By the end of the experiment his digit span had grown to 80 numbers. A later description of the research in The Brain-Targeted Teaching Model for 21st Century Schools (2012) states that SF later expanded his strategy by incorporating ages and years, but his chunks were always familiar, which allowed him to recall them more easily.[13] 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.