TK3333 Software Management Topic 6: Scheduling

Contents  Estimate the duration for each activity  Establish the estimated start time and required completion time for the overall project  Calculate the earliest times at which each activity can start and finish, based on the project’s estimated start time  Calculate the latest times by which each activity must start and finish in order to complete the project by its required completion time  Determine the amount of positive or negative slack  Identify the critical (longest) path of activities

Real World Example Vignette: US Census 2000 Project A US Census takes place every 10 years in the form of a questionnaire from the US Census Bureau. Census information helps the government in making policies regarding health, education, transportation, community services, etc. Census participation takes only a few minutes, but it takes years for the employees of the Census Bureau. Census 2000 was a 13-year project that’s total life cycle cost $65 billion. Planning for data collection is important. 520 local census offices across the US verify and collect as many addresses as possible. Project’s goal was 70% response rate to the 2000 Census. The Bureau implemented a plan to spread the word about the census and stress its importance. A non-response plan was also established to reach those that failed to complete the census. The 2000 Census was considered to be the most accurate population count in US history. For the first time, census data was made available on the Internet.

Real World Example Vignette: High Risk Projects at NASA The Genesis spacecraft returned to earth on September 2, 2004, but the landing didn’t go as planned. The parachute didn’t deploy and the $264 million spacecraft fell toward the earth at an estimated 193 miles per hour. The research contained in the spacecraft was in jeopardy. The team was able to implement the contingency plan they developed in case the parachute didn’t work. Most of the research was salvageable, and the spacecraft remained mostly intact. NASA will apply lessons learned in this project to their next project, Stardust, which takes off in 2006 and will land in the same way as the Genesis.

Activity Duration Estimates The first step in scheduling is to estimate how long each activity will take. The duration estimate is the total elapsed time for the work to be done PLUS any associated waiting time. The person responsible for performing the activity should help make the duration estimate.

Project Start and Finish Times It is necessary to select an estimated start time and a required completion time for the overall project. Part of the project objective and stated in the contract. E.g. – The project will not start before October 1, 2010 and must be completed by Disember 31, 2011 – The project will be completed within 90 days of the signing of the contract.

Schedule Calculations A project schedule includes: – the earliest times (or dates) at which each activity can start and finish, based on the project's estimated start time (or date) – the latest times (or dates) by which each activity must start and finish in order to complete the project by its required completion time (or date)

Earliest Start and Finish Times Earliest start time (ES) is the earliest time at which a particular activity can begin. Earliest finish time (EF) is the earliest time by which a particular activity can be completed. “Move forward”

Earliest Start and Finish Times Activity Description Activity No. Duration ESEF AIB Event No. ESEF Duration Activity Description AOA

Earliest Start and Finish Times EF = ES + Duration Estimate Activity Description Activity No. Duration ESEF Event No. ESEF Duration Activity Description AIB AOA

Earliest Start and Finish Times Rule 1 The earliest start time for an activity must be the same as or later than the latest of all the earliest finish times of all the activities leading directly into that particular activity. Source: pg. 162 The latest EF of the preceding activities ….. …..will become the ES for the subsequent activity AIB

Earliest Start and Finish Times Rule 1 Source: pg. 162 The latest EF of the preceding activities ….. …..will become the ES for the subsequent activity AOA

Latest Start and Finish Times Latest finish time (LF) is the latest time an activity must be finished in order for the entire project to be completed by its completion time. Latest start time (LS) is the latest time an activity must be started in order for the entire project to be completed by its completion time. “Move backward”

Latest Start and Finish Times Activity Description Activity No. Duration LSLF AIB Event No. LSLF Duration Activity Description AOA

Latest Start and Finish Times LS = LF – Duration Estimate Activity Description Activity No. Duration LS LF Event No. LS LF Duration Activity Description AIB AOA

Latest Start and Finish Times Rule 2 The latest finish time for a particular activity must be the same as or earlier than the earliest of all the latest start times of all the activities emerging directly from that particular activity. Source: pg. 169 … the earliest LS of its subsequent activities The LF for an activity will be …. AIB

Latest Start and Finish Times Rule 2 Source: pg. 169 AOA … the earliest LS of its subsequent activities The LF for an activity will be ….

Total Slack Total slack (TS) or float is the difference between the calculated “EF of the very last activity and the project’s required completion time (i.e. its LF)”* Total Slack = LF - EF or LS – ES => Both result in the same value Note*: But, we also calculate total slack for each activity by using its individual LF and EF, or LS and ES. This is to identify the critical path; the activities that are belong to the same path; and free slack.

Total Slack Activity Description Activity No. Duration LS LF AIB Event No. LSLF Duration Activity Description AOA ESEF ESEF

Total Slack If total slack is positive, it is the maximum time the activities on the path can be delayed. If total slack is negative, it is the amount of time the activities on the path must be accelerated. If total slack is 0, the activities on the path do not need to be accelerated, but cannot be delayed. All activities on the same path will have and share the same total slack. – E.g. If the total slack for Activity A, B and C is +5 days each, they are on the same path. They have a “delay buffer” of 5 days (i.e. A, B and C can delay for a total up to 5 days, not 5 days each!)

Critical Path The critical path is the longest path in the diagram (i.e. most time-consuming). The activities that make up the critical path have the least slack (but, not necessarily has to be a negative value e.g. pg. 182) – either least positive or most negative. All activities with this value are on the critical path.

Total Slack & Critical Path Source: pg. 175 Critical path Total Slack = LS – ES or LF - EF

Total Slack & Critical Path What to do with the negative value of critical path? Reduce the development time of any activities on the path, or Extend the project completion time, or Some compromise

Types of Critical Paths A large network diagram normally contains both positive and negative values of total slack. Noncritical paths have positive values of total slack. Critical paths have zero or negative values of total slack. The most critical path is the longest critical path.

Free Slack The amount of time an activity can be delayed without delaying the start of other activities. It is the relative difference between the amounts of total slack for activities entering into the same activity. – Only necessary when there are two or more activities. It is always a positive value.

Free Slack Total Slack for A = =0 Total Slack for B = = -8 The lesser/lowest value = -8 The Free Slack for A = 0-(-8) = 8 days A B C

Scheduling for Information System Development Some common problems that push IS projects past their required completion time:  Failure to identify all user requirements  Logical design flaws  Continuing growth of project scope  Underestimating learning curves for new software packages  …..More on pg. 174 Read an IS example pg

Project Management Software Allows one to perform scheduling functions. Activity durations can be estimated in a variety of ways. – E.g. hours, days, weeks, months, or years. Project start and finish times can be entered in a variety of ways. – E.g. calendar dates (i.e. 31/01/10), overall number of days/weeks/months (i.e. week 50) Can calculate dates, times, total and free slack.

Probability Considerations: Activity Duration Estimates Used when there is high degree of uncertainty about the duration estimates. 3 estimates: – Optimistic time (t o ): time to complete an activity if everything goes perfectly well. – Most likely time (t m ): : time to complete an activity under normal conditions. – Pessimistic time (t p ): : time to complete an activity under adverse circumstances.

Probability Considerations: The Beta Probability Distribution When using three time estimates, it is assumed that they follow a beta probability distribution. The expected duration is calculated using the following formula: t e = (t o + 4(t m ) + t p )/6

Probability Considerations: The Beta Probability Distribution E.g. optimistic time = 1 week; most likely time = 5 weeks; pessimistic time = 15 weeks. The expected duration of the activity: t e = (t o + 4(t m ) + t p )/6 t e = (1 + 4(5) + 15)/6 t e = 6 weeks Beta Probability Distribution

Probability Considerations: Fundamentals Network planning that uses three time estimates for each activity can be considered a stochastic or probabilistic technique, since it allows for uncertainty. Any technique that uses only one time estimate is considered to be a deterministic technique.

The total probability distribution is a normal probability distribution. The variance for the beta probability distribution of an activity is: Variance = s 2 = [(t p – t o )/6] 2 The standard deviation, s, is another measure of the dispersion of a distribution and is equal to the square root of the variance. Probability Considerations: Fundamentals

The total probability distribution of the critical path activities is a normal distribution. The mean = the sum of the individual activity expected durations. The variance = the sum of the individual activity variances. Probability Considerations: Fundamentals Read example pg

Calculating Probability The probability of completing a project before its required completion time: Z = (LF – EF)/s t  LF = the required completion time (latest finish).  EF = the earliest expected finish time (mean of the normal distribution).  s t = the standard deviation of the total distribution of activities on the longest path.  Z measures the number of standard deviations between EF and LF on the normal probability curve.

Thank You Question? Next (05/04) : – Schedule Control