A work instruction (WI) is a document that is used to clearly define the inspection variables for a given inspection. It is designed to be used in conjunction with a procedure or standard. 

A work instruction is normally issued for the following reasons: 

1. To ensure that a particular task is carried out to specific requirements to improve reproducibility and reliability. 

Example: If 100 items of a given component need to be tested over a period of time by numerous technicians. A WI will ensure that all technicians are using the same technique / process. 

2. To be issued to Level 1 Technicians.

As per AS3998/ISO9712 Level1 technicians shall not be responsible for selecting method or technique. A Work Instruction is essentially most of the technique areas normally recorded on a record of test, pre-nominated by a Level 2 or 3 level.