A work instruction is documentation: it tells one person how to perform a task correctly. Standard work is a lean concept: it defines the current agreed best way to do the work, including the sequence, the cycle time, and the standard work in progress.
The distinction that matters is purpose. A work instruction exists so the task is done correctly. Standard work exists as a baseline to be improved against.
What standard work adds
- Takt or cycle time. How long the task should take, which a work instruction usually does not state.
- Sequence as an explicit choice. Not just the order, but the agreed best order given the layout.
- Standard work in progress. How much inventory should sit between steps.
- An improvement baseline. You cannot measure an improvement without a documented current state, which is the main reason standard work exists.
How they relate in practice
Standard work defines what the current best method is. The work instruction is often how that method is communicated at the station. Change the standard work through a kaizen and the work instruction has to be revised to match, which is where the two most often fall out of sync.
Which do you need?
If you are documenting so work is performed correctly and consistently, you need work instructions. If you are running continuous improvement and need a measurable baseline, you need standard work, and your work instructions become the mechanism for deploying each revision to the floor.