如何实现联合hyerprior?

第五章第三节大鼠肿瘤。一个层次模型是适合的,所使用的超先验不是pymc3中包含的密度之一。

超优先级是 a*b*(a+b)^-2.5

import pymc3 as pm
with pm.Model() as model:

    def ab_dist(x):
        #Should be log density, from what I have read
        a = x[0]
        b = x[1]
        return a+b-5/2*(a+b)

    ab = pm.DensityDist('ab', ab_dist, shape = 2, testval=[2,2])

    a = ab[0]
    b = ab[1]

    theta = pm.Beta('theta',alpha = a,beta = b)

    Y= pm.Binomial('y', n = n, p = theta, observed = y)

在这个阶段,我返回了一个错误

ValueError: Input dimension mis-match. (input[0].shape[0] = 71, input[1].shape[0] = 20000)