Decision - Critical Path Analysis Flashcards
What is a precedence table?
A precedence table is a table in which shows when activities must be competed before others are started
But only the immediate dependence is shown
What is another name for a precedence table?
Dependence table
Have a general idea what a precedence table looks like
What is a dependency?
When one activity can’t proceed until another activity is completed it is referred to as a dependency
How can you represent a precedence table as a network?
An activity network
In an activity network what do the arcs represent?
Activities
In an activity network what do the nodes represent?
The completion of the activities
What are activities also called?
Events
What must be on each arc in an activity network?
An arrow to show which way the production process happens
And an activity letter
What is the first node labeled?
What is it called?
0
Source node
What is the final node called?
The sink node
When creating an activity network where do the numbers come from?
You add the numbers once you draw a line from that activity (do them in order from A-Z)
What may be different about the source node sometimes?
It may have a start label of 1 instead of 0
What is the exam tip with activity networks?
You are most likely going to need to do a couple tries so don’t try to do it first time
What is a dummy variable?
A dummy node can show that an event C depends on events A and B, which are not dependent on each other and therefore are impossible to connect on a graph
State the standard example of a dummy variable?
What does a dummy variable have none of?
It has no time or cost
What is a dummy variables sole purpose?
A dummy variables sole purpose is to show dependencies between activities
What does a dummy variable look like?
A dummy variable is represented by a dotted line (with an arrow in the correct way in which you will continue the graph
What is the early event time?
The early event time is the earliest time of arrival at the event allowing for the completion of all preceding activities
What is the late event time?
The late event time is the latest time that the event can be left without extending the time needed for the project
How do you calculate the early event time?
The early event times are calculated starting from 0 at the source node and working towards the sink node by adding the value of each arc to the previous early event time
This is called a forwards pass or forwards scan (you always use the largest number)
How do you calculate the late event time?
The late event times are calculated starting at the sink node and working backwards towards the source node. This called a backwards pass or backwards scan
It is calculated by subtracting the previous late time by the number on the arc (you always choose the smallest number
When doing questions with early and late times what changes with the graph?
You replace the nodes with:
What is a critical activity?
Critical activity - an activity in which any increase in its duration results in a corresponding increase in the duration of the whole project
What is a critical path?
Critical path - a path from the source node to the sink node which entirely follows critical activities
What will be true about a critical path?
It will be the longest path
What is the easy way to see where a critical activity is?
Any node which has an early time = late time is a critical activity
What do you need to mention when explaining why a activity is critical?
An increase in the time to do with activity will lead to an increase in the whole project time
What is the total float of a critical activity?
0
What is the total float of an activity?
The total float of an activity is the amount of time that its start may be delayed without affecting the duration of a project
How do you calculate total float?
Total float = latest finish time - duration - earliest start time
Bottom Right - Middle - Top Left
How do gantt charts represent a critical path?
It is put continuously across the top of the graph
What does a gantt chart do?
A gantt chart provides a graphical way to represent the range of possible start and finish times for all activities on a single diagram
How do you draw a gantt chart?
Draw the boxes of the critical path at the top
Then on the second line draw in a box representing the first non critical activity (you need to draw the start of the box at the earliest start time and then draw dotted box to represent its float which ends at the latest time)
Continue this process
What is a gantt chart also called?
Cascade graph
What is a big mistake when drawing gantt charts?
Forgetting to draw in the float of the activities
If you asked to state which activities must be happening at 10 days, what do you do?
you draw a line down your Gantt chart and any activities that are through the line (and can’t be shifted to the left or right because of their float) are the answer
What are the assumptions of the resource diagrams?
No worker can do more than 1 job at a time
Once a worker has started an activity , they must complete it
The worker becomes avaliabke for another activity immediately after finishing the previous activity
What can resource histograms be used for?
Considering how many workers are required to complete a project in minimum time
When drawing an initial resource histogram what else do you assume.
Each activity starts at the earliest time possible
What is it called when you move around the start times of activities?
Resource leveling
What is resource levelling (definition)
The process of adjusting the start/finish times so that the project can use less workers
What is the first step when drawing a resource histogram?
You draw in your critical path along the bottom
How do you draw a histogram (from a ghant chart)?
Draw critical path along the bottom
From top to bottom on the gantt chart (starting as soon as possible) each event
It may be quicker to draw it day by day
When resource levelling what do you nit want to do (with what exception)?
You don’t want to move an activity outside its float.
If it is necessary, do this with the critical activities
How do you draw a resource histogram from an activity network?
Draw a precedence table then keep an eye on the dependencies and the start and finish times from the activity network
What is a scheduling diagram?
A scheduling diagram is a diagram which shows how all the activities can be completed with the minimum number ow workers
(Probably not a good definition)
How do you draw a scheduling diagram?
Always assume worker 1 does the critical activities
Worker 2 will start with the next activity with the earliest finish time
You start with the top row then work your way down
You shade in any wasted time
Explain what is meant by a critical path?
A chain of activities which have no float
How can you tell from an activity network which activities should be happening on day 20?
Look at the start and end times. If the start time is lower than 20 and the finish time later than 20 then look at the length. Use your brain to see if that has to occur during the 20th day
How do you calculate a lower bound for the number of workers required?
Then round up as you can’t have .3 of a worker
What does it mean when it says to schedule the project?
Draw a scheduling diagram
