What are typical selection criteria for attendees at research summer schools?

Question

In my field there are often summer schools for graduate students/postdocs to learn about a particular research topic in a short amount of time. Given that some of these summer schools are in exotic locations, or with expenses paid, I imagine that they receive a large number of applicants (probably far more than can be accommodated). In this case, how do the organizers typically select who attends? I can think of a few things they might consider:

• selecting students from diverse institutions and countries, in order to maximize the impact of the school
• selecting students of diverse academic backgrounds and skill levels, to facilitate mentoring and sharing of knowledge
• selecting only the "best" students (e.g. by publication record), similar to scholarship/fellowship applications
• selecting the students with the strongest demonstrated skills, so that the school can teach only the most advanced topics
• selecting the first to apply that meet the minimum criteria (first come, first served)
• something else?

Since summer schools primarily exist to teach, I feel they may be distinct from other competitive applications in academia e.g. fellowships or conferences. For those who have served on the organizing committee of such a school, any insights would be appreciated.

Answer 1

Summer schools where you pay a fee have relatively few criteria for selection. They will have some minimum standard that is usually published (they may phrase it as "summer school geared towards people with undergraduate degrees in x, y, z) but even those minimums are flexible. These schools are pure money makers for the universities and the universities accept as many people as possible until the max spaces are filled, usually on a rolling first come/first serve basis. I want to emphasise that these summer schools are not selective because they are not meant to be.

Summer schools which are free usually have posted criteria for selection and are selective. They may specifically serve a particular group (e.g. scholars from lower income countries; scholars working within a particular sector). Many of these require a letter of motivation and a CV (I know of two that even require recommendation letters) and applicants are selected based on how well the student's background and goals mesh with the content and goals of the summer school. They look for people who can really "use" the education from the summer school in interesting ways. They will also have minimum requirements that are less flexible but, too, have some flexibility if the student is otherwise intriguing.

(Note, I've taught on probably about ten different summer schools in about five different European countries)