The key idea of measurement at level 2 is that units can be used to measure objects.
- non-standard
- standard (simple)
Non-standard units are objects which are used because they are known to students and are readily available, for example, paces for length, books for area, and cups for volume. At level 2, students should be provided with many opportunities to measure using these kinds of non-standard units. Non-standard units introduce the students to the idea that units are repeated and counted in order to provide a measure of an attribute of an object. For example, the width of the desk is 4 handspans.
Non-standard units introduce most of the principles associated with measurement:
- Measures are expressed by counting the total number of units used.
- During a measurement activity, the unit must not change.
- Units are repeated across the attribute being measured with no gaps or overlapping of the units.
- Sometimes fractions of a unit (such as half and quarter) need to be used in order to get a more accurate measure.
- Units of measure are not absolute but are chosen for appropriateness. For example, the length of the room could be measured by handspans but a pace is more appropriate.
Prior to introducing standard units, students need to realise that non-standard units tend to be personal and are not the most suitable for communication. For example, my hands are smaller than yours, so telling me to measure a piece of cloth three hands wide may not be useful.
As students are introduced to standard units it is important that they develop personal benchmarks for frequently used units of measure, for example the width of a finger is about a centimetre, and there are about 4 cups in a litre. Estimation can help increase students familiarity with units and prevent errors in measurement.
This key idea develops from the key idea of measurement of level 1, focussed on direct and indirect comparison of objects.
This key idea is extended to the key idea of measurement of level 3 where the students use a standard scale to measure the attributes of objects.