Whole Number thinking

Caitlin is in Year 5 and has recently been introduced to decimal notation. She is treating the decimal part of the number as another (whole) number. For example, the number after 4.9 (4 wholes and 9 parts) is 4.10 (4 wholes and 10 parts). If the predominant discussion in the classroom is with decimals of equal length, her wrong belief is not challenged, and may continue to secondary school. Whole number thinking is one of the longer-is-larger misconceptions.

Look at Caitlin's answers to the Decimal Comparison Test. (You can double-click on the yellow notes for her reasons.)
Interviews with Caitlin
Talking About Place Value Video Images with text
Making the Biggest and Smallest Numbers Video Images with text
Number Between Video Images with text
Hidden Numbers Video Images with text
See how Caitlin would count from 1 to 2 with decimals and compare with other students.
Attempt a short decimal comparison test to check your understanding of Caitlin's thinking. This will help you learn to diagnose this error.
Lesson ideas appropriate for students like Caitlin.
Research on our Australian sample shows that the percentage of students diagnosed with whole number thinking decreases from about 35% in Years 4 to about 3% of Year 10 students. A full description of whole number thinking.