7. Experts approach tasks flexibly, so they recognize when more knowledge is needed and take steps to acquire it while monitoring progress (Bilalić, McLeod, and Gobet, 2008; Metcalfe and Kornell, 2005; Spiro et al., 1991).

8. Within certain physical limits of speed and endurance associated with aging and health status, experts retain domain-related skills through adulthood as long as they are practiced (Krampe and Charness, 2006).

Expertise is usually difficult to achieve—and for a complex skill such as literacy requires many hours of practice over many years—experts tend to have 1,000-10,000 hours of experience in their field of expertise (Chi, Glaser, and Farr, 1988). With respect to literacy expertise taught in schools, an hour per day from kindergarten through twelfth grade amounts to about 2,000 hours in total, after taking out the inevitable days when no real instruction occurs, which is at the low end of the range needed to gain expertise. Adult literacy learners can be assumed to have missed out on many of these hours or to need substantially more practice. Adults bring varied goals to adult literacy education, but it is clear that given the hours of practice needed to develop literacy skills for functioning well in the realms of work, family, education, civic engagement, and so on, instruction needs to be designed to ensure that learning proceeds as efficiently as possible. Efficiency is especially important considering that adolescents and adults live in complex worlds with many competing demands (Riediger, Li, and Lindenberger, 2006).

Learning involves being proficient with the tools needed to complete the tasks to be mastered and so requires practice with using tools. Tools can be anything from a physical tool (pen, computer, textbook, or graphic organizer) to more abstract tools—such as the appropriate lexicon of a particular domain or knowledge of how people in a domain construct written arguments or literature. Tools can contribute to the development of deeper understandings of a concept or idea by presenting learners with varied ways of representing the idea (Eisner, 1994; Paivio, 1986; Siegel, 1995).

The learning principles described in this chapter vary in their attention to explicit and implicit teaching and learning. Both explicit and implicit learning contribute to the development of expertise in complex skills, such as reading and writing, as illustrated in previous chapters. The principles also vary in their emphasis on promoting initial acquisition of knowledge and skills over transfer and generalization of acquired knowledge and skills to new situations. Initial acquisition involves attention to and encoding of relevant material, so that it can be retrieved from memory or applied to problems within short retention intervals. Transfer and generalization are

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