It is the only sample we have... But the you cannot consistently find any evidence that anything else works...There is no data on self defence that we can access.
Yes, there is quite a plethora of data. The Boatman Edged Weapon program has years of statistical data, both in the UK and in the U.S. This is why many L.E. agencies, including mine, have adopted it. No sparring involved. Last statistical data from the North Hamptonshire P.D. for G.B. as a whole indicates that prior to the adoption of the program, officers were injured 78% of the time in an edged weapon altercation (very prevalent in the U.K). After adoption of the program the injury rate dropped to 17%. More importantly, since it is based upon gross motor skill, it is retained in long term memory. This means that remedial training went from annually to every 18 months. Our retraining rate is 12-24 months depending upon the cycle the Deputy is in at the time.
L.E. doesn't use sparring, at least none that I'm aware of when we're discussing in-service training. Rather it is quite often scenario-based training which has already been discussed in other threads.
WWII combatives, possibly the most effective long term program, and certainly amount the most brutal never used sparring. In fact the actual training program was quite brief. Yet again, based upon gross motor skills the retention rate actually spanned into decades. Anyone in the combatives community knows of the effectiveness of the program.
My school never sparred. Yet our own data spanned women preventing date rapes, Correction Officers, Deputies and Baliffs had successful uses-of-force as well as Executive Protection Agents I've taught (one of which is now a fellow Deputy on my shift).
Since we're discussing self defense and not competitions, MMA using sparring as a training platform is not evidence for it's effectiveness or necessity. As noted in this and a multitude of other threads they are two separate animals and one training methodology that is effective in one venue doesn't constitute effectiveness in the other. And as I've pointed out multiple times, one methodology can actually be detrimental for an opposing venue.
Take home point, while sparring may be beneficial in a sport setting, it isn't necessary for self-defense.