Gantt (Cascade) Charts (Edexcel A Level Further Maths): Revision Note

Gantt charts

What is a Gantt (cascade) chart?

• A Gantt chart - also known as a cascade chart - is a graphical display of the activities making up a project

  • a Gantt chart shows

    • the critical activities

    • the total float for non-critical activities

    • the minimum project duration

• Gantt charts can be used in resource levelling and scheduling problems when the number of workers for each activity is known

  • in the first instance, a Gantt chart assumes one worker per activity

How do I draw a Gantt (cascade) chart?

• (Usually) a horizontal axis is drawn for time and this appears at the top of the Gantt chart 

• Activities are then drawn as a series of bars (rectangles) underneath

  • each activity is assumed to commence at its earliest event time

    • i.e. at the early event time of its start node 

  • each activity is assumed to occur in a single block of time

    • i.e.  no breaks!

    • e.g. an activity of duration 5 and early event time 4 would be drawn as a bar starting at 4 and ending at 9

  • Critical activities are all drawn in the same horizontal line

    • these have a total float of zero so are drawn back-to-back

  • Non-critical activities

    • are drawn one per line

    • have their total float indicated by a bar drawn with a dotted line

    • e.g. an activity of duration 4, early event time 7 and total float 3 would be drawn as a (solid) bar starting at 4 and ending at 11, with a dotted bar starting at 11 and ending at 14

  • Bars are labelled with their activity name and duration

    • floats are not labelled

• For the activity network below

  • the critical activities are highlighted and are A, C, G and I

    • (the critical path is A-C-G-I)

  • the minimum project duration is 23 (days)

• The Gantt chart for the project would be constructed with

  • a horizontal axis running from 0 to (at least) 23

    • (0 to 25 keeps things nice!)

  • critical activities A, C, G and I drawn back-to-back on a single line underneath

  • activities B, D, E, F, H and J are each drawn on a separate line

    • e.g.  activity D will be drawn as a solid bar from 4 to 7 with a dotted bar from 7 to 12