Event Tree Analysis

prunierdunnett · Post by **prunierdunnett** » Fri Nov 20, 2009 12:41 pm

Hello Folks

New to this forum so hope this makes sense.

I am studying Part A of the NEBOSH Diploma Course. I am struggling with Event Tree Analysis. If I can get my head round it it should be good for 15/20 marks if a question comes up. The problem is that the ETA in the manual is considerably simpler than those in the past exams. The first past paper I have 2005 only gives a numerical answer. The most recent in 2008 is way more complicated than the manual.

I tried looking at numerous websites but they generally have been written by university professors or students trying to become one. As a result they are VERY VERY!! complicated and impossible to follow.

I was wondering if any one could direct me to a place where there are simple ETA's with only a few eg 2 or 3 branches.

Thanks

Dave

Post by **bernicarey** » Fri Nov 20, 2009 2:29 pm

Hi Dave (can we call you Pru, or is that too informal

)
Welcome to the forums

As I see it (and please somebody correct me if I'm wrong) you're making a basic mistake.

Your post is about EVENT Tree, but the 2008 paper (Q7) was a FAULT Tree Question.

Others have made that mistake, in the exams, in fact the 2008 Examiners Report actually says ' This was not a popular question, but those candidates who had a good understanding of the construction of a fault tree did well. There were some, however, who did not have this understanding, and produced an event rather than a fault tree.'

Please check what you are asking and all of us around here will be happy to contribute - Oh and I'm in the same boat as you, studying for my Part A in January. I hope the rest of your studies are going well

bedds · Post by **bedds** » Fri Nov 20, 2009 3:13 pm

I remeber it as fault trees fall down.. as in they're top to bottom.. can't think of anything witty for event trees..

not much help to your question sorry!

In the same boat as you both btw

Polo · Post by **Polo** » Fri Nov 20, 2009 4:19 pm

Here are my revision notes for those two topics. (Hope the attachment works. It's the first time I've tried it.)

FAULT TREES ETC.ppt: (55.5 KiB) Downloaded 1140 times

Hmm not sure that worked. Can I do it? If not, pm me and I will send it to you.

prunierdunnett · Post by **prunierdunnett** » Fri Nov 20, 2009 4:44 pm

Guys

Thanks but it is definitely the event tree analysis the calculation that goes sideways. It is the way that you manipulate the percentage numbers that I loose track of

Dave

Post by **bernicarey** » Sat Nov 21, 2009 2:08 am

OK, having cleared up the confusion about the 2008 paper (at least I think we have

)...

Event Trees go sideways

.
Scenario. A Factory has a risk of fire once a year.
20% of the time it can quickly spread (so 80% it is contained)
60% of the time the sprinklers work so it is contained (so 40% of the time the sprinklers fail)
50% of the time when Sprinklers do fail, people get trapped and die (so 50% of the time they get out)

What is the Fatality rate at the factory? About 1 fatal incident every 25 years.

If you can't read the text on the image, click on it and it will enlarge.
Along each branch, the probabilities are multiplied to reach the end. All the final probabilities added up, should equal 1, so in this case 0.04+0.04+0.12+0.8=1

Hope this helps.

Post by **Jack Kane** » Sun Nov 22, 2009 1:51 pm

Hey Dave,

Welcome to the forums

Not my strong point I'm afraid, but what's said above looks really good

Glad to have you with us.

Post by **kevlarion** » Sat Dec 05, 2009 4:04 pm

I'm studying via distance learning for Unit A in January and the layout of an event tree is different for the notes I have, consequently I don't think whether you work from top to bottom, left to right, or bottom to top don't really matter. What is important is that you start with an event which has a frequency of happening or a probability of happening... all the examples I have seen are of a fire for some reason.
The events following the fire are then described simply as happening or not happening and a probability is assigned to whether they happen or not. The sum of probabilities for any happens / not happens brach always adds up to 1.
The frequency or probability of an end event is then calculated by multiplying the probabilities from that end point back to the original event.
Have a look at http://www.theiet.org/factfiles/health/" onclick="window.open(this.href);return false;
There are worked examples of Fault Trees, Event Trees, FMEA etc there and they are quite simple cases.