Disclosed are various embodiments for tracking, in at least one computing device, a presentation of a plurality of items for a user, each item being associated with at least one respective category of a plurality of categories, and determining a user histogram that is customized for a user. A scanning unit which moves relative to the scale in a measuring direction so that a first position dependent scanning signal and a second position dependent scanning signal are generated by scanning the first track and the second track by the scanning unit. The thrust is opposite to the thrust applied to a same part by the other of the internal and external working head. The data items appears one-at-a-time in only one cell structure of the spreadsheet at each time in the time sequence. This eliminates the release oil from building up on the fuser roller and transferring to subsequent larger receiver sheets, and reduces non-uniform wear on rollers at the fuser, thus reducing likelihood of image artifacts and machine contamination. The memory devices include semiconductor pillars penetrating interlayer insulating layers and conductive layers alternately stacked on a substrate and electrically connected to the substrate and floating gates selectively interposed between the semiconductor pillars and the conductive layers.