We don't know the true population parameters.

我们不知道真正的人口参数。

We don't know what the true difference of these two population parameters are.

我们不知道这两个人口参数的真正区别是什么。

Maybe there's some huge study and this was the best estimate of what the population parameters are.

也许有一些庞大的研究，这是对人口参数的最佳估计。