Thall and Simon's criterion function for determining the trial decision cutoffs based on the posterior probability.

1 | ```
postprob(y, n, alpha_e, beta_e, alpha_s, beta_s, delta)
``` |

`y` |
the number of responses among |

`n` |
the number of patients treated by the experimental drug at a certain stage of the trial. |

`alpha_e` |
the hyperparameter (shape1) of the Beta prior for the experimental drug. |

`beta_e` |
the hyperparameter (shape2) of the Beta prior for the experimental drug. |

`alpha_s` |
the hyperparameter (shape1) of the Beta prior for the standard drug. |

`beta_s` |
the hyperparameter (shape2) of the Beta prior for the standard drug. |

`delta` |
the minimally acceptable increment of the response rate for the experimental drug compared with the standard drug. |

`prob` |
the posterior probability: |

Thall, P. F., Simon, R. (1994).
Practical Bayesian guidelines for phase IIB clinical trials.
*Biometrics* **50**: 337-349.

Yin, G. (2012).
*Clinical Trial Design: Bayesian and Frequentist Adaptive Methods.*
New York: Wiley.

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