1.Developing a project network
1.1 WBS review
WBS doe not identify:
--All the activities that consume time (e.g. waiting periods)
--Dependencies and interrelationships between activities
--Sequence of activities (e.g. work flow)
--Timing of activities (e.g. how long the project will take)
--What are the most important activities
为了解决顺序、时间和重要activity选择等问题，引入了project network的方法。
1.2 Project network
Project network Is a flow chart that graphically depicts the sequence, interdependencies and start and finish times of the project job plan of activities.
项目网络要描述顺序和起止关系。
Time elements of a project:
(1)Activity--Is some action which requires time
(2)Event--It does not consume time. May also be known as a “milestone”
项目的时间分为瞬时的event和持续的activity
Types of activities
(1)Burst，从一个activity延展到多个
(2)Merge，从多个activity聚集到一个
(3)Parallel，可以同时开展的多个activity
Project network work flow
(1)Path--a sequence of connected, dependent activities.
(2)Critical path--the longest path through the activity network that allows for the completion of all activities; OR the shortest expected time in which the entire project can be completed.
critical path是影响项目成败的路径，通常是最耗时不容有误的一些activities。
Basic rules for developing a project network
(1)从左向右
(2)后面的项目必须在前面项目完成后才能开始
(3)每个activity必须有unique的编号，后续的项目编号一定大于前面的编号
(4)不能有loop，不能有conditional statement
Predecessor--前面的activity；Successor--后面的activity
Network computation process
(1)Forward pass
(2)Backward pass
LF(Late finish) = LS(Late start) + DUR(duration)
EF(Early finish) = ES(Early start) + DUR
Slack/Folat--Is the amount of time that a task in a project network can be delayed without causing a delay.
(1)Total slack--shared by activites along a path, affects project completion date
(2)Free slack--owned by the activity, affects subsequent tasks
Total Start Slack = LS - ES
Total Finish Slack = LF - EF
FS = ES(successor) - EF(current)
我们希望TS > 0 ，当TS = 0的时候变成critical situation，如果TS < 0则已经有滞后，要小心。
Very little slack and lots of critical paths--More sensitive;
Lots of slack and only one critical path--Less sensitive
1.3 Extended network techniques
(1)Laddering
根据工作顺序合理安排工作，例如埋管线：
挖掘组1/3路面；挖掘组1/3路面；挖掘组1/3路面
埋管线1/3路面；埋管线1/3路面；埋管线1/3路面；
回填土1/3路面；回填土1/3路面；回填土1/3路面
(2)Lag
Is the minimum amount of time a dependent activity must be delayed to begin or end
a.Finish-to-start lag relationship
例如订购设备
b.Start-to-start lag relationship
例如可以同时开展的工程任务
c.Finish-to-finish lag relationship
例如项目测试一定在项目结束的时候才能完成，在项目中间的时候测试已经在进行
d.Start-to-finish lag relationship
例如The system documentation cannot end until three days after testing has started.
(3)Hammock activity
An activity that spans over a segment of a project.
比如工程项目中的重要仪器
1.4 Adding statistics
Program Evaluation and Review Technique (PERT)
PERT often uses the 'beta distribution'
as the statistical distribution for each activity’s duration.
The whole project often uses the 'normal distribution'.
