Constructing a Resource Histogram
What is a resource histogram?
- A resource histogram is a graphical way of showing the number of resources needed during each time unit (e.g. day) of a project
- A 'resource' usually refers to a person (called a worker) but could refer to
- a team of workers (e.g. bricklayers, roofers, painters)
- a machine (e.g. cement mixer)
- The number of workers needed will vary as a project progresses and will be determined by
- the activities that need to happen at a particular time
- the number of workers needed per activity
- A resource histogram shows at a glance the maximum (and minimum) number of workers/resources needed for a project
- 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?
- In the first instance, there are three main assumptions used in order to construct a resource histogram
- Each activity begins at its earliest event time
- A worker/resource can only carry out one activity at a time
- 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
- Once a resource histogram is constructed, the assumptions can be reconsidered
- This may be so that the number of workers/resources required at any given time is minimised
- This process is called resource levelling and is covered in the next revision note
- This may be so that the number of workers/resources required at any given time is minimised
How do I draw a resource histogram?
- The horizontal axis is used for time
- However units of time are treated as discrete data
- So "day 1" would be labelled underneath the first column
- The vertical axis is used for the number of resources (number of workers)
- For each unit of time
- a box per worker is drawn with a label of the activity that worker is doing
- multiple boxes are 'stacked' on top of each other
- critical activities are at the bottom of the stack
- other activities appear in alphabetical order
- 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)
