logrank_sample_size()

Total sample size needed for the log-rank test to reach a target power.

Usage

logrank_sample_size(
    hazard_ratio,
    prob_event,
    *,
    power=0.8,
    alpha=0.05,
    allocation=0.5,
    sides=2
)

Computes the number of subjects required to observe enough events for a log-rank test to achieve a target power when detecting a specified hazard ratio. This function combines two calculations:

First, it computes the required number of events using logrank_n_events (Schoenfeld’s formula under proportional hazards).
Then, it converts events to subjects using the expected probability that a subject experiences the event during follow-up.

Workflow: Start here to plan study size. You provide the expected effect size (hazard ratio), the fraction of subjects expected to have the event (based on baseline hazard, accrual, and follow-up time), and your desired power. The result is the total enrollment needed.

Parameters

hazard_ratio: float

The hazard ratio to detect (group 2 versus group 1). Smaller HR (e.g., 0.5 = 50% hazard reduction) requires fewer subjects for a given power; larger HR (e.g., 0.8 = 20% hazard reduction) requires more.

prob_event: float

Probability (or fraction) that a subject experiences the event (death, hospitalization, etc.) during the study. Range: (0, 1]. Typical values depend on the condition and follow-up duration:

Rare disease (annual incidence 1%): 0.01-0.05 per year of follow-up
Common condition (annual incidence 20%): 0.2 per year
Study design: shorter follow-up → lower prob_event; longer follow-up → higher

This is usually estimated from historical data, Kaplan-Meier curves, or clinical judgment. Use sensitivity analysis (try 0.3, 0.4, 0.5) if uncertain.

power: float = 0.8

Target statistical power (default 0.8, i.e., 80%). Interpretation: the probability that the study detects the effect if it truly exists. Common choices:

0.80 (80%): Conventional, implies 20% Type-II error rate
0.90 (90%): Higher confidence, requires more subjects
0.95 (95%): Stringent, requires many more subjects

alpha: float = 0.05

Significance level (Type-I error rate, default 0.05). Probability of rejecting the null hypothesis if it’s true. Use 0.05 for two-sided tests with p < 0.05 threshold; use 0.01 for stricter control or 0.10 for more exploratory studies.

allocation: float = 0.5

Fraction of subjects in one group (default 0.5, balanced design). For unbalanced allocation (e.g., control-to-treatment ratio 2:1), use allocation=0.33. Unbalanced allocation increases total sample size needed; use only if required by design or logistics.

sides: int = 2

1 (one-sided) or 2 (two-sided, default). One-sided tests have higher power and require fewer subjects but test directional hypotheses only. Two-sided tests are standard but require more enrollment.

Returns

int: Total number of subjects (enrollment) needed, rounded up. This is the total across all groups.

Details

Relationship between events and subjects:

n_subjects = ceil(n_events / prob_event)

More subjects are needed when:

prob_event is low (most subjects censored before event): e.g., 50% power, 50 events needed, but if only 25% get the event, you need 200 subjects.
The effect size is smaller: smaller HR requires more events
Power is higher: 90% power requires more events than 80%
Design is unbalanced: 3:1 allocation needs more subjects than 1:1

Estimating prob_event: Use Kaplan-Meier curves from historical data or prior studies, or calculate from baseline rates: \(\text{prob\_event} \approx 1 - \exp(-\lambda_0 \times t_{\text{follow-up}})\).

Sensitivity analysis: If prob_event is uncertain, compute sample size for a range of values (e.g., 0.3 to 0.5) to understand robustness.

Examples

A trial aims to detect a hazard ratio of 0.5 (50% hazard reduction) with 90% power. Based on historical data, about 40% of subjects are expected to have the event during follow-up. How many subjects must be enrolled?

import greenwood as gw

gw.logrank_sample_size(hazard_ratio=0.5, prob_event=0.4, power=0.9)

This sample size (~350) is much larger than the event count from logrank_n_events (~140) because most subjects will be censored before the event occurs.

Perform sensitivity analysis for uncertain event probability. How does sample size change if only 30% or 50% of subjects have events?

for prob in [0.3, 0.4, 0.5]:
    n = gw.logrank_sample_size(hazard_ratio=0.5, prob_event=prob, power=0.9)
    print(f"prob_event={prob}: n={n} subjects, {int(prob * n)} events")

prob_event=0.3: n=292 subjects, 87 events
prob_event=0.4: n=219 subjects, 87 events
prob_event=0.5: n=175 subjects, 87 events

Compare sample size for different effect sizes (smaller HR → fewer subjects):

for hr in [0.5, 0.6, 0.7]:
    n = gw.logrank_sample_size(hazard_ratio=hr, prob_event=0.4, power=0.9)
    print(f"HR {hr}: n={n} subjects")

HR 0.5: n=219 subjects
HR 0.6: n=403 subjects
HR 0.7: n=826 subjects

Use higher power (0.95) if you want to be very confident the effect is detected:

gw.logrank_sample_size(hazard_ratio=0.5, prob_event=0.4, power=0.95)

Use unbalanced allocation (e.g., 2:1 control:treatment) if logistics require it:

gw.logrank_sample_size(hazard_ratio=0.5, prob_event=0.4, power=0.9, allocation=1/3)