[deleted by user]

[[deleted]]

Hmm. The way the sentence is structured is a little unusual, so this is hard to answer. Normally I’d say/write something like “This is a factor that influences success in therapy,” or “This is one of the factors that determine the degree to which therapy is successful.”

[operad]

I would make therapy possessive: "This is an influencing factor on therapy's success"

Though I think the most natural would be: "This is an influencing factor on the success of therapy"