No. It is more difficult. However, it would not be impossible to add a module to an existing survey so that we can ask relevant questions on the subject. I am thinking, for example, of the General Social Survey, which represents a sample of 25,000 people. We could put questions to people who have taken an immersion program and ask them whether they have maintained their skills. The option is there.
On May 23rd, 2013. See this statement in context.