These terms are often used interchangeably but are two different approaches to how you design your products and services. A recent paper on Co-production with Autistic adults3 communicates the differences using this tiered diagram to indicate the levels in which participants are involved:
In Co-design: your participants are treated as equals and have equal say in decision making of the project goals and outcomes.
Participatory design: users give input but ultimately you are the decision maker and hold the power.
A great example of co-design was a project with ARCScotland where young people were asked to design a consultation for other young people. The young people were coached and given advice but the design and final decision was theirs.
Most researchers get input into their research once they have won the funds but Sue Fletcher-Watson1 from the University of Edinburgh sought an Autistic mentor to give input at the grant writing stage to ensure the community has the power to really shape the direction of research about them rather than simply shaping the outputs.
What should I use if I am building technology?
The size of a product can make a difference. If the product is small participatory design with short iteration cycles is still a good approach to testing your product to get feedback to ensure you are building the right tool.
If you have a longer project co-design should reduce the number of iterations a product requires by getting input at an earlier stage. The process enables you to build better relationships with users, learn how they think and increase the opportunity of finding novel and innovative solutions by introducing new perspectives. It reduces the risk of designing the wrong solution from the outset and the risk of investing resources into the wrong place.