Shot Stats

While playing friendly singles with "Z", he was lamenting how he lost most of the points to double faults. 

But I felt otherwise. So, I got him to keep score while I did some mental "Shot Stats" for him.

According to my count, the shot stats for the second and third set were, respectively:
6-12-5-1
4-6-12-2

The first, second, third & fourth numbers respectively indicated, for Z, the:
(1) number of double faults,
(2) number of shots hit into the net,
(3) number of shots hit out, and
(4) others like inability to get to the ball, frame hits, double bounce, etc

Clearly, out of 24 points in each set, he lost between 71-75% of the points to what I would deem as "loss of control" issues - hitting into the net or out of court.

As Z was racket testing as well, he used a different stick for each set.

Assuming he was consistent, and adopting the ceteris paribus principle, the numbers seem to indicate, that either: 
(1) the racket used in the second set was a little too low-powered for him, or the stringbed's rebound angle was too low, or the strings were dead, or
(2) the racket used for the third set was too powerful, had too high a rebound angle, or was tensioned too low.

Whichever case it was, I still think his serve was very decent for someone who has only been playing casual tennis (mainly rallying) for only six months.