Gantt Charts (AQA Level 3 Mathematical Studies (Core Maths)): Revision Note

Gantt Charts

What is a Gantt (cascade) chart?

• A Gantt chart (also known as a cascade diagram), 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 bottom of the Gantt chart 

• Activities are then drawn as a series of bars (rectangles), or horizontal lines, underneath

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

  • Each activity is assumed to occur in a single block of time, with no breaks

    • E.g. an activity of duration 5 and earliest start time 4 would be drawn as a bar starting at 4 and ending at 9

  • Critical activities are often (but not always) drawn in the same horizontal line

    • These have a total float of zero so can be drawn back-to-back

  • Non-critical activities are drawn one per line

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

    • E.g. an activity of duration 4, earliest start 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

    • The dotted float bar can be seen as room that the activity bar can slide back and forth along to vary its start and end time

  • 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, I and K

    • The critical path is A-C-G-I-K

  • The minimum project duration is 28 (days)

• The Gantt chart for the project would be constructed with

  • A horizontal axis running from 0 to (at least) 28 at the bottom

  • Critical activities A, C, G, I and K drawn back-to-back on a single line just above the axis

  • 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