The difference between assessment and recommendation is that in the case of assessment the whole knowledge structure is searched and the information about user current knowledge is extracted while in the case of recommendation each problem that user hasn't answered correctly is selected. The probabilistic function to balance this two is used:

$p = \frac {Prob} {M}$ p is the probability that the assessment will be chosen and 1-p is that recommendation will be used. Prob stands for the amount of problems that user visited totally, and M is the parameter controlling transition between recommendation-favoring and assessment-favoring.