Again, you are the one suggesting creation of hurdles for the exercise of a Constitutional right. That places the onus squarely upon you to prove that it would have any benefit at all, let alone such a degree of benefit that it would justify creation of said hurdles.
If that means proving a negative, that's too bad.
The relative accident rate would not require proving a negative; it would require you to look up and compare per capita accident rates in, say, Vermont (constitutional carry) and neighboring New York, or some other state that has training requirements and has had them for some period of time.
Again, you want the change, you prove the benefit.