What is Measurement? Exact Approximate

What is Measurement?
Measurement is the process of attaching numbers to certain qualities of objects and events.
Counting involves discrete objects
(How many?)
Measuring involves continuous properties
(How much?)
What is Measurable?
A measurable attribute of an object or event is a characteristic that can be quantified by
comparing it to a unit.
Temperature Time
Acceleration Pressure
What is the Process of Measurement?
Choose an appropriate unit for the attribute being measured;
Compare the attribute of the object or event to the unit;
Obtain a measurement — a number AND a unit.
What is a common instructional sequence?
Develop the meaning of the attribute being measured through activities involving
perception and direct comparison. (Which is taller?)
Children begin to measure using arbitrary or nonstandard units. (How tall?)
Children measure and estimate using standard units. (How tall in Feet and Inches.)
Related experiences: Learning to use measuring instruments, read scales, develop formulas, use
proportional reasoning to solve problems.
Accuracy of Measurement
“To apply care to”
When a person puts a lot of care into a measurement, the result is likely to be accurate.
Precision of Measurement
“to cut off in front”
The smallest unit of measure used to express an approximate value (the exact value is “cut off”
Precision in Measurement
An aspect of defining an “appropriate unit” in measurement.
Introduction Recall that the process of direct measurement of a characteristic involves
matching that characteristic of the object by repeating a unit, which is another object that has
that same characteristic. So, when determining an appropriate unit for a measurement, one first
has to find a good match between the characteristic of the object being measured and the unit
used to do the measuring (It is hard to measure area with a measuring cup). Once the match is
made, the next most important consideration may have to do with precision.
Discussion question Suppose a length is reported as 2 ¾ inches, to the nearest ¼ inch. How
long and how short could it actually be?
Commentary on Precision, from Reconceptualizing Mathematics, Center for Research in
Mathematics and Science Education, San Diego State University.
Unfortunately, much school work with measurement treats the values as exact, so
one can easily form the erroneous impression that the values are exact. Like the
ideal constructs of “line segment” and “angle,” theoretically perfect
measurements can be discussed, of course. We can imagine a square with sides
exactly 2 centimeters long, for example, even though drawing one with even a
finely sharpened pencil is not possible. We can imagine a line segment with
length exactly 2 cm although we cannot literally produce one.
Which of these two ways would give a better measurement for the length of a
board: measuring to the nearest inch, or measuring to the nearest foot? In a direct
measurement, choosing the smaller unit naturally allows the matching to be done
more closely. So usually a measurement with a smaller unit narrows the range of
possible values for the measurement and lessens the error.
The numerical part of the value, however, can also imply something about the
accuracy of the measurement. Suppose you have these measurements, all reported
for the length of the same object: 2.75 feet; 2 feet 9 inches; and 33.0 inches.
Which one is most accurate, or are they the same? A measurement given as 2.75
feet, say, is implying that the measurement was carried out to the nearest onehundredth of a foot, so it would be more accurate than a measurement reported as
2 feet, 9 inches, which implies that the measurement was carried out only to the
nearest inch, or one-twelfth of a foot. On the other hand, 33.0 inches would imply
a measurement to the nearest one-tenth of an inch, or 1/120 of a foot, so 33.0
inches would be the best approximation of the three given here—2.75 feet, 2 feet
9 inches, and 33.0 inches.
1. Different units produce different numerical measures of the same attribute of an object.
2. Smaller units of measure provide a more precise measure.
Related CA Standards
1.1 Compare the length, weight and capacity of objects by making direct comparisons or using
reference objects (e.g., shorter/longer/taller, lighter/heavier, which holds more?)
1st Grade
1.1 compare the length, weight and volume of two or more objects using direct comparison or
a non-standard unit
2 Grade
1.1 measure the length of objects by iterating (repeating) a non-standard or standard unit
1.2 use different units to measure the same object and predict whether the measure will be
greater or smaller when a different unit is used
1.3 measure the length of an object to the nearest inch and/or centimeter
3rd Grade
1.1 choose appropriate units (metric and U.S. customary) and tools, and estimate and measure
length, liquid volume and weight/mass
4th Grade
1.1 measure the area of rectangular shapes, using appropriate units
5th Grade
1.4 differentiate between and use appropriate units of measures for, two- and three-dimensional
objects (perimeter, area and volume)
Area by Comparison
Related CA Standards
Note that the CA Standards contain very specific language about measuring area with respect to
squares and rectangles. Why is this so?
3rd Grade
1.2 estimate or determine the area and volume of solid figures by covering them with squares
or by counting the number of cubes that would fill them
4th Grade
1.1 measure the area of rectangular shapes, using appropriate units
5th Grade
1.1 derive and use the formula for the area of right triangles and of parallelograms by
comparing with the area of rectangles (i.e., two of the same triangles make a rectangle with
twice the area; a parallelogram is compared to a rectangle with the same area found by
cutting and pasting a right triangle
Historical Note: Edward G. Begle1
During the late 1950s, in the midst of the Cold War, there was a perception that the
United States had fallen behind the Soviet Union in basic research and education in
mathematics and the sciences. In 1958 the National Science Foundation (NSF) provided
$100,000 to initiate a revision of the secondary mathematics curriculum. The project was
called the School Mathematics Study Group, or SMSG, and Edward G. Begle of Yale
University was given the responsibility for directing its efforts.
SMSG developed sample textbooks for grades K-12 that were field tested during the
early 1960s across the nation. While many of the revisions that were suggested became
the target of criticism (see, for example, Morris Kline, Why Johnny Can’t Add, New
York: Vintage Books, 1973), the SMSG postulate set for geometry received positive
reviews and became, in various forms, widely used throughout the United States and
The SMSG postulates include a few that directly address the concept of Area:
Postulate 17. To every polygonal region there corresponds a unique positive number (called the
area of the polygonal region).
Postulate 18. If two triangles are congruent, then the triangular regions have the same area.
Postulate 19. Suppose that the region R is the union of two regions R1 and R2. Suppose also that
R1 and R2 intersect at most in a finite number of segments and points. Then the area of R is the
sum of the areas R1 and R2.
Postulate 20. The area of a rectangle is equal to the product of the length of its base and the
length of its altitude.
Wallace, E. C., S. F. West. Roads to Geometry, 2nd ed. Upper Saddle River, NJ: Prentice-Hall, 1998: 58.
The postulates on the previous page essentially indicate that all area formulas for shapes in the
plane can be derived from the definition of the area of a rectangle as given in postulate 20 (in
mathematics we start with a set of ideas and build from there). IN FACT, in Calculus we even
calculate areas of regions in the plane with wildly curved boundaries using by adding up the
areas of lots of little rectangles that approximately cover the region.
The activity below illustrates how one uses the rather simple idea of comparison to make
connections between rectangular areas and areas of other polygons.
Activity 1: Rectangles To Circles
Use paper folding and transformations to derive the areas of basic shapes by relating them to the
area of a rectangle.
Mark a point on the top of the rectangle and connect the two base vertices to the point. This gives
you a triangle. Cut off the excess area outside the triangle.
What is the base and height of the triangle in relation to the original rectangle?
Fit the excess area that was cut off onto the triangle.
What does this tell you about the area of the triangle?
Write the formula for the area of a triangle.
Mark a point on the top of the rectangle and connect one of the base vertices to the point. Cut
along this line and translate the cut triangle to the opposite side of the rectangle, and tape to form
a parallelogram.
What is the base and height of the parallelogram in relation to the original rectangle?
What does this tell you about the area of the parallelogram?
Write the formula for the area of a parallelogram.
Fold the rectangle in half lengthwise, crease and unfold. Mark two points anywhere on the top of
the rectangle and connect each one to the closest endpoint of the creased line. Cut off the two
small triangles formed in the top corners. Rotate them 180° about the endpoints and tape to get a
What dimensions of the trapezoid relate to the original rectangle?
What does this tell you about the area of the trapezoid?
Write the formula for the area of a trapezoid.
Cut out a large circle.
Fold it in half and crease.
Continue to fold and crease until you have 16 sectors.
Cut the circle into 16 sectors.
Rearrange the sectors, fitting the straight sides together, placing the curved sides alternating up
and down.
What shape does this approximate?
Cut the sectors into narrower sectors and arrange as before.
What shape will be approached?
What is the base and height of this shape in relation to the original circle?
What does this tell you about the area of the circle?
Write the formula for the area of a circle.