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What size schedule is best for Schedule Risk Analy

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Colin Cropley
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If you have the choice of modelling the uncertainty of your project with a 25 activity schedule, a 250 activity schedule or a 25,000 activity schedule (to choose some numbers arbitrarily), and assuming you are able to model the significant dependencies in the project appropriately in all of those schedule sizes, which size of schedule will you choose and why?

What determines the best size of schedule to use for SRA?
How do you know you are right?


Oliver Melling
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I agree the best option is to brain-storm the most important risks with expert and possibly use quick-risk for modelling the general uncertainty you have in estimates, but I dont think the number of activities should be limited as the effect of a risk(s) will need to be assessed on a full plan.


Peter Hibberd
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My view is that of practicality, because the ranges of a 3 point estimate are built up by the qualitative feeling for the risks of an activity, it would be very difficult and time consuming to do 3 point ranges for too many activities. Most often these are conducted at a workshop of knowledgable people and a consensus is achieved on the perceived risk. If you have too many activities the chances are that your project will be finished before you have the time to complete the risk analysis. In place of consensus you could use the ’quick risk’ in Pertmaster to apply the 3 point range, but then it is merely an academic exercise. My view is that anything more than about 100 activities takes away from the perception of the risk to a mathematical exercise. Also if you have too many swings and roundabouts you will always end up with an even distribution converging on your deterministic target.


Oliver Melling
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Confidence in schedule risk analysis can surely only match the confidence that you have in your baseline estimates? In a contract programme you should plan down to a level that makes you confident you can control and deliver your baseline.
Analysing risk at summary level or in high level programmes is inherently less accurate due to the fact you are basing the analysis on a less consolidated estimate, or in other words, you are trying to predict risk events on several larger elements made up of detailed activity estimates.
Can you find risk software that will model risk events at summary level? or do they usually apply different scenarios at activity level to produce P50, P90 programmes etc, my guess is they do the later for a reason.

Hope I made sense..

Trevor Rabey
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On the one hand, (Vladimir says) Risk is Risk regardless of the number of Activities/Tasks. The modeling determines the correct number of Tasks and the Risk assessment follows from that.
On the other hand, more detail generally = more control.
Also, more detail=more Tasks=shorter Durations, tends to compartmentalise or quarantine the schedule risk.
Having lots of detail, by itself and because it leads to more control, mitigates risk.
Once you have the detail to whatever level of granularity suits the project, I can’t see any reason to sacrifice that by summarising or merging Tasks back together.
Raja Izat Raja Ib...
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Hi All,

Schedule Analysis is based on what you Think, experiences.
Its not matter of how many activities you have done but to understand the risk u have to understand the sequence and the what Material to relate with and what are the schedule interfacing with. So that is the risk.

From my point of view.
Gwen Blair
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We did our BRISK schedule of late and was limited to 250 activities for a project of hull, topsides, turret, integration, gas export pipeline, subsea installation. That also had to include all the owners scope such as HSSE, operations, Project Assurance, QA, Drilling & Completion and Subsurface. Thats includes Engineering, procurement as well.
We got penalised for being too scant with our detail (I wont bother to tell you the replies, I am sure you can imagine!)
Philip Rawlings
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If you are creating a schedule especially for SRA then I would recommend a few hundred activities (so that the risk estimator can examine reasonably-sized durations). If you are taking an existing schedule then the highest-level at which logic etc. is consistent. This may be a lot of activities but then you need to concentrate on critical path and/or sensitive activities.

As usual, there is no absolute answer (in my belief).
Bernard Ertl
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Colin, I think the answer is dependent upon the method you are employing to analyze risks. It would appear that your method is sensitive to the structure of the schedule.

Did you see my post about RiskyProject in the Software for Risk Assessment thread? They are using an Event Chain Methodology that looks interesting and (as I understand it) will be indifferent to the level of detail in the schedule.

Bernard Ertl
InterPlan Systems - eTaskMaker Project Planning Software
I have never seen a schedule that being summarised keeps all the logic, resource requirements and all other necessary information. In that case I don’t understand the reason why it was described in more details.
I don’t understand why SRA results depend on activity durations. If correlations are not taken into consideration the results will be unreliable in any case.
Summarising schedules you will loose additional necessary information.
The only purpose of summarisation that I could understand is receiving the same results produciong SRA next time. The errors that you will have the first time will repeat on second.

Colin Cropley
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When I am asked to do a Schedule Risk Analysis (SRA), some of the first questions I have are concerned with the schedule:
Is it a good model of the project?
Does it have appropriate logic?
How many activities are there?
How long are the activity durations?
If there are too many activities and the durations are generally short, as commented in my discussion thread about Correlation, the SRA analysis is likely to produce a duration uncertainty that is too narrow.
So in a number of instances, it may be necessary to summarise the schedule before performing the SRA.
If meaningful and representative logic links can be preserved as the schedule is increasingly summarised and the summary activities are lengthened, the duration uncertainty of the project is likely to increase, unless duration correlation has been used in the more detailed schedules.

The question is, how much summarisation is enough?

Colin Cropley
I cannot understand your question. Schedule Risk Analysis is applied to the existing schedule and it may contain 25, 250 or 250000 activities. A number of schedule activities does not depend on your wishes.