z1.iloc[idx]).mean()
    p0_b = fit_b.predict(df_z0.iloc[idx]).mean()
    boot_rds[b] = p1_b - p0_b

se_marg_rd = boot_rds.std(ddof=1)
ci_marg_rd = (marg_rd - 1.96*se_marg_rd, marg_rd + 1.96*se_marg_rd)

R equivalent (preferred for confirmatory):

library(marginaleffects)
fit <- glm(y ~ z + x1 + x2 + x3, family = binomial, data = df)
avg_comparisons(fit, variables = 'z', vcov = 'HC3')
# Returns marginal RD with HC3 sandwich SE; CIs; deltamethod_inference

Tsiatis et al 2008 robustness guarantee: under randomisation Z ⊥ X, the g-computation marginal estimator is consistent for marginal ATE EVEN IF the outcome model is misspecified. Efficiency depends on model quality; consistency does not. This is why FDA 2023 accepts marginal RD via g-computation without requiring proof of correct logistic mean structure.

Marginal vs Conditional Effects -- The Core ICH E9(R1) Question

For logistic regression, the maximum-likelihood coefficient on Z in glm(Y ~ Z + X, family=binomial) is the conditional log odds ratio -- the OR comparing Z=1 to Z=0 holding X fixed. Because the OR is non-collapsible, this is different from the marginal log OR even under perfect randomisation. Gail, Wieand & Piantadosi 1984 (Biometrika 71:431) first showed that treatment-effect estimates from nonlinear models are biased for the marginal effect when prognostic covariates are omitted, even under randomisation; this non-collapsibility is what FDA 2023 addresses by targeting a marginal estimand.

# Conditional OR (the default; biased toward null vs marginal OR)
import statsmodels.formula.api as smf
import numpy as np

model = smf.logit('y ~ z + x1 + x2', data=df).fit()
cond_or = np.exp(model.params['z'])

# Marginal RD via g-computation / standardisation:
df_z1, df_z0 = df.assign(z=1), df.assign(z=0)
p_z1 = model.predict(df_z1).mean()
p_z0 = model.predict(df_z0).mean()
marg_rd = p_z1 - p_z0
marg_rr = p_z1 / p_z0
marg_or = (p_z1 / (1-p_z1)) / (p_z0 / (1-p_z0))

# Variance via influence function or bootstrap (RobinCar / marginaleffects)
# R: marginaleffects::avg_comparisons(model, variables='z', vcov='HC3')

The reporting standard post-FDA-2023: marginal estimand as primary (RD with HC3 sandwich SE), conditional as supportive. The two are different parameters, not different estimates of the same parameter. See clinical-biostatistics/logistic-regression for full g-computation machinery and Tsiatis et al 2008 Stat Med 27:4658.

Non-collapsibility -- the standard explanation

A conditional (adjusted) OR typically differs from the marginal (unadjusted) OR even WITHOUT confounding. Conditioning on a prognostic covariate (one that predicts outcome but is unrelated to treatment) changes the OR by a mathematical property -- not bias. In most settings, the conditional OR is further from the null than the marginal OR. This is what postdocs argue about: Frank Harrell argues conditional effects are more clinically meaningful (they apply to individuals); FDA via Permutt argues marginal effects are what regulatory submissions need (the population-level decision). FDA 2023 settles it for regulatory reporting: marginal primary, conditional supportive.

Risk ratios and risk differences are collapsible. Marginal and conditional RR/RD are equal absent confounding. This is why FDA 2023 favours RD as the primary estimand for binary outcomes -- the conditional vs marginal distinction disappears.

Reconciliation: When Methods Disagree