Scheduling WorkScheduling Work

I love deadlines.

I love the sound they make as they fly by.-- Douglas Adams

Why is software late?

Under-Estimate of effort.

Unrealistic deadlines imposed by outsiders.

Changing requirements.

Risks that were not considered.

Technical problems that could not be foreseen.

Failure to recognized that the project is falling behind schedule and lack of action to correct the problem.

Software Engineering : A Practitioner’s Approach by Pressman

How to build a schedule:

1. Determine dependencies of activities found in WBS.

2. Use dependencies to build activity network diagram (eg PERT).

3. Determine critical path.

4. Map network to a timeline (eg Gantt Chart) using the effort estimates.

Use a software tool to help do this work.

Why Determine Dependencies?

Dependencies determine deadlines and when work can begin.

The WBS shows hierarchy, not dependency.

Planners must find all dependencies. Surprises are very bad!

Dependency Types

External Dependency Other projects Stakeholders Sub-Contractors

Internal Dependency Audits (design, testing, documentation, …) Sign-Offs at end of phase

Dependency Types

Start Finish

FS - T2 cannot start until T1 is finished. coding must wait on design to be finished

SS - T1 and T2 must start at same time. documentation and coding start together

FF - T1 and T2 finish at same time. configuration management ends when coding

is done

Dependency Types

Lag – T2 cannot start until a given amount of time after T1 is done. Start on-site training one week after final demo to

customer management group.

Soft – T2 should start after T1, but it is not completely essential that T1 be finished. Populate the new database after the database has

been built. If the new database is not completely finished we can still populate the few finished tables.

Activity Networks

CPM – Critical Path Method activities are graph nodes (circles) single estimate for duration of tasks

PERT – Program Evaluation and Review Technique activities are graph arcs (lines) activity times are ranges and include probabilities

http://www.cs.unc.edu/~stotts/COMP145/homes/map/images/pert.gif

http://www.mckinnonsc.vic.edu.au/la/it/ipmnotes/ganttpert/pert_chart.gif

Example Activity Network – Activity on Node

T1

T2

T3

T4 T6

T5

T7

T8 T9

1wk

2wks

4wks

3wks

1wks

1wk

3wks

2wks 1wk

Questions:• Minimum time to complete the project?• Critical Path?• For T2, what are the earliest start time, latest start, earliest finish, latest finish?

es=2ndwk ls=4thwk ef=3rdwk lf=5thwk

Resulting Gantt Chart

1 2 3 4 5 6 7 8 9 10 11 12 13 14

T1

T2

T3

T4

T5

T6

T7

T8

Managing Uncertainty

Q: How confident can we be in our milestone dates and the delivery date?

A: Using confidence and Statistics for each task, determine pessimistic and optimistic times

if PT-OT is small for all tasks, then great!!!

compute average PT-OT, and std dev

if a task's PT-OT is outside the std dev, worry about that task

Managing Uncertainty

Q: Where do we schedule uncertainty time?

A1: spread it out by bumping each milestone. used by Critical Path

A2: lump it together as one activity at end. used by Critical Chain

Common PERT Problems

Missing dependencies

Misinterpretation

Example

K – integration testing L – Install Software M – Write Manual N – Train Users

3 4 521K L M N

Example

K – integration testing L – Install Software M – Write Manual N – Train Users

But wait, we can shorten the project if we write the manual right after testing is finished. (L and M can be done in parallel.) So…

3 4 521K L M N

Example

K – integration testing L – Install Software M – Write Manual N – Train Users

But wait, the system needs to be installed before we can use it to train people. So…

3

4 5

21K

L

M N

Example

K – integration testing L – Install Software M – Write Manual N – Train Users

But what is that activity between dates 3 and 4? So…

3

4521

KL

M N

Example

K – integration testing L – Install Software M – Write Manual N – Train Users

Dashed line is a dummy activity.

3

4521

KL

M N

Example

Note: dummies not needed with "activity on node" graphs.

K N M

L

K – integration testing L – Install Software M – Write Manual N – Train Users

Resource LevelingResource Leveling

Activity Shifting to avoid using resources during a peak time. use slack in start and end dates

Activity Splitting to shorten duration break activity into pieces when more

resources are available

Optimize the System, not individual tasks

Leveling usually yields longer projects

Example

What is the completion time? Since F is so long, what would be the completion time

if we break it into two parallel parts of 2wks and 3wks?

A3wks

B2wk

E2wks

C2wks

F5wks

G3wks

H3wks

I2wks

D2wks

Theory of Constraints

“The Goal” by Goldratt in 1997 Traditional approach is to divide and divide into

manageable parts, then optimize these parts. TOC looks at the whole system.

1. Identify: In order to manage a constraint, it is first necessary to identify it.

2. Exploit: Focus on how to get more production within the existing capacity limitations.

3. Subordinate: Prevent the materials needed next from waiting in a queue at a non-constraint resource.

4. Elevate: If, after fully exploiting this process, it still cannot produce enough products to meet market demand, find other ways to increase capacity.

5. Go back to Step 1.

Scheduling Tools

Gantt Chart producers MS Project

Web access Supports multiple projects Dependencies Billing support Resources leveling Gantt charts, activity network, …

MS Project

Open WorkBench

ATC Professional

And finally…