This clinical trial is planned as two parts:In the first part,consecutive patients would be recruited and randomized to have colonoscopy examination performed by inexperienced colonoscopists (having performed less than 200 procedures) either with or without the assistance of magnetic scope navigation system. The second part would be conducted as a pilot study,we would first construct a simple colon force measurement device;then colonoscopist would be randomized to perform colonoscopy on a colon model either with or without an MEI view while the force measurement device being used to collect data of colon force variation meanwhile. All force variation data would be recorded by a image storage device.No patients would be involved in the second part.In this pilot study, we plan to perform 40 colonoscopy procedures with the assistance of MEI and 40 colonoscopy procedures without the assistance of MEI.