'experimental, test failure in solve_power for effect_size'

def _solve_effect_size(self, effect_size=None, nobs=None, alpha=None, power=None, k_groups=2):

    def func(x):
        effect_size = x
        return self._power_identity(effect_size=effect_size, nobs=nobs, alpha=alpha, k_groups=k_groups, power=power)
    (val, r) = optimize.brentq(func, 1e-08, (1 - 1e-08), full_output=True)
    if (not r.converged):
        print(r)
    return val
