Units of Measurement and
Accuracy vs. Precision
Vocabulary
• qualitative
• quantitative
• standard
• accuracy
• precision
• Imperial system
• metric system
• SI units
“Target with Four Faces” – 1955
Jasper Johns
Measurement
A lot of science is qualitative (observations that do not
involve measurement.)
Examples:
Classifying
organisms
Interpreting rock
strata
Understanding the
Skeletal System
Measurement
However, a very important aspect of science is that much
of it is quantitative (observations that involve
measurements)
Examples:
Determining the
amount of CO2 in
the atmosphere
Measuring the age
of the earth
Calculating the
amount of energy
produced by uranium
Standardization
In order for scientists to communicate quantitative
measurements, there must be standardized units (units that
are agreed upon between scientists.)
Imperial (or English or Standard)
System
Imperial units are the units we use most of the time here in
the US.
What are some examples?
Length: ft, mi, yd, in, fathoms, furlongs, rods, etc.
Mass: lb, oz, ton, dram, pennyweight, grain, etc.
Volume: gal, pt, qt, cp, fl.oz., tsp, tbls, gill, peck, bushel, etc.
Imperial (or English or Standard)
System
To convert between these units require conversion factors.
For instance:
1 foot = _____
12 inches
4 quarts
1 gallon = _____
60
1 min = _____sec
1 mile = 5280
_____ feet
Imperial (or English or Standard)
System
More conversion factors (and this is just for volume!)
Imperial (or English or Standard)
System
Even though the units are standardized, the complicated and
inconsistent nature of the Imperial System is why only 3 countries out
of 192 in the world use it.
Countries that use the
Imperial system:
•The United States
•Liberia
•Myanmar (Burma)
SI (International System)
All 189 other countries use the more consistent and logical
SI units. They were adopted in 1960 by the General
Conference of Weights and Measures and are founded on
some base units:
length: meter (m)
mass: kilogram (kg)
time: seconds (s)
and some others we won’t cover right now
SI (International System)
What are some other SI units?
length: m, km, cm, mm, etc.
mass: kg, g, cg, mg, etc.
time: s, ms, ks, cs, etc.
volume: L, mL, kL, cL, etc.
SI (International System)
What are some other SI units?
length: m, km, cm, mm, etc.
mass: kg, g, cg, mg, etc.
time: s, ms, ks, cs, etc.
volume: L, mL, kL, cL, etc.
Notice that each type of measurement can
have the same prefixes: kilo, centi, milli, etc.
SI (International System)
That’s what makes the SI units so appealing.
1 meter = 1000 millimeters
1 gram = 1000 milligrams
1 liter = 1000 milliliters
1 second = 1000 milliseconds
The prefixes are consistent no matter what measurement you
are making and all conversions are based on factors of 10.
SI
Prefixes
SI Prefixes
However these are the only ones you
will need to know.
When making measurements
Scientists don’t just need to know about
units. They need to use them correctly and
make accurate and precise measurements.
Imagine a Dart Board
If you were to throw a
dart at the dart board,
what you consider an
accurate throw?
Imagine a Dart Board
2
1
3
Which of these
darts would you
consider
accurate?
Imagine a Dart Board
1
#1 is accurate.
Accuracy: the
closeness of a
measured value
to an accepted
value.
Imagine a Dart Board
1
Because the
center of the
board is the
accepted value
for success, dart
#1 is accurate.
Imagine some boiling water
If you read the
thermometer and
recorded a value of
102o, is your reading
accurate? Why or
why not?
It would not be
accurate because the
boiling point of
water is 100oC at
sea level and lower
at higher elevations.
Now let’s talk about precision
Precision has to do
with how closely
multiple measured
values agree with
each other.
Imagine a dart board
Would you say that
the three darts on the
dart board are
precise?
Imagine a dart board
No, they are not
precise because they
are not close to one
another. They are
inconsistent.
Comment on the accuracy and
precision of the following darts
They are not
accurate or
precise.
Comment on the accuracy and
precision of the following darts
They are both
accurate and
precise.
Comment on the accuracy and
precision of the following darts
They are precise
but not accurate.
Comment on the accuracy and
precision of the following darts
The average is
accurate (perhaps)
but they are not
precise.
Back to the water
Three students each perform
three trials to find the boiling
point of water at sea level.
Their data is shown below.
Trial 1
Trial 2
Trial 3
Student 1
Student 2
99o
94o
100o
94o
100o
94o
Student 3
93o
98o
95o
Back to the water
Which student was both
accurate and precise?
Student #1
Trial 1
Trial 2
Trial 3
Student 1
Student 2
99o
94o
100o
94o
100o
94o
Student 3
93o
98o
95o
Back to the water
Which student performed a
good experiment but probably
had a faulty thermometer?
Student #2
Trial 1
Trial 2
Trial 3
Student 1
Student 2
99o
94o
100o
94o
100o
94o
Student 3
93o
98o
95o
Back to the water
Which student probably had
multiple variables in his lab
between trials?
Student #3
Trial 1
Trial 2
Trial 3
Student 1
Student 2
99o
94o
100o
94o
100o
94o
Student 3
93o
98o
95o