Hung reviews the various models of PBL, Barrow's original concept or "pure PBL", the Hybrid PBL and lecture based learning with problem solving activities.[3] In general these models form a continuum where the level of instruction and lecture are inversely proportional to the amount of self-directed learning. The individual Problem design, or "trigger" must ultimately guide students to obtain the learning objectives. Azer reviews the detailed objectives for constructing "the problem" for PBL.[37] Facilitator selection, training and development is very important to PBL. Students respond better to motivated and enthusiastic facilitators.[38] It is the facilitator's role to direct students during the tutorials. Guiding students learning entails much more time then simply giving students the answers.[3] Kol et al.(2008) reported PBL facilitator-student contact time was 3-4 times greater than instructors in traditional methods.