Precedence Tables (SQA National 5 Applications of Mathematics): Revision Note

Precedence tables

What is a precedence table?

• A precedence table shows a list of the activities for a project

  • e.g. the activities needed to make a cup of tea

• Each row corresponds to one activity

• A precedence table usually has four columns:

  • Activity

    • Each activity is labelled by a different letter

  • Description

    • This is a short description of the activity

  • Preceding activities

    • These are the activities that need to happen before the current activity can happen

    • Activities might have no preceding activities

  • Time

    • The time that the activity takes

    • This could be in seconds, minutes, hours, days, etc

Example of a precedence table

• Here is a precedence table for organising a charity bake sale

How do I fill in a diagram from a precedence table?

• You will be given a mostly blank diagram and need to fill in the remaining activities

  • You will normally be given either the starting activity or the final activity

• Identify the starting activity or activities

  • These are on the left on the diagram

  • These do not have any preceding tasks

    • e.g. activity G has no preceding activities

• Identify the final activity or activities

  • These are on the right on the diagram

  • These do not appear as preceding activities for any activity

    • e.g. activity F does not precede any activity

• Identify the different routes from the starting activity to the final activity

  • Start with the starting activity

  • Then find an activity which has the starting activity as a preceding activity

  • Continue this until you reach the final activity

    • e.g. G - B - D - C - A - F

    • e.g. G - H - E - F

    • e.g. G - I - A - F

• Use the lengths of the routes to place some activities

  • G - H - E - F and G - I - A - F both contain 4 activities

  • G - B - D - C - A - F contains 6 activities

    • This can be placed in the diagram straightaway

• Identify activities which appear in more than one route (other than the start and final activities)

  • G - B - D - C - A - F and G - I - A - F both contain activity A

    • These routes must merge at A

  • G - H - E - F does not share any activities with the other routes

    • This route can be by itself

• If you include the times as well as the letters in the boxes, it will make answering later parts of the question easier

How do I find the minimum time to complete a project from a precedence table?

• You need to find the time taken to complete each of the possible routes

  • G - B - D - C - A - F takes 20+3+20+15+5+8=71 minutes

  • G - H - E - F takes 20+7+5+8=40 minutes

  • G - I - A - F takes 20+5+5+8=38 minutes

• You need to select the biggest time

  • This is to ensure there has been enough to complete all the other routes

    • e.g. the minimum time for the project above is 71 minutes