Worksheet No.2/July- 2011
Part A: ACCURACY and PRECISION
Least Count:
The least count of an instrument is the smallest amount it is capable of measuring. For example, the least
count of a metre scale is 1 mm.
We have 10 divisions between 0 to 1 so we can say
that the least count is 1/ 10 or 0.1cm or 1mm.
Accuracy:
Accuracy refers to the closeness of the measured value to the true value. For example, if the true value of the
length of a block is 20.6 cm and the value measured with a metre scale ‘A’ is 20.4 cm whereas the value
measured by metre scale ‘B’ is 20.5 cm, then we say that the accuracy of the reading made with metre scale B
is more than that made with metre scale A.
True value = 3.459 cm
Is least count the same as accuracy? Discuss
…………………………………………………………………
…………………………………………………………………
…………………………………………………………………
…………………………………………………………………
Precision of measurements and of the instrument:
Let us consider two vernier calipers A and B, which are used to
measure the length of a block. Let the true length of the block
be 3.459 cm. The calipers are used to take five readings of the
length of this block and the results are as
follows:
Which calipers result is more precise (consistent)
set of readings? ................
Now compare the readings taken with a meter
scale (LC = 0.1 cm), vernier calipers (LC = 0.01
cm) and a screw gauge (LC = 0.001 cm)
Fill in the blanks:
The readings with each instrument are quite
consistent (………………..).
In addition, the screw gauge has the smallest
…………………and hence the
best…………………...
[screw gauge; precise; least count; precision; accuracy;
vernier calipers;
meter scale;
size]
Precision thus refers to two things:
1. The closeness of measured values to each other (consistency of measured values).
2. The least count of the instrument. Smaller the least count, the more precise is the instrument.
Accuracy and precision understood usinq the dart-board illustration:
(a)
(b)
(c)
(d)
The clustering of darts together represent 'precision' while the tendency of the darts to fall near or around the
'bull's eye' represents 'accuracy'.
Write the correct diagram alphabet next to each case shown:
Most accurate and least precise
………
Most accurate and precise
………
Least accurate but quite precise
………
Neither accurate nor precise
………
Table of comparison of accuracy and precision:
ACCURACY
PRECISION
•
•
•
Accuracy expresses the closeness of the
measurement to the true value.
Accuracy can be improved by recalibration of
the instrument or by reducing human errors.
•
•
Precision of a set of measurements expresses
the 'closeness' of the measurements to each
other.
Precision of an instrument indicates the least
count of the instrument.
Precision of an instrument cannot be
improved as it is inherent in the instrument.