NETWORK DIAGRAMS
Moving house
Think of yourself moving into new accommodation. If you liked the new place better than the old one, it would not be surprising if your mind were focused primarily on how things would look when they were complete, when all the things that needed to be done had been done.
“Vision” – an ability to represent the future internally, often in some detail – is an important quality in a good manager, especially a senior manager. Without clarity of purpose it can be hard to stay motivated and persist until the goal is achieved.
Oh the work!
Then I look around me and realise that there is a long way to go before that final stage is reached. I need to:
pack all my things
arrange someone to transport them
get the cash to pay them on the day
get my mail redirected
register with a GP in the area I am moving to
ensure that the telephone lines and broadband are arranged ready for the day of entry
notify the electricity, gas, and water suppliers that I am the new tenant of the premises and set up an account in my name
inform my bank, employer, friends, family and anyone else that I thought that I have moved and my new address
close my electricity, gas, and water accounts for my old address
continue this list and draw up more detailed plans for each item
Usually, we do all this in our heads, without bothering to write it down. This is because the bulk of the tasks can be accomplished in a couple of days at the most. But consider now the person who is moving from much larger accommodation. In a sense, for them the whole process is actually much easier:
pick up the phone
tell a removals company what you want
wait for them to tell you what needs to happen when
But this is business
Here, of course, we put ourselves not in the position of the consumer using the removal company’s services, but in the position of the removals company.
Fleet
One thing that can be said about removals (which is part of the “Haulage” sector) is that it involves running a fleet of trucks, each with a driver (who requires special training to get a licence to drive a truck of that size), and a crew of labourers ready to load all the goods from the old house to the new one.
Profit
Since the purpose of business is profit (= revenue – costs) the bigger the costs are, the smaller the profit. To reap the biggest profit, or perhaps in order to be able to charge competitively low prices, the costs need to be kept as low as possible without alienating the workforce (although many organisations forget how important it is not to alienate the workforce).
Variable costs
Not thinking for the moment about fixed costs (which will have to be paid anyway), think instead about the variable costs of a move for a haulage company. Things like the amount of fuel, the number of kilometres travelled, the time taken to load and unload the truck, the amount of damage done to the client’s goods, and delay will all greatly affect the variable costs associated with each job. To get these down to the lowest possible level, the haulier will have to ensure that each task gets done in the minimum time.
Getting it down on paper
So, the question really is, “What is the order I should do them in to ensure that time is not wasted?”
This is the point at which it becomes useful to draw diagrams, because it can be difficult to hold a whole process in one’s mind. Having it on paper or on a screen helps fill in details missing from my mental image.
Convention
The diagram used in this context is called a “Network Diagram”. So that anyone can read any network diagram, certain conventions are used. These are shown below.
Each new step in a process is represented by a circle and an arrow or line. The arrow represents a task, while the numbers in the circle provide information about the tasks before and after it.
Network Diagram Conventions
Each node in a process is numbered from left to right.
Each node is divided into two halves
The left task shows the node number in the sequence
The task that will commence at that node is represented by the arrow or line after it
The Earliest start time for each task is shown in the upper right quadrant
The latest time for the task before to finish is shown in the lower right quadrant
The task label sits above the line representing the task
The position of the task duration is flexible, but is mostly shown below the line
A NODE
Drawing Network Diagrams
The first step in drawing a network diagram is to specify the tasks and put them in order.
Task: Survey client’s residence and goods to be moved.
Sub task: Note special requirements by room
Task No Task Time taken (minutes)
a Greet client, explain procedure, photograph room, and open picture on PC. 5 – 20
b Click all objects to remain 2 – 10
c Click any fragile, or valuable items, any items that will need special treatment to move out (e.g. disconnection from electricity supply, extraction through window or garden, unusually heavy object, etc.) 1 – 30
d Estimate number of boxes required for smaller items 1 – 6
Total 9 – 66
We can use this information to draw the diagram below.
Starting at Time 00 at the first node, simply add the minimum time taken by the task after it to calculate the EST at the next node. The minimum time for task a starting at Node A is 5 minutes. Hence the EST at node B is 0 + 5 = 05.
If 66 minutes is accepted as the possible duration of the process, then starting with the last task and working backwards, we can find the LFT at a node by subtracting the duration of the task after it from the node to the right of it. Thus, since task d lasts a maximum of 6 minutes, the LFT at node D is:
LFT at node E – duration of Task d = 66 – 6 = 60
Preparing a mug of instant coffee: Task List
Task Task code Time (s) Task Task code Time (s)
Pour one mug cold water into kettle a 10
Boil kettle b 45 Pour boiling water into mug while stirring g 6
Wash and dry mug c 18 Pour required milk into mug while stirring h 4
Wash and dry spoon d 5 Stir coffee i 2
Put coffee in mug e 5 Replace milk in fridge j 2
Get milk ready to pour in f 10 Wash spoon, dry and put away k 5
Network diagram for preparing a mug of instant coffee
The critical path
Network diagrams are useful when planning an operation, such as a production process, in detail. By comparing the Earliest Start Times and Latest Finish Times at various points along the diagram, we can find out which particular tasks will, if they are delayed, delay the whole process, operation, or project. This sequence of tasks along the diagram that can delay the whole thing is called the “Critical Path”. (So network diagrams are used for ‘critical path analysis’.)
Float
Where the EST at a node is earlier than the LFT, the spare time at that point is termed the “float” or “free float”. Another purpose of critical path analysis, apart from identifying the critical path itself, is to determine where there is float and how much. This tells us how much flexibility there is in deadlines. Projects are obviously best organised in a way that preserves the maximum free float. This allows for unforeseen delays.
Questions and Exercises
1) What is the value of critical path analysis to an organisation?
2) Analyse how a T-shirt printing company might benefit from representing its operations in a network diagram.
3) Imagine you are the Owner-Manager of a mail-order business. One of the ways you compete is by reliably getting your products to your customers in the shortest time possible.
Represent the process between customer order and despatch in a network diagram. (You will have to estimate the times taken.)
4) Draw a network diagram for the following:
