Accuracy vs
Precision
Accuracy vs Precision
 ACCURACY:
a measure of how closely a result
approaches the true value.
(How close can you get to the actual value?)
 PRECISION:
is a measure of how closely the results
within a set of results agree with one another.
(How “repeatable” are your measurements?)
Accuracy vs Precision
1
Precise but
not
accurate
Not precise
and not
accurate
Accurate
but not
precise
Precise and
accurate
Accuracy vs Precision:
Measuring volume in the laboratory
Volume can be measured using:
 a beaker
 a graduated cylinder
 a buret
 Which
glassware would give you the most precise
volume measurement? (The most sig digs?)
 ** remember: the LAST digit is when taking a
measurement is always a guess!
Accuracy vs Precision:
Measuring volume in the laboratory
A
rule of thumb: Read the volume to 1/10 or 0.1 of
the smallest division.
(This rule applies to any measurement.)
This means that the error in reading is ±1/10 or
0.1 of the smallest division on the glassware.
Beaker
 The
smallest division is 10 mL,
so we can read the volume to
1/10 of 10 mL or 1 mL. The
volume we read from the
beaker has a reading error of
±1 mL.
 Volume = 47 ± 1 mL
You could estimate
48 mL OR 47 mL OR 46 mL
(any reading is
acceptable – but pick one!)
 2 sig digs
Graduated Cylinder
 The
smallest division of this
graduated cylinder is 1 mL.
Therefore, our reading error will be
± 0.1 mL or 1/10 of the smallest
division.
 Volume = 36.5 ± 0.1 mL
36.4 mL OR 36.5 mL
OR 36.6 mL (Pick one!)
** NOTE: the curved
surface of the water
This is called a MENISCUS  3
We always read the
BOTTOM of the meniscus
sig digs
Buret
 The
smallest division in this buret is
0.1 mL. Therefore, our reading error
is ± 0.01 mL.
 Volume = 20.38 mL ± 0.01 mL
 4 sig digs
** NOTE: the numbers go UP as you go down the buret.
This is because the liquid comes out the bottom.
NOTICE – The LAST digit of every reading was a GUESS
 So….Which
glassware would give you the most
precise volume measurement? (The most sig digs?)



Buret is the most precise
Graduated cylinder also has good precision
Beaker is poor – never use these to measure volume
precisely!!
 How
do you know the ACCURACY of your
glassware?

You don’t, unless you calibrate it (compare it against
a similar instrument you know is accurate)