4.1. Proposed SMSRS Model
The proposed SMSRS model incorporates SRA with the MSRS algorithm of [
11] to generate a resource schedule with a more realistic project completion duration. In the SMSRS model, we developed an SRA model with the aid of Microsoft excel using triangular probability distribution and Monte Carlo simulation. Thereafter, the SRA model was integrated with the MSRS algorithm.
Figure 1 shows an overview of the methodology for the existing MSRS and the proposed SMSRS models. To demonstrate the integration procedure, a sample project (project A) obtained from [
11] was used to set up the model. Note that, the daily resource availability of the project was modified in order to formulate a suitable resource substitution rule adopted for the project.
Figure 2 depicts the network diagram of project A, while
Table 2 shows the project information which includes the activities, precedence relationships, activity durations, activity resource requirements, daily resources availability, and the resource substitution rule for the project. First, SRA was conducted to generate realistic activity durations. Then, the results obtained are used to conduct MSRS for the project so that realistic project completion time can be determined considering activity duration uncertainty.
4.1.1. Development of SRA Model
SRA model was developed in MS excel following the steps below:
- (i)
Determine the optimistic a, and pessimistic b durations of each activity duration
The optimistic a, most likely m and pessimistic b durations for each activity were established. Note that, in the triangular probability distribution, a, m and b values are required to generate random variables. Here, mi is the given duration for each activity while ai and bi can be determined using expert judgment through survey, historical records, etc. However, in this study we determined ai and bi of each activity by multiplying m with fictitious values 0.8 and 1.5, respectively.
- (ii)
Generate random numbers between 0 and 1
Random numbers between 0 and 1 were generated using the excel function RAND () representing the probability occurrence of each variable.
- (iii)
Generate a random activity duration for each activity
Unlike other probability distributions whose random variables generating functions are available in excel, the triangular probability distribution function is not. Therefore, Equation (1) [
18] was used in MS excel to generate the random duration for each activity.
where
Randi is the random number generated for activity
i and
DurRandi is the random duration for activity
i.
Each random activity duration generated was simulated 1000 times using the Monte Carlo simulation method. The average of 1000 iterations for each activity was calculated, which is the expected duration for each activity. Each time F9 is pressed on the keyboard, a new set of random durations are generated. In this way, the risk and uncertainty which may affect the activity durations were accounted for. The resultant expected activity durations (stochastic) were used as input for the development of the stochastic multi-skilled resource schedule. The results of these procedures are shown in
Table 3.
4.1.2. Integrating the SRA Model with MSRS
This section explains the integration of the SRA model with the MSRS algorithm. To integrate the SRA with the MSRS algorithm, the stochastic activity durations obtained from the SRA model were used as input for the computational steps involved in the integration as described below:
- (i)
Determine project duration, start and finish times of each activity
The project duration, start and finish times based on the stochastic durations of each activity were computed using the forward and backward pass approach with the aid of Microsoft excel using Equations (2a)–(2d). The result is shown in
Table 4.
where Pred., EST, EFT, LST, and LFT are predecessor, early start time, early finish time, late start time, and late finish time of each activity.
- (ii)
Schedule each activity
Each activity was scheduled accordingly using the stochastic activity durations. MSRS algorithm was adopted at the instances of resource conflicts based on the resource substitution rule of the project in
Table 2. After scheduling all the activities considering the resource constraints the project duration was 19.49 days as shown in
Table 5. This duration is the expected project duration considering risk and uncertainty and resource constraints.
4.2. Scheduling Procedure
Table 5 shows the details of the scheduling. In this schedule, the longest activity duration (LAD) PR was used for the scheduling based on the project information in
Table 2;
Table 4. In
Table 5, Column 1 (from the left) shows the starting time of the eligible activities while column 9 shows the corresponding finish times of the activities in each scheduling cycle. Column 2 shows the eligible activities while columns 3, 4, and 5 show the resources and the availability per day. The activity durations, the PR applied and the decision made in each cycle is shown in columns 6, 7, and 8, respectively. The last column shows the substitution rule used to resolve the resource shortage.
In the first cycle, i.e., at the beginning of the project (time = 0) as shown in
Table 5 only activities C, B, and A were eligible and enlisted for scheduling. The activities were sorted base on LAD. Activities C and B were scheduled to be completed on days 5.71 and 4.27, respectively. Activity A was delayed because of insufficient resources and the substitution rule could not resolve the resource shortages for R1and R2.
Then, in the second cycle (time = 4.27), activity B has been completed while activity C continued as scheduled in the previous cycle. The completion of activity B consequently made activity F eligible for scheduling in this cycle. Activity F and A were started accordingly and to be completed on days 8.6 and 7.58, respectively. Activity A was started by applying the resource substitution rule 2R3 = 1R2 in columns 4 and 5. The total R2 required by activities C, F, and A start is 9 of R2 which is 2 units above the daily availability (7 units). Therefore, 2 units of R2 were subtracted from column 4, and a corresponding 4 units of R3 were added in column 5. This way activity A was scheduled. Note that at the end of each cycle, the number of resources used by the activities either in starting or in progress should not exceed the availability per day of each resource. Additionally, notice that the start times for the next cycle is the earliest finished time of the previously scheduled activities. For example, in the 2nd cycle, the finish times for activities C, F, and A are 5.71, 8.6, and 7.58, respectively. Thus, the next cycle starts on day 5.71.
In the third cycle (time = 5.71), activity C was completed thereby making activities H and I eligible. Activities F and A continued as scheduled in the last cycle while activity H was delayed until the next cycle because resources were not enough to start them and the substitution rule could not resolve the resource conflict of R1 and R2. Thus, activity I started and to be finished on day 10.03 using the substitution rules 2R3 = 1R1 and 2R3 = 1R2. The shortage in R1 (2 units) and R2 (1 unit) in columns 3 and 4, respectively, were resolved in column 5 (6 units of R3), so activity I was started. This scheduling procedure was continued till the last activity was scheduled resulting in a project duration of 19.49 days.
Similarly, two case projects obtained from literature namely; projects B [
11,
39] and C [
25] were analyzed using the SMSRS model, and results were obtained. The project information and the network diagram are shown in
Table 6;
Table 7 and
Figure 3;
Figure 4 respectively. Additionally, the substitution rules adopted for these projects are shown in
Table 6;
Table 7 while PR used in scheduling was LST. We applied the LST rule to have an unbiased basis for comparison with existing studies where the two projects (A and B) were studied.
Then, the SMSRS model results were validated through a comparison with results obtained from single-skilled resource scheduling (SSRS) software (RESCON) and the existing deterministic MSRS model. RESCON is a resource-constrained scheduling educational software based on a single-skilled resource strategy.
Table 8 shows the schedule summary results of the three projects based on SSRS software, MSRS, and SMSRS models. Next, the project delay and percentage delay for the projects were computed using Equation (3). The results are shown in
Table 9;
Table 10 with the summary depicted in
Figure 5. Finally, the average percentage delay error for the three projects was computed as 10.50%.
where %
Di is the percentage delay of schedule
i;
Pi is the project duration for schedule
i and
PB is the baseline project duration i.e., deterministic project duration.