He did this three to five times a week for 20 months until he could repeat as many as 79 digits. While his capacity on this trained task had improved, his working memory: the ability to store information, as described above had not. This was most clearly demonstrated when, asked to repeat letters instead of numbers, this same student with over 320 hrs of practice at recalling digits could recall only six letters at a time: a normal to below average performance. The effect of the training was not to improve the working memory system but to change the information being stored: the student had learned multiple methods of grouping numbers and relating them to similar figures already in his long-term memory. In reality, his working memory capacity had not increased. This study and others like it contributed to the prevailing assumption in the scientific community that working memory is a set characteristic that cannot be improved.