:AACE CERTIFICATION 2009-2010:

The schedule risk assessment are consisted on two analysis from the critical path and the highest risk path (ref. Humphrey’s books on Chapter 19 , “Schedule Risk Assessment”). The steps involved in assessing the risk of a schedule are as follows :

Schedule risk assessment include estimating the likelihood of overrunning the baseline schedule and identifying paths that are potential problem areas. With CPM, the critical path is identified and with risk analysis, the distribution of the various paths at the end of the project can be compared explicitly. CPM is just the beginning of a complete schedule risk analysis and does not explore the likelihood of other durations occurring where the highest risk path are generally identified at long duration and risky, but may not be the critical path

Highest Risk Path Analysis using PERT

The highest risk path can be identified by comparing path distribution’s S-curves to see which has the longest potential duration. Again, a risk analysis does not accept the duration estimates as accurate. Path durations are distributions. S-curve is to tell how likely it is overrun or under run a particular estimated duration. The S-curve posing the greatest risk to the scheduled finish date identifies the highest risk path. The amount of risk in the CPM estimate depends on the shape of the S-curve and its position relative to the other curves.

There are three different approaches that have been used to assess the risk in all schedule which includes the Program Evaluation and Review Technique (PERT), Monte Carlo simulation and Analytical Approximation. PERT was used to forecast probabilities associated with project duration and cost. PERT was intended to determine the mean and standard deviation of the completion time of the project. As kind of projects are particularly risky because little or no historical data are available, In practice, we need to quantify the risk factors using expert judgment when data are not available.

This is indicated in above Figure that there are 4 activities have more gradual shape of S-curve show the high of risk path and three of them are located in the critical path i.e. : drilling and blasting, sub base construction, load-haul overburden and ore but they are still located toward the left of the graph. However, the activity of bench sloping and clean up ore is not critical path but it is near critical as the curve toward the right of the graph and risky gradually sloped S-curve. Considering all of the factors, activity of bench sloping and clean up ore is called the highest risk path. It is explained that as the overall high of bench slope is 20 meter and required to cut batter the slope to allow the loading equipment working with maximum 10 meter bench high affect that the productivity of main loading equipment will be depended by small excavators and dozers which work to cut batter slope and the bench to feed the main loading equipment. Other, whenever ore layer exposed, the top of ore layer is required to clean up by small excavator prior mining the ore to minimize dilution of ore grade during mining. As 20 meter width box cut of design road is excatly the minimum mining width to allow heavy equipments safely work at the loading point, the ore expose defined in the road section should be cleaned up and mined for continuing of construction the next road sections. When there is a delayed on the small excavator to clean up the ore and cut batter of slope will affect the loading and hauling equipments is unable to continue (loading and hauling equipments standby) and result the clean up ore and sloping the bench activity will be on the critical path.
Well, It seems the picture of schedule risk assessment is going to appear what is the highest risk activtiy path of the project. I believe the analysis will be completed by comparison with Monte Carlo simulation using Palisade software @5.5 industrial for excel download from internet.