Criteria for appropriate selection of OWIs in a WAS:
A set of criteria should first be set to determine what indicators to include for evaluating farmed animal welfare. This set of criteria should fulfil the following theoretical and practical requirements for on-farm welfare assessment:  

• Comprehensive, but minimalistic - The set list of WIs should collectively cover all key aspects of welfare, while avoiding redundancy, overlap, or irrelevant measures.
• Independent - Each WI must be mutually exclusive and provide distinct information, and their interpretation must not rely on or be overtly linked to other WIs.
• Repeatability - WIs must be consistently measurable in a farm setting.
• Validity and feasibility - WIs should be scientifically validated, reliable, auditable, and practical enough for on-farm use by standard farm operators. 

As discussed in Module 3, the choice of OWIs depends on factors such as production system, life stage of the animals, and the specific goals of the assessment. Each OWI has its own strengths, weaknesses, and sampling considerations, all of which must be carefully evaluated before their selection. Additionally, different approaches to WASs are likely to require specific ways of structuring and applying OWIs.