Review Request Fields

Hi

I am a PhD student, working on the topic of recommender systems in code review process. I am using mozilla data for my research. In the exploratory analysis phase of my work, I am having problem in distinguishing between accepted patches, open ad reject patches. I have extracted data from Review Board. there are two fields in each patches that seems to be relevant, "approval_failure" and "approved". based on my understanding, when the status of "approved" is "true" and "approval_failure" is "null", that means the patch has been accepted. but in the case of having "approved" filed equal to "false", there might be different status for "approval_failure". I am not sure which one represents the rejected patches, and which ones are open patches. and if my assumptions are correct at all or not. 
 I appreciate if someone can help me in this regards.

thanks.