This knowledge base is represented by some formalism (rules, frames, Bayesian networks,etc.)
and is built by the knowledge engineer from elicited expert knowledge and, later, validated by the expert.
Evidently, the system is subject to and limited by the amount of knowledge entered, that is, represented in its knowledge base.
And, precisely, the bottleneck in expert system construction is knowledge elicitation, a phase conditioned by countless constraints ranging from the number of available experts, or how much expertise the experts have, to the complexity of the actual knowledge elicitation process.
Recently, automatic knowledge acquisition techniques have attracted a lot of interest as they are potentially a big help for remedying this bottleneck.
The knowledge discovery in databases (KDD) process, especially data mining techniques, is used to automatically discover knowledge from data.