Description Usage Arguments Details Value Note References See Also Examples
Power calculation for Cox proportional hazards regression with two covariates for epidemiological Studies. The covariate of interest should be a binary variable. The other covariate can be either binary or non-binary. The formula takes into account competing risks and the correlation between the two covariates. Some parameters will be estimated based on a pilot data set.
1 | powerEpi(X1, X2, failureFlag, n, theta, alpha = 0.05)
|
X1 |
numeric. a |
X2 |
numeric. a |
failureFlag |
numeric. a |
n |
integer. total number of subjects |
theta |
numeric. postulated hazard ratio |
alpha |
numeric. type I error rate. |
This is an implementation of the power calculation formula derived by Latouche et al. (2004) for the following Cox proportional hazards regression in the epidemiological studies:
h(t|x_1, x_2)=h_0(t)\exp(β_1 x_1+β_2 x_2),
where the covariate X_1 is of our interest. The covariate X_1 should be a binary variable taking two possible values: zero and one, while the covariate X_2 can be binary or continuous.
Suppose we want to check if the hazard of X_1=1 is equal to the hazard of X_1=0 or not. Equivalently, we want to check if the hazard ratio of X_1=1 to X_1=0 is equal to 1 or is equal to \exp(β_1)=θ. Given the type I error rate α for a two-sided test, the power required to detect a hazard ratio as small as \exp(β_1)=θ is
power=Φ≤ft(-z_{1-α/2}+√{n[\log(θ)]^2 p (1-p) ψ (1-ρ^2)}\right),
where z_{a} is the 100 a-th percentile of the standard normal distribution, ψ is the proportion of subjects died of the disease of interest, and
ρ=corr(X_1, X_2)=(p_1-p_0)\times √{\frac{q(1-q)}{p(1-p)}},
and p=Pr(X_1=1), q=Pr(X_2=1), p_0=Pr(X_1=1|X_2=0), and p_1=Pr(X_1=1 | X_2=1).
p, ρ^2, and ψ will be estimated from a pilot data set.
power |
the power of the test. |
p |
proportion of subjects taking X_1=1. |
rho2 |
square of the correlation between X_1 and X_2. |
psi |
proportion of subjects died of the disease of interest. |
(1) The formula can be used to calculate
power for a randomized trial study by setting rho2=0
.
(2) When ρ^2=0, the formula derived by Latouche et al. (2004) looks the same as that derived by Schoenfeld (1983). Latouche et al. (2004) pointed out that in this situation, the interpretations are different hence the two formulae are actually different. In Latouched et al. (2004), the hazard ratio θ measures the difference of effect of a covariate at two different levels on the subdistribution hazard for a particular failure, while in Schoenfeld (1983), the hazard ratio θ measures the difference of effect on the cause-specific hazard.
Schoenfeld DA. (1983). Sample-size formula for the proportional-hazards regression model. Biometrics. 39:499-503.
Latouche A., Porcher R. and Chevret S. (2004). Sample size formula for proportional hazards modelling of competing risks. Statistics in Medicine. 23:3263-3274.
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