90% of the people who sign up for online classes don't complete them! It seems that this happens because these are courses where people can start and finish the course whenever they want. But what happens? Life gets in the middle, and then people forget about the course.

That's why now you see more and more online courses return to a system with a start and end date. By having an end date when you have to finish the course, it gets people motivated to finish instead of thinking: "Oh, I'll do that later. This other thing is more urgent".

It's interesting to me to see that this isn't only something done for online courses but also for community groups. A church in the Swiss German part of Switzerland has a system of small groups of interest. The idea is that meeting for the Sunday service is a good start but that in order to grow spiritually, you want to have conversations with a smaller group of people with whom you share common interests. What's special here is that each of these groups has a kind of "expiration date". They all start on the same date and finish one year later. People can then decide to leave a group or continue to be a part of it. 

This is a first draft of a principle that might end up in a book of the "Service Design Principles" series.