看资料时看到一段话，有一句话我不是特别理解，还望大神解疑答惑，谢谢

Normally propensity score matching only includes baseline variables. And nonbaseline variables (e.g.mechanical ventilation) shouldn't be included in the propensity score matching. This is a way to match for the trajectory of the disease dusing the length and may introduce the bias.

这个trajectory of disease 是什么意思啊，具体的不能纳入的原因，是会引入什么样的偏椅，又应该如何处理这样的变量呢，谢谢大神