LESSON 2 TIME ESTIMATES

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LESSON 2 TIME ESTIMATES

The term “time estimation” refers to the assessment of the number of hours needed to complete a task or a series of tasks. To evaluate the volume of time for a project as a whole, you should do the following:

 Identify the amount of work – develop a detailed work breakdown structure and create a list of tasks for achieving significant project milestones and deliverables;

 Assess how many hours, days or weeks each of the project activities may take and sum up individual estimates to get the grand total of time necessary to complete the project.

As for schedule delays, there are a plethora of other variables to consider:

 It is always challenging to determine deadlines and stick to them in massive and complex projects;

 Sometimes delays may happen due to third parties’ actions or environmental factors that are out of your direct control;

 In other situations, they may be an outcome of unforeseen changes in project scope (i.e., scope creep) and poor initial project plans.

Nevertheless, the provided statistics demonstrate that inadequate time estimation increases the risks for project underperformance and unsatisfactory outcomes.

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Types of time estimation:

There are three types of estimation of times involved

Expected time: The average time taken for completion of job or activity is

called Expected time

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Example 1:

For a particular activity of a project, time estimates received from two engineers X and Y are as follows:

Name Optimistic Time (days)

Most likely Time (days)

Pessimistic Time (days)

Engineer X 4 6 8

Engineer Y 3 5 8

State who is more certain about the time of completion of the job.

Calculate the expected time for the activity.

Expected time for engineer X, t

EX

=

^to+4tL+tp

6

=

^4+4x6+8

6

= 6 days (Ans.)

Exercise 1:

State who is more certain about the time of completion of the job and then calculate the expected time for the activity for a particular activity of a project, from the following time estimates.

Name Optimistic Time (days)

Most likely Time (days)

Pessimistic Time (days)

Engineer P 3 5 8

Engineer Q 3 6 9

Engineer R 4 5 8

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TOOLS OR TECHNIQUES OF PROJECT SCHEDULING i) Network diagram

ii) Bar chart

iii) Mile stone chart

NETWORK DIAGRAM:

i) PERT Network diagram ii) CPM Network diagram

Difference between PERT and CPM Network

PERT CPM

PERT stands for Project Evaluation and Review Technique

CPM stands for Critical Path Method

PERT uses event oriented network. CPM use activity oriented network diagram In PERT system time estimated is not so

accurate and definite

In CPM, time estimates for completion of activities are with fail degree of accuracy It is assumed that cost varies directly with

time

Cost is not proportional to time It is used in those projects where there is an

extreme degree of uncertainty and where control over time over weights control over cost.

It is used in those projects where time can estimated fairly well and when cost can be calculated in advance.

ELEMENTS OF NETWORK

A network is a flow diagram consisting activities and events connected logically and sequentially.

Fig. 1: PERT Network Diagram

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Fig. 2: CPM Network Diagram

Event:

The commencement or completion of an activity is called an event.

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Specifying the Events:

Fig. 3: Tail Events

Head Event:

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Fig. 4: Head Events

Dual role event:

Fig. 5: Dual Role Events

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Successor Event:

Predecessor Event:

Activity:

An activity is a recognizable part of a work project that requires time and resources for its completion.

Characteristics of Activity:

Predecessor Activity:

Successor Activity:

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Dummy:

Fig. 6: Network Diagram with Dummy

Network Rules:

1) Initial node has only outgoing arrows. These must be only single initial node in a network.

2) An event can’t occur until all the activities leading to it are completed.

3) An event can’t occur twice.

4) There must not be only dead end left expect the final node.

5) No activity can start until its tail end event has occurred.

6) Any arrow should represent singular situation.

7) Representation of activity should be such that every activity is completed to reach end objective.

8) All constraints and interdependencies should be shown properly on the network by use of appropriate dummies.

9) It is usual practice to show time flow from left to right.

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Example 2: Draw network diagram by PERT method

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Example 3: Draw network diagram by PERT method

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Example 4: Draw the network diagram by both PERT & CPM method from the following activities according to predecessors relation.

Activity Predecessors

A -

B A

C A

D B

E C

F D,E

Solution:

PERT Network Diagram

CPM Network Diagram

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Example 5:

Draw the both PERT and CPM diagram from the following activity relations.

Activity ID (Activity Details) Predecessors

A = Footing layout -

B = Earth excavation work A

C = Brick flat soling B

D = Reinforcement fabrication or binding B

E = Concrete casting C,D

F = Formwork removing E

G = Earth filling F

Solution:

PERT Network Diagram

CPM Network Diagram

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Critical Path:

How to Find the Critical Path of a network diagram?

Step 1: Find Activities

Activities for this project are as below (output from Define Activities process) – A. Assemble two-tier bridge

B. Join winding tracks

C. Assemble and add train station D. Place standalone items around E. Assemble and add construction site F. Join train engine and bogies

G. Place the train on the track H. Start the engine and let it chug!

Step 2: Build Schedule Network Diagram

Sequence activities and build schedule network diagram (output from Sequence Activities process). This is how it looks, with individual activity duration in minutes –

Fig. 7: Assemble train set – schedule network diagram

16 Step 3: Find all Possible Paths

Find all possible paths through the diagram, there are 3 in our case – A -> B -> F -> G -> H

A -> B -> C -> D -> G -> H A -> B -> C -> E -> G -> H

Step 4: Calculate Duration for Each Path Let us see the duration for each of these paths –

A -> B -> F -> G -> H —> 10+20+4+2+2 = 38 minutes

A -> B -> C -> D -> G -> H —> 10+20+5+10+2+2 = 49 minutes A -> B -> C -> E -> G -> H —> 10+20+5+2+2+2= 41 minutes The network path with longest total duration is the critical path!

Critical path is the shortest duration required to complete the project successfully.

In our example this is the second path: A -> B -> C -> D -> G -> H, which comes to 49 minutes.

Fig. 8: Assemble train set – Critical Path

A path through the network is a route made up of nodes and arcs that traverses the network from the start node to the finish node.

The length of a path is the sum of the durations of the activities on the nodes along the path.

In this simple example, there are two paths through the network:

The project duration will be no longer than the longest path through the network. Therefore, the total time required to complete the project equals the length of the longest path through the network and this longest path is called the critical path.

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Example 6:

What is the total time required to complete the project if no delays occur?

Write the critical path of the network diagram by CPM and PERT.

Activity Duration (weeks) Predecessor (s)

A 2 none

B 3 A

C 3 A

D 4 C

E 8 D

F 6 B, E

G 2 F

Solution:

By CPM

CPM Network Diagram

 start -> A -> B -> F -> G -> finish, with a length of 13

 start -> A -> C -> D -> E -> F -> G -> finish, with a length of 25 The longest path A -> C -> D -> E -> F -> G is the critical path.

The total time to complete the project should be 25 weeks if no delays occur.

By PERT

PERT Network Diagram (Option 1)

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PERT Network Diagram (Option 2)

 start -> A -> B -> F -> G -> finish, with a length of 13

 start -> A -> C -> D -> E -> F -> G -> finish, with a length of 25 The longest path A -> C -> D -> E -> F -> G is the critical path.

The total time to complete the project should be 25 weeks if no delays occur.

FLOAT

Float is the amount of time that an activity can be delayed without causing project delays or causing delays in other activities. The Float also called as Slack.

There are 3 types of float. Let’s review them one-by-one.

Total Float: Total Float is the amount of time an activity can be delayed without delaying the project end date or an intermediary milestone.

Float here helps you to see how long you can delay an activity, without affecting the project duration.

Free float: Free float is the amount of time an activity can be delayed without delaying the early start date.

Early start date of an activity shows the earliest data that an activity can be started. Because, an activity will depend on other activities, constraints etc. Therefore, it will have an early start date. And free float is the amount of time that an activity can be delayed, without delaying the early start date.

Project Float: This is the amount of time an activity can be delayed without delaying externally imposed project completion date.

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Note that critical path activities should have zero floats. This is because the critical path activities show the longest path of the project to complete, and therefore any activity on the critical path will have zero floats. Any delay on a critical path activity will cause project delays respectively.

How does the float of an activity Calculated?

Ans: Float of an activity can be calculated in two ways. Either Early Start is subtracted from the late start, or early finish is subtracted from the late finish.

Float = Late Start - Early Start = LS - ES Float = Late Finish - Early Finish = LF - EF

Example 7:

Calculate a single deterministic early and late finish date for each activity based on specified sequential network logic and a single duration estimate.

Activity Duration (days) Predecessor (s)

A 10 none

B 20 A

C 5 B

D 10 C

E 20 D,G,H

F 15 A

G 5 C,F

H 15 A

Solution:

Activity, Duration EF = ES + Duration - 1 LS = LF - Duration + 1

ES EF

LS LF

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Step 2

 start -> A -> F -> G -> E finish, with a length of 50 days

 start -> A -> B -> C -> D -> E finish, with a length of 65 days

 start -> A -> B -> C -> G -> E finish, with a length of 60 days

 start -> A -> H -> E finish, with a length of 45 days The longest path A -> B -> C -> D -> E is the critical path.

The total time to complete the project should be 65 days if no delays occur.

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Exercise:

Calculate a single deterministic early and late finish date for each activity based on specified sequential network logic and a single duration estimate.

Activity Predecessor (s)

Duration

(days) Network diagram

A none 3

B A 4

C A 2

D B 5

E C 1

F C 2

G D,E 4

H F,G 3

Now complete the rest portion of the diagram with early and late finish date for each activity.

Slack: Slack may be simply defined as the difference between the latest allowable time and the earliest expected time of an event.

S = TL-TE

22 BAR CHART

A bar chart or bar graph is a chart or graph that presents categorical data with rectangular bars with heights or lengths proportional to the values that they represent. The bars can be plotted vertically or horizontally. A vertical bar chart is sometimes called a column chart.

Bar graphs/charts provide a visual presentation of categorical data. Categorical data is a grouping of data into discrete groups, such as months of the year, age group, shoe sizes, and animals. These categories are usually qualitative. In a column (vertical) bar chart, categories appear along the horizontal axis and the height of the bar corresponds to the value of each category.

Gantt chart: A Gantt chart is a type of bar chart that illustrates a project schedule, named after its inventor, Henry Gantt (1861–1919), who designed such a chart around the years 1910–1915. Modern Gantt charts also show the dependency relationships between activities and the current schedule status.

A Gantt chart is a type of bar chart that illustrates a project schedule. This chart lists the tasks to be performed on the vertical axis, and time intervals on the horizontal axis. The width of the horizontal bars in the

graph shows the duration of each activity. Gantt charts illustrate the start and finish dates of the terminal elements and summary elements of a project. Terminal elements and summary elements constitute the work breakdown structure of the project. Modern Gantt charts also show the dependency (i.e., precedence network) relationships between activities.

Gantt charts are usually created initially using an early start time approach, where each task is scheduled to start immediately when its prerequisites are complete. This method maximizes the float time available for all tasks.

23 A template of Gantt Chart for Construction of a Lift

MILESTONE SCHEDULE

A milestone schedule, or milestone chart, is simply a timeline that uses milestones to divide a project schedule into major phases. Due to its simplicity, it’s used when project managers or sponsors need to share an overview of the project schedule with stakeholders or team members without going over every detail.

All projects have start dates and end dates. But along the way, how do teams check to be sure they’re on track to meet deadlines, finish on time and stay within budget? The answer: project milestones.

As the name implies, milestones are markers in the project’s schedule signifying important events or goals. On a Gantt chart, they are called Gantt chart milestones. Gantt chart milestones are an excellent way to estimate how long certain phases of a project should take.

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To that end, milestones are an effective management tool. By setting project milestones, a project manager can track the status of different project phases and decide if adjustments should be made to stay on schedule. Further, milestone charts track and highlight major successes. This can improve team efficacy and overall team morale. The result is happier team members who feel appreciated and can see the direct results of their efforts.

A template of Gantt Chart Milestone for Construction work

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Questions:

1. What do you mean by optimistic time estimate and pessimistic time estimate?

Or, Write short note on optimistic time estimate and pessimistic time estimate.

2. Write short note on most likely time estimate.

3. What do you mean by expected time of a project?

4. Differentiate between PERT and CPM.

5. What is Network and draw a typical network diagram?

6. Define Event, Activity, Dummy of network.

7. What do you mean by event? Describe the classification of event with figures.

8. What do you mean by Head event and Tail event?

Or, Write short note on Head event and Tail event.

9. Write short note on dual role event.

10. What is the difference between successor event and predecessor event?

11. What is the difference between successor activity and predecessor activity?

12. Write down the rules of Network diagram.

13. What is Bar chart, Gantt Chart and Milestone chart?