Constructing a resource histogram
What is a resource histogram?
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A resource histogram is a graphical way of showing the number of resources needed during each time unit (e.g. day) of a project
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A ‘resource’ usually refers to a person (called a worker) but could refer to
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a team of workers (e.g. bricklayers, roofers, painters)
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a machine (e.g. cement mixer)
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The number of workers needed will vary as a project progresses and will be determined by
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the activities that need to happen at a particular time
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the number of workers needed per activity
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A resource histogram shows at a glance the maximum (and minimum) number of workers/resources needed for a project
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Resource histograms can be used to help minimise the number of workers at any particular time during a project
What assumptions are made in drawing a resource histogram?
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In the first instance, there are three main assumptions used in order to construct a resource histogram
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Each activity begins at its earliest event time
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A worker/resource can only carry out one activity at a time
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Once an activity has begun, it must be completed in one go e.g. If an activity C, say, of duration 5 days, starts on day 7, it has to take place on days 7, 8, 9, 10, 11 It cannot be done on days 7, 8, 12, 13, 14 for example
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Once a resource histogram is constructed, the assumptions can be reconsidered
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This may be so that the number of workers/resources required at any given time is minimised
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This process is called resource levelling and is covered in the next revision note
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How do I draw a resource histogram?
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The horizontal axis is used for time
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However units of time are treated as discrete data
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So “day 1” would be labelled underneath the first column
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The vertical axis is used for the number of resources (number of workers)
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For each unit of time
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a box per worker is drawn with a label of the activity that worker is doing
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multiple boxes are ‘stacked’ on top of each other
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critical activities are at the bottom of the stack
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other activities appear in alphabetical order
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Each activity commences at its earliest event time
Worked Example
The Gantt chart for a project is shown below, with the duration of each activity measured in days.
The table below shows the number of workers required for each activity.
|
Activity |
No. of workers |
|
A |
2 |
|
B |
1 |
|
C |
3 |
|
D |
2 |
|
E |
1 |
|
F |
2 |
|
G |
3 |
|
H |
1 |
|
I |
2 |
|
J |
2 |
Construct a resource histogram for the project, assuming that each activity starts at the earliest possible time.
Activity A requires 2 workers and lasts for 5 days
Add the rest of the critical activities with their number of workers
Other activities can be added in order
Activity B is straightforward but activity D gets ‘bumped’ up for days 6 and 7 days due to activity C requiring 3 workers (whilst activity A only needed 2 workers)
Add the other activities carefully, one at a time, considering where any are ‘bumped’ up or down
It is always worth double checking – especially any ‘bumped’ activities – and check the minimum duration time (23 days)
Responses