These terms are often used interchangeably but they are actually two different approaches to design,
Co-design, design with users, not for them.
Participatory design you design for users and get their input.
- Co-design: you ask Autistic users to create survey questions which are used for product feedback
- Participatory design: you create the survey and ask users for product feedback
The goal of co-design is ultimately to become invisible. You facilitate users who have the first-hand expertise but do not have necessarily have experience in technology and design to know how their solutions might be constrained.
Co-design can involve coaching (I.e what technology and designs are possible within a budget) and involving them right at the start of the design process where ideas are generated.
In academia this can go as far as including the community in the grant writing stage so ideas are co-designed from the outset1.
Ultimately Co-design can give you a new perspective on solutions to problems you have considered. It can reduce the number of product iterations and the risk of an idea not working in practice.
In Participatory design you tend to create the product based on your own feedback. The lotus of control and decision on the product primarily sits with you however you aim to get feedback at each stage e.g after wire-framing or prototyping. You then iterate based on this feedback and this process gives enables you to test your assumptions.
Keep in mind that some groups might struggle with low cost testing approaches such as pen/paper prototyping.
Which to use?
The size of a product can make a difference. If the product is small participatory design and testing small iterations can be the most cost effective approach of reaching product market fit.
Co-design can take time to build relationships, if you are working with new groups and can take time to setup new processes to cater for accessibility needs. On the flip side if a project is large or the problem space complex – co-design reduces risk of building the wrong solution.
Keep in mind though that you can use both! Ask users to co-design the product but still use participatory design with a different set of users to ensure all assumptions are tested and you are building the best possible solution to users problems.