Here is an example of a remarkably symmetric pattern of wear and use, but most assuredly not bell-shaped. The pattern on the top of this trash bin shows two prominent areas of wear at the left and right side of the opening. These two areas show greater wear than a large fairly uniform area of use in the center between the peaks. The two extreme areas of use tell us something about the modes of customers’ and restaurant workers’ actions.
Fast food is often delivered on plastic serving trays. As diners leave, they collect the assorted packaging and wrappings from their meals and deposit them in the trash bin near the exit. The diners then return their serving trays to the top of the trash bin. The plastic trays have small raised ridges on bottom of each corner. These small ridged “feet” act to provide a tiny gap between stacked trays to make them easier to separate.
When the top of the trash bin is empty, trays are returned by sliding them back along the front edge of the bin. The plastic feet on the bottom of the trays scrape along the top of the bin. This leaves prominent peaks in the wear pattern on the bin. As the trays are slid further the central portion of the trays sag and also scrape the bin to produce the pattern of use showing almost uniform wear between these two peaks.
Of course we would expect to produce this type of wear mainly when the top of the bin is empty, allowing the sliding tray to wear down the top. Later trays may not produce any wear along these edges if they are just placed on top of trays already in position. But here is where the restaurant’s workers contribute to the pattern.
After awhile, the trays stack up and must be returned for, what is hoped, a good washing. As they are retrieved, the pile of trays is slid forward to be picked up. This produces the uniform center wear and the peaks along the right-hand and left-hand edges as the trays and their feet again scrape the top of the bin. These actions produce the pattern of nearly equal left and right peaks of use with more uniform wear in between, resulting in a symmetric, but bi-modal frequency distribution.