Process flow and timing
A process is a set of steps done in a planned order to reach a result. The order of steps is called the flow. Each step also takes time, so we study flow and timing together.
Two steps can be related in two ways:
- Dependent: step B cannot start until step A is finished (you cannot pour tea before it is steeped).
- Independent: B does not need A (cutting ginger does not need the water to be hot).
When all steps are done one after another, the total time is the sum of all step times.
Flow charts: symbols and drawing
A flow chart is a picture of a process. Standard shapes let everyone read it the same way:
- Oval (terminator): Start or End.
- Rectangle (process): an action or step.
- Diamond (decision): a yes/no question with two arrows leaving, labelled YES and NO.
- Arrow (flow line): the direction of flow.
How to draw: write the steps in order, put Start at the top, draw each step top to bottom or left to right, use a diamond wherever the path can split, and join all paths to one End. Keep each box to a few words, beginning with a verb.
A loop is made by an arrow that goes back to an earlier box, for example "change the bulb, then ask again".
Timing bars
A timing chart (also called a bar chart or Gantt chart) draws each step as a bar on a time line. The bar length shows how long the step takes, and where it starts shows when it begins. If steps run one after another, the bars form a staircase. Steps that run together sit side by side, in the same time.
Optimisation and the critical path
Optimising a process means making it better: faster, cheaper or with fewer mistakes. A common way is to run independent steps in parallel.
The critical path is the longest chain of dependent steps. It decides the total time. In the tea example: boil water (5) → steep (3) → pour (1) = 9 minutes. Cutting ginger (2) and setting cups (1) are not on the critical path: they have spare time, so they can start any time before the steep.
To shorten a job you must shorten the critical path; speeding up a step that is not on it does not help. Other ways to optimise: remove unneeded steps, use better tools, add a worker for a long step.
Try it
Draw a flow chart of how you get ready for school, with at least one decision diamond (for example, "Is it raining?"). Put a time on each step. Find two steps you can do at the same time, and work out how many minutes you save. Then use the slider in the last 3D step.
Key formulas and definitions
- Total time (one after another) = sum of all step times
- Total time with parallel steps = length of the critical path (longest chain)
- Time saved = old total − new total
- Shapes: oval = start/end, rectangle = action, diamond = decision, arrow = flow
Worked examples
1. Three steps take 4, 3 and 2 minutes and depend on each other in a chain. How long does the job take?
They are dependent, so the total is 4 + 3 + 2 = 9 minutes.
2. Tea: boil water 5, ginger 2, cups 1, steep 3 (after boil, ginger and cups), pour 1. Ginger and cups run beside the boiling. Total time?
Boil, ginger and cups start together; they finish by minute 5 (the longest). Then steep 3 and pour 1: 5 + 3 + 1 = 9 minutes. Saving: 12 − 9 = 3 minutes.
3. Draw in words a flow chart for 'Is the fan working?'
Start → Switch on fan → Diamond: Does it spin? YES → End. NO → Check the plug → back to Switch on fan (loop).
Common mistakes
- Using a box for a question. A question with YES/NO paths needs a diamond.
- Forgetting the labels YES and NO on the diamond arrows.
- Doing a dependent step in parallel (pouring while the tea is not ready).
- Speeding up a step that is not on the critical path and expecting the total to fall.