A few random thoughts and questions - bearing in mind I've not done a DT course and I'm in a different country.
Firstly, I'm assuming there are minimum standards required to become a police officer relating to things like fitness and other physical condition - is that assumption valid?
A few pages ago there was a statement that there have been 8 officer deaths caused by assault in the last 8 years, so on average one per year - I have to question this figure... Even excluding firearm assaults, figures on wiki suggest that "death by other assault" was closer to 40 last year alone.
I haven't dug into those figures further, but it seems there's a pretty tight selection criteria going on to claim an average of 1.
Even so, surviving an assault isn't really a valid proof of efficacy. How many of the assaults were really carried out with causing death as the primary motivation? Much more likely to me is that "not get arrested" would be the main aim, and causing a death would be an unintended outcome.
I know plenty of people who have survived an assault with zero training, so I have to completely discount that especially if those minimum standards count because good physical condition gives you an immediate advantage.
And then there's what DT is really intended to achieve as well. It's been mentioned that it doesn't just cover defence.
I know that there's a huge bag of monkeys difference between holding someone down, and holding someone down and cuffing them.
So, it must have an effect there. It must teach a few things that are specifically useful in that situation?
Are figures available to show failure to defend that didn't involve death? For instance, an officer sees someone breaking into a car and challenges them. They punch or shove the officer and run away.
That's a failure to detain, and a failure to defend. Effectively, a failure of DT.
Is there anything other than "we all use it" available to show that DT really improves the outcome in that sort of situation?