Sample size calculation testing interaction effect for Cox proportional hazards regression with two covariates for Epidemiological Studies. Both covariates should be binary variables. The formula takes into account the correlation between the two covariates.

1 | ```
ssizeEpiInt2(power, theta, psi, mya, myb, myc, myd, alpha = 0.05)
``` |

`power` |
postulated power. |

`theta` |
postulated hazard ratio. |

`psi` |
proportion of subjects died of the disease of interest. |

`mya` |
number of subjects taking values |

`myb` |
number of subjects taking values |

`myc` |
number of subjects taking values |

`myd` |
proportion of subjects taking values |

`alpha` |
type I error rate. |

This is an implementation of the sample size calculation formula derived by Schmoor et al. (2000) 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 + γ (x_1 x_2)),*

where both covariates *X_1* and *X_2* are binary variables.

Suppose we want to check if
the hazard ratio of the interaction effect *X_1 X_2=1* to *X_1 X_2=0* is equal to *1*
or is equal to *\exp(γ)=θ*.
Given the type I error rate *α* for a two-sided test, the total
number of subjects required to achieve a power of *1-β* is

*n=\frac{≤ft(z_{1-α/2}+z_{1-β}\right)^2 G}{
[\log(θ)]^2 ψ (1-p) p (1-ρ^2)
},*

where *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)*, and

*G=\frac{[(1-q)(1-p_0)p_0+q(1-p_1)p_1]^2}{(1-q)q (1-p_0)p_0 (1-p_1) p_1},*

and
*p0=Pr(X_1=1 | X_2=0)=myc/(mya+myc)*,
*p1=Pr(X_1=1 | X_2=1)=myd/(myb+myd)*,
*p=Pr(X_1=1)=(myc+myd)/n*,
*q=Pr(X_2=1)=(myb+myd)/n*,
*n=mya+myb+myc+myd*.

*p_{00}=Pr(X_1=0,\mbox{and}, X_2=0)*,
*p_{01}=Pr(X_1=0,\mbox{and}, X_2=1)*,
*p_{10}=Pr(X_1=1,\mbox{and}, X_2=0)*,
*p_{11}=Pr(X_1=1,\mbox{and}, X_2=1)*.

The total number of subjects required.

Schmoor C., Sauerbrei W., and Schumacher M. (2000).
Sample size considerations for the evaluation of prognostic factors in survival analysis.
*Statistics in Medicine*. 19:441-452.

`ssizeEpiInt.default0`

, `ssizeEpiInt.default1`

1 2 3 4 5 6 | ```
# Example at the end of Section 4 of Schmoor et al. (2000).
# mya, myb, myc, and myd are obtained from Table III on page 448
# of Schmoor et al. (2000).
ssizeEpiInt2(power = 0.8227, theta = 3, psi = 139 / 184,
mya = 50, myb = 21, myc = 78, myd = 35, alpha = 0.05)
``` |

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