This is a subtype of interval variables where ratio of scales is
used for measurement.
For Example, Water representation in chemistry is H2O which
represents two molecules of hydrogen and one molecule of
oxygen. Thus, the ratio of elements is 2: 1.
The discrepancy between the measured value and actual value
in terms of number is called measurement error.
For Example While buying fruits from a vendor in kilograms, if
we wanted 1 kilogram of fruits and the vendor’s weighing
machine showed 1 kilogram when we brought the same. After
checking the same in another machine, if the measured value
shows 0.1 kilogram less than expected then this difference is
what we call as measurement error.
Reliability implies whether the instrument gives consistent
result across different conditions.
For example, if we test the same value twice on the same entity
then the results from the instrument should remain same if it has
to be reliable. Such tests are known as test-retest.