01 See #TiSDD chapter 3, Basic service design tools, for a brief introduction to personas. For a comprehensive introduction to how to create and use personas see, for example, Goodwin, K. (2011). Designing for the Digital Age: How to Create Human-Centered Products and Services. John Wiley & Sons.
02 See, for example, the Wired article “Netflix’s Grand, Daring, Maybe Crazy Plan to Conquer the World” from March 27, 2016, that quotes Todd Yellin, Netflix’s VP of product innovation: “There’s a mountain of data that we have at our disposal. That mountain is composed of two things. Garbage is 99 percent of that mountain. Gold is one percent … Geography, age, and gender? We put that in the garbage heap.”
03 See method description Co-creating personas for a detailed description of how to run co-creative workshops for this purpose.
04 This is a tip by Phillippa Rose. See also her case study on how to use personas in a service design project: #TiSDD 5.4.3, Case: Developing and using valuable personas.
Personas  usually represent a group of people with shared interests, common behavior patterns, or demographic and geographical similarities. However, demographic information such as age, gender, or residency is often rather misleading, so be careful to avoid stereotypes.  You can either use existing market segments or use the opportunity to challenge current segmentation and try more meaningful criteria.
When developing customer personas, you should aim to create approximately 3–7 core personas representing your main market segments that could be used company-wide. If you create more than this number of personas, it is unlikely that you will really use them in your work simply because people won’t remember all of them. We often see these core personas used throughout a company – they become like friends. Employees remember their background stories, including their different expectations and behavior patterns.
Following the principle of “design for the average – test with extremes,” you can have many more “edge-of-the-curve” personas to test ideas and prototypes with people from rather extreme ends of your user spectrum. Although you’ll mainly use your core personas during a design process, it makes sense to test ideas as early as possible with these extreme cases, too. Such extreme or edge-of-the-curve personas could, for example, be people who would never use your offerings. You might be able to tweak a concept to cover these and thus increase its usefulness not only within your core target group, but also beyond it.
In a project, you often mix different approaches to create personas – for example, starting with some quick, assumption-based personas on your own, then inviting frontline staff and other stakeholders to a co-creative workshop  to develop some more assumption-based personas. In a third step, these assumption-based personas are then aggregated, enriched, and backed with research-based data.