SF later expanded his strategy by incorporating ages and years, but his chunks were always familiar, which allowed him to recall them more easily. It is important to note that a person who does not have knowledge in the expert domain (e.g. being familiar with mile/marathon times) would have difficulty chunking with race times and ultimately be unable to memorize as many numbers using this method. Chunking in motor learning Chunking is a method of learning that can be applied in a number of contexts and is not limited to learning verbal material. Karl Lashley, in his classic paper on serial order, argued that the sequential responses that appear to be organized in a linear and flat fashion concealed an underlying hierarchical structure. This was then demonstrated in motor control by Rosenbaum et al. in 1983. Thus sequences can consist of sub-sequences and these can, in turn, consist of sub-sub-sequences. Hierarchical representations of sequences have an advantage over linear representations: They combine efficient local action at low hierarchical levels while maintaining the guidance of an overall structure. While the representation of a linear sequence is simple from a storage point of view, there can be potential problems during retrieval. For instance, if there is a break in the sequence chain, subsequent elements will become inaccessible. On the other hand, a hierarchical representation would have multiple levels of representation. A break in the link between lower-level nodes does not render any part of the sequence inaccessible, since the control nodes (chunk nodes) at the higher level would still be able to facilitate access to the lower-level nodes. Chunks in motor learning are identified by pauses between successive actions in Terrace (2001). It is also suggested that during the sequence performance stage (after learning), participants download list items as chunks during pauses. He also argued for an operational definition of chunks suggesting a distinction between the notions of input and output chunks from the ideas of short-term and long-term memory. Input chunks reflect the limitation of working memory during the encoding of new information (how new information is stored in long-term memory), and how it is retrieved during subsequent recall. Output chunks reflect the organization of over-learned motor programs that are generated on-line in working memory. Sakai et al. (2003) showed that participants spontaneously organize a sequence into a number of chunks across a few sets and that these chunks were distinct