Project Scheduling

WBS: quick recapTypes: Process, product, hybridFormats: Outline or graphical org chartHigh-level WBS does not show dependencies or durationsWhat hurts most is whats missingBecomes input to many things, esp. schedule

WBS exampleshttp://www.dir.state.tx.us/eod/qa/planning/wbs.htmThe next two wbs: two (fictitious) wbs taken from PMBOK

Project SchedulingSplit project into tasks (= create a WBS)Estimate time and resources required to complete each task.Organize tasks concurrently to make optimal use of workforce.Minimize task dependencies to avoid delays caused by one task waiting for another to complete.Dependent on project managers intuition and experience.

EstimationThe single most important task of a project: setting realistic expectations. Unrealistic expectations based on inaccurate estimates are the single largest cause of software failure.Futrell, Shafer, Shafer, Quality Software Project Management

now: just a couple of slides to keep going.In the next few lessons: more detailed and accurate description (tools and techniques)

EstimationActivities/task characterized by:

Effort: how much work will the activity need to be completedResources: how many resources will be working on the activityDuration: how long will the activity last for

estimation technique provide (at least) two of the quantities specified above

EffortYour best shot for providing estimations (how complex/how much work does the activity require?)A.K.A. WorkMeasured in man/month (3 m-m = 1 person working for 3 months; 3 people working for one 1 month)Mind you though: communication increases the time to complete activities

DurationHow much time will the activity last forMeasured in (work-)hours, (work-)days, (work-)months, Calendar time != duration: calendar time includes non-working days, holidays, Usually:A duration of 5 days == 40 hours (8 hours a day) = 1 calendar week (sat and sun rest time)

Duration (II)Some more relationships:In some countries/contexts: 1 calendar week = 36 work-hours (7.12 hours/day)A (work-month) maybe:20 days17 days( where are the missing days?)

A (simplicistic) viewD = E / M

Fix any two among D, E, and M, and you get the third(once again) a simplification, but good enough for us (we embed the non linearity in the effort)

A (simplicistic) view (II)When working with planning tools, you change one variable at a time.Standard characterisation:Fixed Unit. A task in which the assigned resources is a fixed value and any changes to the amount of work or the tasks duration do not affect the tasks units. (Duration x Units = Work).Fixed Work. A task in which the amount of work is a fixed value and any changes to the tasks duration or the number of assigned resources do not affect the tasks work. (Duration x Units = Work).Fixed Duration. A task in which the duration is a fixed value and any changes to the work or the assigned resources, don't affect the tasks duration. (Duration x Units = Work).

Some general considerationsEstimating the difficulty of problems and hence the cost of developing a solution is hard.Productivity is not proportional to the number of people working on a task.Adding people to a late project makes it later because of communication overheads.The unexpected always happens. Always allow contingency in planning.

SchedulingOnce tasks (from the WBS) and size/effort (from estimation) are known: then schedule

Primary objectivesBest timeLeast costLeast riskSecondary objectivesEvaluation of schedule alternativesEffective use of resourcesCommunications

Some rules of the thumbOrganize tasks concurrently to make optimal use of workforce.Minimize task dependencies to avoid delays caused by one task waiting for another to complete.Dependent on project managers intuition and experience.

Scheduling (II) according to Sommerville:

while project has not been completed or cancelled loopDraw up project scheduleInitiate activities according to schedule Wait ( for a while ) Review project progress Revise estimates of project parameters Update the project schedule Re-negotiate project constraints and deliverables if ( problems arise ) then Initiate technical review and possible revision end ifend loop

TerminologyPrecedence: A task that must occur before another is said to have precedence of the otherConcurrence:Concurrent tasks are those that can occur at the same time (in parallel)Leads & Lag TimeDelays between activitiesTime required before or after a given task

TerminologyMilestonesHave a duration of zeroIdentify critical points in your scheduleShown as inverted triangle or a diamondOften used at review or delivery timesOr at end or beginning of phasesEx: Software Requirements Review (SRR)Ex: User Sign-offCan be tied to contract terms

TerminologyExampleMilestones

TerminologyDeliverable:a deliverable is a measurable and verifiable work products (we saw it already!)

in current practice sometimes milestone and deliverable are used interchangeably (both used to identify products - milestones may represent key-products)

TerminologySlack & FloatFloat & Slack: synonymous termsFree SlackSlack an activity has before it delays next taskTotal SlackSlack an activity has before delaying whole projectSlack Time TS = TL TETE = earliest time an event can take placeTL = latest date it can occur w/o extending projects completion date

Scheduling TechniquesMathematical AnalysisNetwork DiagramsPERTCPMGERTBar ChartsMilestone ChartGantt Chart

Network DiagramsDeveloped in the 1950sA graphical representation of the tasks necessary to complete a projectVisualizes the flow of tasks & relationships

Mathematical AnalysisPERTProgram Evaluation and Review TechniqueCPMCritical Path MethodSometimes treated synonymouslyAll are models using network diagrams

MS-Project Example

Network DiagramsTwo classic formatsAOA: Activity on ArrowAON: Activity on NodeEach task labeled withIdentifier (usually a letter/code)Duration (in std. unit like days)There are other variations of labelingThere is 1 start & 1 end eventTime goes from left to right

Node Formats

Network DiagramsAOA consists ofCircles representing EventsSuch as start or end of a given taskLines representing TasksThing being done Build UIa.k.a. Arrow Diagramming Method (ADM)AONTasks on NodesNodes can be circles or rectangles (usually latter)Task information written on nodeArrows are dependencies between tasksa.k.a. Precedence Diagramming Method (PDM)

Critical PathThe specific set of sequential tasks upon which the project completion date dependsor the longest full pathAll projects have a Critical PathAccelerating non-critical tasks do not directly shorten the schedule

Critical Path Example

CPMCritical Path MethodThe process for determining and optimizing the critical pathNon-CP tasks can start earlier or later w/o impacting completion dateNote: Critical Path may change to another as you shorten the current

Example Step 1

Forward PassTo determine early start (ES) and early finish (EF) times for each taskWork from left to rightAdding times in each pathRule: when several tasks converge, the ES for the next task is the largest of preceding EF times

Example Step 2

Backward PassTo determine the last finish (LF) and last start (LS) timesStart at the end nodeCompute the bottom pair of numbersSubtract duration from connecting nodes earliest start time

Example Step 3

Example Step 4

Slack & ReserveHow can slack be negative?What does that mean?How can you address that situation?

Slack & Reserve

Network DiagramsAdvantagesShow precedence wellReveal interdependencies not shown in other techniquesAbility to calculate critical pathAbility to perform what if exercisesDisadvantagesDefault model assumes resources are unlimitedYou need to incorporate this yourself (Resource Dependencies) when determining the real Critical PathDifficult to follow on large projects

4 Task Dependency TypesMandatory DependenciesHard logic dependenciesNature of the work dictates an orderingEx: Coding has to precede testingEx: UI design precedes UI implementationDiscretionary DependenciesSoft logic dependenciesDetermined by the project management teamProcess-drivenEx: Discretionary order of creating certain modules

4 Task Dependency TypesExternal DependenciesOutside of the project itselfEx: Release of 3rd party product; contract signoffEx: stakeholders, suppliers, Y2K, year endResource DependenciesTwo task rely on the same resourceEx: You have only one DBA but multiple DB tasks

Task Dependency RelationshipsFinish-to-Start (FS)B cannot start till A finishesA: Construct fence; B: Paint FenceStart-to-Start (SS)B cannot start till A startsA: Pour foundation; B: Level concreteFinish-to-Finish (FF)B cannot finish till A finishesA: Add wiring; B: Inspect electricalStart-to-Finish (SF)B cannot finish till A starts (rare)

PERTProgram Evaluation and Review TechniqueBased on idea that estimates are uncertainTherefore uses duration rangesAnd the probability of falling to a given rangeUses an expected value (or weighted average) to determine durationsUse the following methods to calculate the expected durations, then use as input to your network diagram

PERTStart with 3 estimatesOptimisticWould likely occur 1 time in 20Most likelyModal value of the distributionPessimisticWould be exceeded only one time in 20

PERT FormulaCombined to estimate a task duration

PERT FormulaConfidence Interval can be determinedBased on a standard deviation of the expected timeUsing a bell curve (normal distribution)

For the whole critical path use

PERT ExampleConfidence interval for P2 is 4 times wider than P1 for a given probabilityEx: 68% probability of 9.7 to 11.7 days (P1) vs. 9.5-13.5 days (P2)

DescriptionPlanner 1Planner 2m 10d10da9d9db12d20dPERT time10.16d11.5dStd. Dev.0.5d1.8d

PERTAdvantagesAccounts for uncertaintyDisadvantagesTime and labor intensiveAssumption of unlimited resources is big issueLack of functional ownership of estimatesMostly only used on large, complex projectGet PERT software to calculate it for you

CPM vs. PERTBoth use Network DiagramsCPM: deterministicPERT: probabilistic CPM: one estimate, PERT, three estimatesPERT is infrequently used

Milestone ChartSometimes called a bar chartsSimple Gantt chartEither showing just highest summary barsOr milestones only

Bar Chart

Gantt Chart

Gantt ChartDisadvantagesDoes not show interdependencies wellDoes not uncertainty of a given activity (as does PERT)AdvantagesEasily understoodEasily created and maintainedNote: Software now shows dependencies among tasks in Gantt chartsIn the old days Gantt charts did not show these dependencies, bar charts typically do not

Reducing Project DurationHow can you shorten the schedule?ViaReducing scope (or quality)Adding resourcesConcurrency (perform tasks in parallel)Substitution of activities

Compression TechniquesShorten the overall duration of the projectCrashingLooks at cost and schedule tradeoffsGain greatest compression with least costAdd resources to critical path tasksChanging the sequence of tasksFast TrackingOverlapping of phases, activities or tasks that would otherwise be sequentialInvolves some riskMay cause rework

Mythical Man-MonthBook: The Mythical Man-MonthAuthor: Fred BrooksThe classic book on the human elements of software engineeringFirst two chapters are full of terrific insight (and quotes)

Mythical Man-MonthCost varies as product of men and months, progress does not.Hence the man-month as a unit for measuring the size of job is a dangerous and deceptive mythAdding manpower to a late project makes it later

Mythical Man-MonthWhy is software project disaster so common?1. Estimation techniques are poor & assume things will go well (an unvoiced assumption)2. Estimation techniques fallaciously confuse effort with progress, hiding the assumption that men and months are interchangeable3. software managers often lack the courteous stubbornness of Antoine's chef.4. Schedule progress is poorly monitored5. When schedule slippage is recognized, the natural response is to add manpower. Which, is like dousing a fire with gasoline.

Mythical Man-MonthOptimismAll programmers are optimists1st false assumption: all will go well or each task takes only as long as it ought to takeThe Fix: Consider the larger probabilitiesCost (overhead) of communication (and training)His formula: n(n-1)/2How long does a 12 month project take?1 person: 12 month2 persons = 7 months (1 man-months extra)3 persons = 5 months (1 man-months extra)Fix: dont assume adding people will solve the problem

Mythical Man-MonthSequential nature of the processThe bearing of a child takes nine months, no matter how many women are assignedWhat is the most mis-scheduled part of process?Testing (the most linear process)Why is this particularly bad?Occurs late in process and w/o warningHigher costs: primary and secondaryFix: Allocate more test timeUnderstand task dependencies

Mythical Man-MonthQ: How does a project get to be a year late?A: One day at a timeStudiesEach task: twice as long as estimatedOnly 50% of work week was programmingFixesNo fuzzy milestones (get the true status)Reduce the role of conflictIdentify the true status

One final wordThe seeds of major software disasters are usually sown in the first three months of commencing the software project. Hasty scheduling,irrational commitments, unprofessional estimating techniques,and carelessness of the project management function are the factors that tend to introduce terminal problems. Once a project blindly lurches forward toward an impossible delivery date, the rest of the disaster will occur almost inevitably.T. Capers Jones

draw graphsTraining == ramp-upSoftware is a systems effort

draw graphsTraining == ramp-upSoftware is a systems effort

I dont agree with his formulaHe advocates 50% of schedule to TestLateHighest cost: most staffed timeChanges cost moreSecondary costsYou need to understand Critical Path and other dependencieswhat does Brooks mean by this?Termites not tornadoesImperceptibly but inexorablyHard to recognizeReduce conflict: status vs. action meetings