W14_Reginald Nwachukwu_Determining Activity Durations

1. Problem Definition or Opportunity Statement: 

To estimate the Activity Duration for the construction of 27.0m Length Precast Box Culvert.

It required to estimate and determine the duration required to complete the construction of 27.0m length precast box culvert. The lead scheduler for the project have provided some historical data, and have asked me to evaluate them and make probable duration estimates to for construction of precast box culvert.

Figure 1- 1 Installation of Precast Box Unit on a Highway Project (Source, Author)

Table 1-1 Historial Duration Data from Past Project

2. Development of Feasible Alternatives:

Development of feasible alternatives for determining activity duration will be based on the techniques or  methods illustrated AACE International Recommended Practice No. 32R-04. These techniques have been evaluated and recommended as practice guides for calculating durations. These techniques constitutes the alternatives and includes:

(a) Unconstrained Activity Duration:

(i)  PERT Calculations
(ii) Descriptive Statistics
(iii) Normal Distribution Curve
(iv) Original Distribution (OD)
(iii) CPM Method (PDM Method)
(iv) Cost Estimates

(b) Constrained Activity Duration

(c) Revised Activity Duration to Meet Project Requirements

For the purpose of brevity and clarity, only Unconstrained Activity Duration technique have been used for analysis and evaluation of feasible alternatives.

3. Development of Outcome for Each Alternative:

The various techniques/methods under Unconstrianed Activity Duration were evaluated using Project Historical Data provided and their durations impacts as shown below viz:

(i) Descriptive Statistics:  
Basic descriptive statistical calculations were performed to determine durations as shown on Figure 1-2

Figure 1-2 Determination of Activity Duration Using Statistical Description

Figure 1-3 Frequency Polygon to Determine of Activity Duration Using Statistical Description

(i) Normal Distribution Curve: 

This technique uses the Normal Distribution functions in Microsoft Excel 2007 (NORMDIST, AVERAGE, STDEV, NORMINV, etc.) to compute activity duration using the Normal Bell-shaped curve.

Figure 1-4 50% Percentile showing 20 Workdays 

Figure 1-4 95% Percentile showing 23 Workdays 

(iii) Original Duration Calculations:

4. Selection of Acceptable Criteria:

Selection of acceptable criteria that may be applied to determining activity durations will be a criteria that is realistic and reflect the size, scope and contract requirements defined for that activity. Methods used to develop the outcomes are both Stochastic and Deterministic in nature. To this end, any technique that considered pertinent activity variables at the minimum possible duration will be selected.

5. Analysis and Comparison of Each Alternative:

(i) PERT Calculation :

Mean  = (17 + (4*20) + 23) / 6 = 20 Days
Range = 23 - 17 = 6 Days
Variance = Range/6 or Range/4 .........................[2]
Variance = 6/6 0r 6/4 = 1.5
Standard Deviation = (1.5)^2 = 2.25 Days

Duration (90% Confidence) = Mean + 1.3*S.D
                                             =  20 + 1.3*1.5  = 22 Days

(ii) OD Original Duration :    Duration  = 39 Days

(iii) Normal distribution Curve :    Duration  = 23 Days

(iv) CPM Method (PDM) :    Duration  = 23 Days

(v) Cost estimates :    Duration  = N/A

6. Selection of Preferred Alternative:
 Based on the activity duration determine, the preferred method will be the Original Duration that considered both crew size and workload factors which can greatly impact the duration of any activity.

7. Performance Monitoring and Evaluation of Results:

Performance and monitoring can effectively be implementated using the various tools developed for determining activity durations. This is achieved by collecting, recording and monitoring the actual duration expended to complete the activity been considered or under study.

References:

1. Humphreys G.C. (2002) Project Management Using Earned Value (2nd Edition) pp. 143 - 144 Orange CA Humphreys & Associates

2. AACE International (2012) Skills and Knowledge of Cost Engineering (5th Edition Revised) pp. 28.3-28.7  AACE International Morgantown W.V. 

3. AACE International (2008) Planning and Scheduling Professional certification Guide (First Edition Revised) pp. 143-145  AACE International Morgantown W.V. 

4. AACE International Recommended Practice No. 49R-06 (2012) Determining Activity Duration  pp. 1 - 12 AACE International Morgantown W.V.