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Question of the Day

How important is it to be able to estimate the cost of a software project?

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exam preview – 150 pts – 10/11/07

1. (30 pts) requirementscreate scenarios for a problem

2. (30 pts) PERT diagrams3. (20 pts) Cost Estimation4. (40 pts) definitions and short discussions5. (30 pts) 10 multiple choice

One crib sheet (8.5 by 11, handwritten) allowed, turn in with exam

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PERT Diagrams

Program Evaluation and Review Technique

a1

a2

a3

a4

a5

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Critical Path

If there are N tasks with dependencies between the tasks, what set of tasks determines the minimum completion time for the whole project?

This is the critical path.

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Critical Path Algorithm (part 1)

For each node (until completion time of all nodes are calculated)

» if the predecessors are completed, then take latest completions time and add required time for this node;

» this is the completion time.

The node with the latest completion time determines the earliest completion time for project.

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Example

Node pred time start finish critA 5B A 8C B 7D A 4E D 6

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Critical Path Algorithm (part 2)

Starting with the node with latest completion time, mark it as critical.

Select predecessor node with latest completion time, mark it as critical.

Continue until reaching start node.

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Example

Node pred time start finish critA 5 0 5 *B A 8 5 13 *C B 7 13 20 *D A 4 5 9E D 6 9 15

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Critical Path Algorithm (part 3)

Slack time is the amount of time that a specific task can be delayed without changing the project delivery time

For each noncritical node starting with the last node, determine the earliest of the latest start times of the successor nodes. This is the latest completion time. Make the latest start time for the node to reflect this time.

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Example

Node pred time start finish critA 5 0 5 *B A 8 5 13 *C B 7 13 20 *D A 4 5,10 9,14E D 6 9,14 15,20

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TTYP

Node pred time start finish crit A 3 0 B A 2 C A 10 D B,C 14 E D 4

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Cost Estimation

An estimate of the effort and duration, associated costs of equipment, travel and

training and the rationale for the calculations

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Recommendation

If you are involved with cost estimation, I recommend the following book

Tom DeMarco, Controlling Software Projects, Yourdon Press, NY c1982

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Definition of Estimate (DeMarco)

Default:– "An estimate is the most optimistic prediction

that has a non-zero probability of coming true" Proposed:

– "An estimate is a prediction that is equally likely to be above or below the actual result"

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Estimates should not become goals

DeMarco argues that the estimation and the management decision about pricing or goals should be separate.

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Estimate

cost

estimate price

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Cost Estimate

should not be a contract bid should not be a schedule should be input into the management

decision to determine the above should be re-estimated frequently

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TTYP1 Given the following data, what would be

your estimate for a project that would be about 200K? What would you use for a cost estimation formula?

Historical data– project 1 100K 50 prog-months– project 2 20K 10 prog-months– project 3 14K 7 prog-months– project 4 16K 8 prog-months

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Confidence in your estimate

How confidence are you in this estimate? Would you stake your job/grade on this

estimate? What would be your concerns? What could you do to increase your

confidence? What would you consider to be a successful

estimate?

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Problems Programmer variability Product complexity Variability of goal Conflict of goals Lack of detailed product description Variation in effort to reuse code Emergence of new processes, methods and

tools

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Cost Estimation Models

LOC models– Boehm's COCOMO– Price S– Putnam's Model (SLIM)

non-LOC models– Function Points

combination– COCOMO2

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Prediction Formulas

E=X

>1

<1

=1

TTYP2 – which one did Henry Ford experience?Which one happens in your project?

Size

cost

A non-zero gamma would shift line up or down

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Tuesday, 10/2

Read

SOS section 4.4.3