you didn't really cover the effect of lack of time which in my opinion has a great deal to do with "poor" investigation of lower severity incidents (which can then create an ingrained, and often unconscious, attitude/culture of how investigations are approached). Rarely are these lower severity incidents investigated by someone independent to the work, and so someone needs to work overtime to complete the investigation - after all the expectation is to be 100% productive when were on the clock right so the extra time to investigate has to be over and above their normal workload. Which I have seen in a lot of cases end with the injured person taking the blame to reduce the burden on everyone involved (or taking it one step earlier, the infamous non-reporting of incidents). This is generally only made worse with mandates of strict 24 hour / 7 days / etc. timeframes to close out an investigation (and sometimes even the associated corrective actions). No wonder we end up with the bare minimum to pass investigation reports (even if we give them a "good" investigation framework). And as I said this then can effect how higher severity incidents are investigated with the focus ingrained on the process of the investigation rather than the purpose.

But to me the true real killer of "good" investigations is when nothing happens as a result of them (or at least any changes made don't visibly make work better for those affected). I would be interested to know from the forum members the split (gut feeling) between "fighting tooth-and-nail" vs "barely an inconvenience" to close out significant corrective actions that actually have a chance of effecting the root cause of incidents?
Why put the effort into investigating something well, if that effort has a good chance of being wasted? Especially if that effort has to be done over and above your normal workload and you (likely) don't even get any recognition for putting in that effort!
Unfortunately (like the investigations themselves) the scope of what is looked at when trying to imporve investigation techniques tend to focus on the processes and tools rather than any wider root cause organisational factors (ironically).

In one training room there was a poster that said, "The only exercise some people get is jumping to conclusions." Isn't that one of the biggest challenges of getting useful outcomes from investigations?

Most people don't even realise that they app[ly their own biases and don't know how to ask useful questions that advance the investigation. My experience has been - including when delivering training - that most people start with an idea of what the cause is and then shape the investigation to support what they already think.

I taught people a simple hierarchy of questions (and then got trainees to do an exercise to practice using them, giving them feedback when they went off track, which was surprisingly often!) - start with a big, very open question that invites the other person to tell a story. Then ask a probing question about a specific part of that story (which is where most people jump into assumptions and closed questions), then a closed question to confirm a fact. I used to draw a funnel to show the levels of questions wide at the top, narrowing, then "a single drop of truth" coming out at the bottom, which then leads to the next open question.

Most people don't really get trained in how to follow an effective line of questioning for an investigation. If you start using closed questions too early in your line of questioning, you have introduced bias and limited the direction of the investigation.

It's also quite likely that many people find investigations tedious and are only interested in arriving at a conclusion in order to close out the investigation, rather than the wider intention of preventing future events.

I have usually found that adopting a mindset of curiosity about the event is really helpful - it becomes a task of exploring (open mind) rather than seeking (looking for proof to support what you have already decided). Usually the many "curious" questions come quite naturally from this mindset. I can recall investigating a serious harm incident (that barely missed being a fatality) using this mindset, and the whole process was rather like seeing a 3D model take shape in front of me. I went in with no assumptions, just asked a lot of questions and did a lot of listening. While it might have been easy for senior managers to blame the guy who was injured, it very quickly became clear that the site layout, lockout process and other systems did not lend themselves to good compliance with machinery isolation and confined space entry. There were also other psycho-social factors and organisational culture factors that strongly influenced behaviours and also needed to be reviewed and addressed to prevent future incidents. There were definitely many layers to that incident.

At the moment, I am working on about 4 investigations, working closely with site personnel and coaching them through the process, rather than doing it for them. We have some great tools for investigation, but we still need to facilitate for people to ask enough of the right questions and avoid premature conclusions - instead of just trying to expedite the report to tick the box and get it off our plate, we have a really great opportunity to coach, lead and support others to learn, which then builds their knowledge and skills to lead safety efforts on the sites.

While I don't agree with all his politics Jocko Willink did a great Ted Talk on what he calls "Extreme Ownership". Basically what could I have done as a manager to stop this.
https://www.youtube.com/watch?v=Qnr_VW-AuI4

Instead of "He didn't know what he was doing" - Think I didn't make sure he was effectively train.
Instead of "He was just an idiot - Think Why do I make poor hiring decisions.
Instead of "we aren't given the right resources. - Think. Why have I been unable to convince senior management to resources this appropriately.

1. Despite the fact that we INTUITIVELY feel that investigating small incidents prevents serious ones, ("look after cents and dollars will look after themselves"??); that theory was postulated, together with a cute little pyramid with DEATH at the top, in the middle of LAST century. Has anyone EVER conducted ANY research to demonstrate its truth?
The SAFEST construction sites I audit are a) neat, well organised and tidy and b) populated by cheerful, confident workpersons. Documents? Nope! Apart from the most basic requirements of relevant training and competence, they simply don't equate at all. A good Accident Investigation can DEFINITELY dig out anything that needs to change.

2. I tell my clients: TAKE RESPONSIBILITY!!
"He was an idiot"? Why did YOU not make his task "idiot proof"?;
"We aren't given the right resource?" - "Here is a list of the vital resources I asked for by formal requisition on (date!)."
NB My favourite is: "inattention" they LOVE that one! And my answer is always WHY? Beginning a difficult task before a break? Low Blood sugar? Who was supervising??

3. NOTE: (having already written something disparaging about Impac prequal i may'swell continue on a roll!) So-called 'experts' are STILL bleating about 'root cause'. This has proved to be a nonsense because EVERY cause contributes and NONE can legitimately be identified as 'root". cf (for eg) Daniel Kahneman "Thinking Fast and Slow")