A measure of creativity from mind maps I have an experiment with 500 students who where asked to solve a general problem.

Each student had the assignment to find as many solutions as possible to the problem in 15 minutes. Solutions where submitted in the form of mind maps (where each node represents a possible solution and connections between nodes are also possible). 500 maps are available for evaluation. All the students were previously trained about the use of mind maps.

The objective would be to get a reliable proxy of each student creativity from their outputs. I'm not sure if this is possible or if the approach is just wrong and one should use a more accepted test to measure creativity. No other test scores are available for this sample. 

We are thinking about different approaches such as:


counting the number of generated ideas;
using experts to evaluate the characteristics of the creative outputs.


Is there any paper related to the issue? Or any accepted method that could be used to evaluate creativty of the students?
You are not far off from one of the classic tests of creativity!  The Torrance Test of Creative Thinking scores for fluency (number of responses) as well as originality (statistical rarity of responses) and elaboration (level of detail) on a variety of different tasks--some of which are similar to your mind-mapping task.  The validity of the TTCT has been examined with several long-term studies, so there is a fair amount of data on its measurements as well as the form of the test itself.

Torrance, E. P. (1981b). Empirical validation of criterion-referenced indicators of creative ability through a longitudinal study. Creative Child and Adult Quarterly, 6, 136-140.