Evidence to Convict?

[RoadRunner79]

How would you honestly answer, This is only about this scenario, not considering anything else.

Surely it's a straightforward question to answer, namely that if I saw you smash somebody over the head I'd make a statement to the police saying exactly that, but if I didn't see you do it I'd tell them that instead.
Or am I missing some hidden depth or something in your question?

Thanks,

It not so much about giving your testimony, but what can other's reason from it.

Your comment brings to mind however;  if testimony is not evidence at all, then why would the police bother gathering it and have procedures to keep it from becoming contaminated. 
The second part of the question is if it is sufficient to come to a reasonable conclusion based on testimony alone.  In this case, we have the testimony of a number of independent witnesses.