Practical estimating and scheduling Critical Path Pert Charts

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Practical estimating and scheduling-----Critical
Path /Pert Charts
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• A good first-phase estimate can be done in a day.

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• Estimation is an exercise in disassembly (and is
  therefore integral with design), and
  interpersonal skills.
• NOT a negotiation (negotiation implies
  compromise, but there should be no compromise of
  reality).
• Time and cost sacrifices must be offset by
  features and effort.

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• Estimates are done and redone at the end of
  requirements, analysis, and design.
• Lines-of-code?
• of people? length of task?
• The LOC charts are good, but per-person
  productivity history for your company is better.

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• Danger - there is a temptation to up the
  LOC/person/day just to lower estimates.
• Cultivate an attitude of stunned disbelief when
  asked to adjust an estimate on non-technical
  factors. Practice in front of a mirror.

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• Estimating is not negotiation.
• Estimating is not adjusting numbers to the known
  right answer.
• Estimates not to be trusted those responsible
  (will be conservative) and those who interface to
  the customer (will be optimistic). Use those
  estimates, but reduce the first by 30, increase
  the second by 30. Why 30? Its the universal
  feel-good multiplier.

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• The usual estimation path
• Vague description of job
• Told by marketing/management what is the absolute
  maximum that can be spent, or youre fired
• You agree to it and stress out

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• Avoid gamesmanship -
• do not pad estimates
• do not expand work to fill available time
• do not code-like-crazy from day 1

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Cocomo Model

• Take the line of code estimate
• Adjust it for Scale. The 5 Scale Drivers are
• Precedentedness
• Development Flexibility
• Architecture / Risk Resolution
• Team Cohesion
• Process Maturity

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• Factor in a Cost Driver COCOMO II has 17 cost
  drivers you assess your project, development
  environment, and team to set each cost driver.
  The cost drivers are multiplicative factors that
  determine the effort required to complete your
  software project. For example, if your project
  will develop software that controls an airplane's
  flight, you would set the Required Software
  Reliability (RELY) cost driver to Very High. That
  rating corresponds to an effort multiplier of
  1.26, meaning that your project will require 26
  more effort than a typical software project.

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• Adjust it for level of effort The COCOMO II
  model makes its estimates of required effort
  (measured in Person-Months PM) based primarily
  on your estimate of the software project's size
  (as measured in thousands of SourceLOC or
  KSLOC))
• Effort 2.94 EAF (KSLOC)E
• Where
• EAF Is the Effort Adjustment Factor derived
  from the Cost Drivers
• E Is an exponent derived from the five Scale
  Drivers

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• As an example, a project with all Nominal Cost
  Drivers and Scale Drivers would have an EAF of
  1.00 and exponent, E, of 1.0997. Assuming that
  the project is projected to consist of 8,000
  source lines of code, COCOMO II estimates that
  28.9 Person-Months of effort is required to
  complete it
• Effort 2.94 (1.0) (8)1.0997 28.9
  Person-Months

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• Adjust it again The Effort Adjustment Factor in
  the effort equation is simply the product of the
  effort multipliers corresponding to each of the
  cost drivers for your project.
• For example, if your project is rated Very High
  for Complexity (effort multiplier of 1.34), and
  Low for Language Tools Experience (effort
  multiplier of 1.09), and all of the other cost
  drivers are rated to be Nominal (effort
  multiplier of 1.00), the EAF is the product of
  1.34 and 1.09.
• Effort Adjustment Factor EAF 1.34 1.09
  1.46
• Effort 2.94 (1.46) (8)1.0997 42.3
  Person-Months

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• The COCOMO II schedule equation predicts the
  number of months required to complete your
  software project. The duration of a project is
  based on the effort predicted by the effort
  equation
• Duration 3.67 (Effort)SE
• Where
• Effort - Is the effort from the COCOMO II effort
  equation
• SE - Is the schedule equation exponent derived
  from the five Scale Drivers

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• Continuing the example, and substituting the
  exponent of 0.3179 that is calculated from the
  scale drivers, yields an estimate of just over a
  year, and an average staffing of between 3 and 4
  people
• Duration 3.67 (42.3)0.3179 12.1 months
• Average staffing (42.3 Person-Months) / (12.1
  Months) 3.5 people

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• um, adjust it again The COCOMO cost driver for
  Required Development Schedule (SCED) is unique.
  The SCED cost driver is used to account for the
  observation that a project developed on an
  accelerated schedule will require more effort
  than a project developed on its optimum schedule.
  A SCED rating of Very Low corresponds to an
  Effort Multiplier of 1.43 (in the COCOMO II.2000
  model) and means that you intend to finish your
  project in 75 of the optimum schedule (as
  determined by a previous COCOMO estimate).

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• Continuing the example used earlier, but assuming
  that SCED has a rating of Very Low, COCOMO
  produces these estimates
• Duration 75 12.1 Months 9.1 Months
• Effort Adjustment Factor EAF 1.34 1.09
  1.43 2.09
• Effort 2.94 (2.09) (8)1.0997 60.4
  Person-Months
• Average staffing (60.4 Person-Months) /
  (9.1 Months) 6.7 people

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• Notice that the calculation of duration isn't
  based directly on the effort (number of
  Person-Months) instead it's based on the
  schedule that would have been required for the
  project assuming it had been developed on the
  nominal schedule. Remember that the SCED cost
  driver means "accelerated from the nominal
  schedule".

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Phase 3 - Do a WBS and schedule for your project

• Work Breakdown Structure
• Every trackable non-free task
• Everything
• How many people, how much time

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some tasks we forget

• meetings
• document preparation
• status and project management
• customer training

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typical - done once

• Customer meetings
• System Spec
• Review
• SRS
• Review
• Top Level Design
• Test Plan
• Review

• Integration
• Documentation
• Customer Site Deployment

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typical - done for each module

• Detailed Design
• Code
• Walk-thru / Review
• Unit Test

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A Project Management Tool

• If youre not doing Critical Paths, youre not
  doing project management.
• Shows the time sequence of events, including
  parallel efforts.
• Determines length (calendar time) of a project.

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Required Data

• List of Activities
• Activity Duration (best estimate)
• Sequential Activity Relationships
• what precedes a task
• what can be done at the same time
• what follows a task

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Output Information

• Activity and Project Completion Time
• Critical Path / Critical Activities / A delay
  here will delay the entire project
• Activity Slack a single activity not in the
  critical path has extra time to complete

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Advantages

• Shows activity relationships and dependencies
• Aids coordination of activities
• Facilitates contingency planning

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Disadvantages

• Complex
• Time consuming
• Hard to Maintain
• Project Management Tools (e.g. Microsoft Project)
  help

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Pert Chart

• tasks a and b can start simultaneously
• task c can start after a is complete
• task d can start after b is complete
• c and d must complete before e can begin

A
c
a
e
CD
E
0
B
b
d
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How to

The Next Task
The Task
The Completion of the Task
Duration
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Draw a Pert Chart for this job

• Task Title Duration Precedence
• start 0 -
• think 1 a
• plan 2 b
• procrastinate 5 a
• assign to someone else 1 c,d
• hide 3 e
• finish 1 e
• grab credit 1 f,g

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b
a
c
e
f
A
B
CD
E
F
FG
H
C
1
2
1
3
1
0
g
d
1
G
5
D
Critical Path A D E F H slack in B C,
G