On an interval
scale, measurements are not only classified and ordered therefore
having the properties of the two previous scales, but the
distances between each interval on the scale are equal right
along the scale from the low end to the high end.
Two points next to each other on the scale, no matter whether
they are high or low, are separated by the same distance,
so when you measure temperature in centigrade the distance
between 0 and 10°C is the same as between 90 and 100.
What you must remember though is that for interval
scales, a measurement of 100°C does not
mean that the temperature is 10 times hotter than something
measuring 10°C even though the value given on the scale
is 10 times as large.
That's because there is no absolute zero, the zero is arbitrary.
On the centigrade temperature scale, the zero value is taken
as the point at which water freezes and the 100°C value
when water begins to boil and between these extreme values
the scale is divided into a 100 equal divisions. Temperatures
below 0°C are designated negative numbers. So the arbitrary
0°C does not mean 'no temperature'. But when expressed
on the kelvin scale, a ratio
scale, a measure of 0K equivalent to -273°C does indeed
mean no temperature!
Calendar years are another example of an interval
measurement. An arbitrary 0 (or 1 depending on your viewpoint)
was assigned when Christ was born, and time before this is
given the prefix BC. Other than these examples interval
measurements are rare.
