A good guess: Estimation and individual differences
The ability to estimate an approximate answer to an arithmetic problem and the ability to evaluate the reasonableness of an arithmetical estimate are important aspects of arithmetical understanding. Estimation experience is likely to play a role in developing awareness of, and resourcefulness with, number relationships (Sauble, 1955). Estimation is also of considerable practical importance (Orton & Frobisher, 1996) and would seem essential to the appropriate use of calculators. Its educational importance has been increasingly emphasized in recent years, both in the United Kingdom (Cockcroft, 1982; DfEE, 1999) and in the United States (Kilpatrick, Swafford, & Findell, 2002; Sowder 1992).