What is the difference between an SOP and a work instruction?

An SOP describes a process across multiple roles, with a trigger, sequence, and decision points. A work instruction describes one task performed by one person at one station, in enough detail to complete it with nothing else open, including tolerances and settings.

What is the difference between a procedure and a work instruction?

In ISO usage a procedure and an SOP sit at the same level, so the answer is the same one: the procedure says who performs a step within a process, and the work instruction says how that step is performed at one station. If you hold documents called both, check they are not two names for the same thing.

What is the difference between an SOP and a job aid?

A job aid is a short prompt for someone already trained: a card at the bench, a one-page decision tree, a checklist on a door. It has no roles, no trigger, and no approval history, so it is never the source of truth. An SOP is. A job aid should name the SOP it was derived from.

SOP vs work instructions: the difference

An SOP describes a process across roles. A work instruction tells one person how to perform one task. A job aid is the reminder for someone who already knows.

An SOP covers a process across roles. A work instruction covers one task at one station. That is the whole difference, and everything below is the consequence of it.

In full: an SOP describes what triggers a process, who is involved, the sequence of steps across those roles, and what happens at each decision point. A work instruction describes a single task in enough detail that one person can perform it correctly at their station, including tolerances, settings, and the specific tools.

Put simply: the SOP says the Quality Inspector performs the final inspection. The work instruction tells that inspector exactly how, with which gauge, against which tolerance.

The practical difference

Scope. An SOP spans multiple roles. A work instruction covers one person doing one task.
Level of detail. An SOP names the step. A work instruction specifies how to execute it, down to values and settings.
Decision points. SOPs contain branches, because processes fork. Work instructions are usually linear, because a task at a station either succeeds or is escalated.
Audience. An SOP is read by anyone who needs to understand the process. A work instruction is read at the point of work by the person doing it.
Change frequency. Work instructions change when tooling, materials, or drawings change, which is often. SOPs change when the process itself changes, which is rarer.

Which one do you need?

Write an SOP when the question is "how does this get done here, and by whom". Write a work instruction when the question is "how do I do this specific thing correctly".

Most teams need both, connected: the SOP references the work instruction at the step where it applies. That reference is also what an ISO 9001 auditor looks for, because it demonstrates the document hierarchy actually exists rather than being asserted.

Procedure vs work instruction, and SOP vs job aid

Two neighbouring questions come up constantly, and both are answered by the same hierarchy.

Procedure vs work instruction: in ISO usage a procedure and an SOP are the same level of document, so the answer is the one above. The procedure says the Quality Inspector performs the final inspection; the work instruction tells that inspector how. If you have documents called both, check they are not two names for one thing.

SOP vs job aid: a job aid is neither of the above. It is a short reminder for someone who already knows how to do the task, a laminated card at the bench, a one-page decision tree by the phone, a checklist on the back of a door. It carries no roles, no trigger, and no approval history, and it is not the source of truth. It is a pointer to the SOP or work instruction that is.

SOP: how the process runs here, across roles, with decision points. Approved, owned, and reviewed.
Work instruction: how one person performs one task correctly. Approved, and revised whenever tooling or materials change.
Job aid: a prompt at the point of work for someone already trained. Derived from the other two, never a replacement for them.

The mistake worth avoiding

Writing one document that tries to be both. It ends up too general to follow at the bench and too detailed to read as a process, so the operator ignores it and the manager cannot use it to explain the workflow. If your document has a torque value in it and also describes a handoff between two departments, it is two documents.