Gantt (Cascade) Charts (Edexcel International AS Maths) : Revision Note

Gantt Charts

What is a Gantt (cascade) chart?

• A Gantt 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 scheduling problems 

  • A Gantt chart assumes one worker per activity

• A Gantt chart is also known as a cascade chart

How do I draw a Gantt (cascade) chart?

• A horizontal axis is drawn for time 

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

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

    • The earliest start time is the early event time of its start node 

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

    • There are no breaks in an activity

    • 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

• Each non-critical activities is drawn on a separate line

  • Their total float is indicated by a bar drawn with a dotted line

    • The dotted float bar can be seen as the room that the activity bar can slide back and forth along

  • The start and end times of a non-critical activity are flexible

  • 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 the following

  • 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 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