What does "Ss" mean in a psychology journal?

I’m going through a bunch of psychology journals on PubMed for an APA project and I keep coming across “Ss” and I don’t know what it means. Here’s an example:

Although 69% of Ss with panic disorder experienced depersonalization or derealization during their panic attacks, panic disorder Ss were no more likely to experience dissociative experiences as assessed by the Dissociative Experience Scale than Ss with other anxiety disorders.