cross-over指什么？

There has been substantial heterogeneity in steroid dosing, timing, and duration in clinical trials studying steroids in ARDS, and results have been mixed. In 1 trial comparing the early administration of very high-dose methylprednisolone (120 mg/ kg over 24 hours in divided doses) with placebo in 99 patients with ARDS, no difference was found in the 45-day mortality rate, or in the reversal of ARDS.19However, a small subsequent placebo-controlled trial in 24 patients assessing a prolonged course of lower dose methylprednisolone (2 mg/kg/d with tapering over a maximum of 32 days) in unresolving ARDS (?7 days of mechanical ventilation) reported that steroids were associated with improvements in both physiologic parameters and mortality (ICU and hospital), although there was substantial cross-over in this trial.

在类固醇治疗急性呼吸窘迫综合征的临床研究中，在其剂量、时间和持续时间方面存在很大的异质性，结果也不一。在一项99例ARDS患者早期应用大剂量甲基强的松龙(24小时剂量120mg/kg，分次给药)与安慰剂进行对照研究中，未发现45天死亡率、ARDS 逆转的差异。然而，随后在24名患者中进行的一项小型安慰剂对照研究，评估了小剂量甲基强的松龙(2 mg/kg/d，最长32天内逐渐减量)长疗程在未缓解ARDS(机械通气7天)中的治疗作用，结果显示类固醇与生理参数和死亡率(ICU和医院)的改善存在相关性，尽管在这项试验中有很大的交叉。

cross-over指什么？指生理参数和死亡率有时可以互相代表？