“…Unfortunately, current instructional methods have not been particularly effective at supporting estimation knowledge. It is well documented that a large majority of students have difficulty estimating the answers to problems in their heads (Case & Sowder, 1990;Hope & Sherrill, 1987;Reys, Bestgen, Rybolt, & Wyatt, 1980;Sowder, 1992).Given the challenges of mentally computing estimates, it is especially important to have a broad repertoire of estimation strategies and to select the most appropriate (often, computationally easiest) strategy for a given problem and goal (Dowker, Flood, Griffiths, Harriss, & Hook, 1996;LeFevre, Greenham, & Waheed, 1993;Lemaire, Lecacheur, & Farioli, 2000). Thus, students who lack strategic flexibility with computational estimation may experience difficulties in this domain.…”