Places become available when another participant cancels his or her registration or if a registrant does not confirm his or her application by paying the course fees within five weeks of the reception of the invoice (shorter delays for last minute registrations).
Our experience shows that your chance of being accepted when placed on the waiting list is decreasing over time as the Summer School approaches. During a first period (the first five to ten weeks after registration opens, i.e. February to April/May, later for less popular workshops) become free: Some participants cancel for personal reasons (planning) or they were unable to get a grant for this year and/or they did not pay the fee within five weeks despite an e-mail reminder.
As the Summer School approaches, cancellations are usually linked to much rarer, unforseen events (personal life). (Late cancellation means that the workshop fees are not reimbursed (fully or partially). [Cancellation rules on this page]