This is what I have understood, 

In order for patients to be enrolled in these clinical studies, they have to *first* satisfy certain criteria. The ***previous step(s)*** referred to are those previously mentioned in the Guideline; i.e. **4.1** *In vitro studies*, **4.2** *Determination of the need for in vivo studies* and **4.3** *In vivo studies*.

 The **(s)** in step(s) suggests that one or more steps may be taken into account. 

The more productive (or positive) the results are (**commensurate with the level of evidence**), the easier it is to compare those results (**support comparability**) from the two different studies 1. Non-clinical studies 2. Clinical studies 

Therefore, only those patients who meet this requirement are normally selected (**enrolled**) .