Power Analysis for Logistic Regression in R

So far every outcome has been continuous — a satisfaction score, a wellbeing index. When the outcome is binary (recovered or not, churned or stayed, clicked or ignored), the right model is logistic regression. The only change from the previous rungs is a single constructor argument.

The model

We study whether a treatment improves recovery, controlling for age. The outcome recovered is 0 or 1, so we need a GLM with a logit link. Pass family = "logit" directly to the constructor — it is not a setter method, and it is not added via $set_* after construction. Logistic regression requires one additional piece of information that linear regression does not: the baseline event rate. model$set_baseline_probability(0.3) tells the engine that 30% of the reference group is expected to recover without treatment. Without this call the model cannot generate realistic binary outcomes, so the call is required for family = "logit".

Effects are still standardised and act as shifts on the log-odds scale. The benchmarks for binary or factor predictors differ from continuous ones: small 0.20, medium 0.50, large 0.80 — see [[concepts/effect-sizes|effect sizes]] for the reasoning behind those values.

How much power at n = 300?

library(mcpower)

model <- MCPower$new("recovered ~ treatment + age", family = "logit")
model$set_variable_type("treatment=binary")
model$set_baseline_probability(0.3)
model$set_effects("treatment=0.5, age=0.3")

result <- model$find_power(sample_size = 300, target_test = "all", verbose = FALSE)
print(summary(result))

==================================================
  MCPower · Power Analysis
==================================================
formula: recovered ~ treatment + age
estimator: GLM  N=300  sims=1600  α=0.05  target=80%
effects: treatment=0.50, age=0.30

Per-test power
───────────────────────────────────
Test                 Power   Target
───────────────────────────────────
LR χ²                82.3%      80%
treatment            54.4%      80%
age                  69.4%      80%
───────────────────────────────────

Power & 95% CI
───────────────────────────────────────────
Test                 Power           CI 95%
───────────────────────────────────────────
LR χ²                82.3%   [80.4%, 84.1%]
treatment            54.4%   [51.9%, 56.8%]
age                  69.4%   [67.1%, 71.6%]
───────────────────────────────────────────
95% CIs are Monte-Carlo (Wilson), n_sims=1600.

Joint significance distribution
────────────────────────
k     Exactly   At least
────────────────────────
0       14.6%       100%
1       47.0%      85.4%
2       38.4%      38.4%
────────────────────────

Estimator details
  baseline_prob_realized: nan
  singular_fit_rate: 0
  tau_squared_hat_mean: nan

Plots: plot(result) to view, plot(result, 'chart.png') to save.

treatment lands at 54.4% and age at 69.4% — both well below the 80% target despite a sample size that would be more than adequate for OLS. The likelihood-ratio omnibus test (LR χ²) reaches 82.3%, just above the target on its own.

The joint significance distribution tells the fuller story: only 38.4% of simulations see both effects detected in the same study, so jointly powering both tests will require a substantially larger N. Use find_sample_size to find the crossing point.

next → Mixed models