A shuffling machine is a machine for randomly shuffling packs of playing cards. Because standard shuffling techniques are seen as weak, and in order to avoid "inside jobs" where employees collaborate with gamblers by performing inadequate shuffles, many casinos employ automatic shuffling machines to shuffle the cards before dealing. These machines are also used to reduce repetitive motion stress injuries to a dealer. Shuffling machines have to be carefully designed, as they can generate biased shuffles otherwise: the most recent shuffling machines are computer-controlled. The randomness or otherwise of cards produced from automatic shuffling machines is the subject of considerable interest to both gamblers and casinos. Shuffling machines come in two main varieties: continuous shuffling machines (CSMs), which shuffle one or more packs continuously, and batch shufflers or automatic shuffling machines (ASMs), which shuffle an entire single pack in a single operation. Batch shufflers are more expensive, but can avoid the problems associated with some continuous shufflers, whereby the shuffling operation only slowly changes the state of the deck, and new cards may be taken before shuffling has sufficiently randomized the pack, allowing some players to "shuffle track" cards through the shuffling process. A widely reported, but unpublished, study by Persi Diaconis and Susan Holmes in 2000 resulted in the redesign of many shuffling machines. SIAM News later published a reasonably detailed discussion of its results. Early mechanical card shufflers Patents regarding card shuffling devices started to appear in the United States around the end of the 19th century. It is unclear whether these devices were converted to commercial products or were discarded. These machines were often complex with many mechanical parts to achieve card retrieval, shuffling and distribution with pseudo-randomness. In 1878, Henry Ash proposed an apparatus to shuffle cards. His device was a box with an open top where the operator would place the deck. The operator would then slightly shake the box to make the cards fall through a comb at the bottom of the box. About half of the cards would fall into the lower compartment while the rest were still in the upper compartment. The operator would take these upper cards, pack them together and do the same with the lower cards. The two packs would be placed upon each other to form a new deck and the operation could be repeated for better shuffling. In 1887, Silvanus Tingley and Charles Stetson patented their "card