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Accuracy, Precision,
Percent Error
101
MEASUREMENT
Learning Objectives
The Learners Will (TLW) collect data and make measurements with accuracy and precision, and will be able to calculate percent error
TEKS 2F
Agenda
Part 1 – Units of Measurements A. Qualitative Measurements vs. Quantitative Measurements B. Number versus Quantity C. Experimental Value D. Review SI Units E. Derived Units
Part 2 – Using Measurement A. Accuracy vs. Precision B. Precision of Measurement C. Percent Error
Mini-Lab
I. Units of Measurement
Everyone makes and uses measurements Examples from the studio audience
Measurements are fundamental to experimental sciences as well
Important to be able to make measurements and to decide whether it is correct
A. Qualitative vs. Quantitative Measurement
Qualitative Measurement – a measurement that gives descriptive, nonnumeric results Examples –
Touching a person’s forehead to check for fever
Determining the winner of the Kentucky Derby by watching the horses cross finish line
Others from learners Can vary by experimenter
A. Qualitative vs. Quantitative Measurement
Quantitative Measurement – a measurement that gives definite, usually numeric results that also includes units Examples –
Using a thermometer to measure a person’s temperature at 99.2 °F (37.2 °C)
Using a photogate to capture time horses cross finish line at Kentucky Derby
Others from learners
B. Number vs. Quantity
Quantity = number + unit
UNITS MATTER!!
C. Experimental Value
A quantitative value measured during an experiment Examples –
Mass of rubber stopper Volume displaced when stopper was
dropped into graduated cylinder of water Temperature change of a reaction
D. SI Units
Quantity Base Unit Abbrev.
Length
Mass
Time
Temp
meterkilogramsecond
Celsius or Kelvin
mkgs
°C or °K
Amount mole mol
Symbol
l
m
tTn
D. SI Units
mega- M 106
deci- d 10-1
centi- c 10-2
milli- m 10-3
Prefix Symbol Factor
micro- 10-6
nano- n 10-9
pico- p 10-12
kilo- k 103
BASE UNIT --- 100
E. Derived Units
Combination of base units.
Volume (m3 or cm3) height width length
D = MV
1 cm3 = 1 mL1 dm3 = 1 L
Density (kg/m3 or g/cm3)mass per volume
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II. Using Measurements
Trasketball
Need 5 volunteers – 5 shots eact
A. Accuracy vs. Precision
Accuracy - how close a measurement is to the accepted value (published, target)
Precision - how close a series of measurements are to each other
ACCURATE = CORRECT
PRECISE = CONSISTENT
A. Accuracy vs. Precision
PRECISE – a golfer hits 20 balls from the same spot out of the sand trap onto the fringe of the green. Each shot is within 5 inches of one another. Wow – that’s CONSISTENT
ACCURATE – the golfer’s 20 shots aren’t very accurate, because they need to be much closer to the hole so she can score easily – that would be CORRECT
Audience Participation
Let’s Play
The Accuracy or Precision Game
Calculating Precision
Determine the average of all data values Determine the spread between highest
and lowest values (highest – lowest = spread)
Divide the spread by 2 This is the approximate range + from the
average Precision of measurement is shown as
average + range
Calculating Precision - Example
Distance 1 Distance 2 Distance 3 Distance 4 Distance 5 Distance 6
1.5 m 2.0 m 1.5 m 1.7 m 1.6 m 1.3 m
Average distance = 9.6 m total / 6 readings = 1.6 m
Highest value (2.0 m) – Lowest value (1.3 m) = 0.7 m 0.7 m is the spread
Divide by 2 = 0.35 m (range from average)
Precision of measurement is 1.6 m + 0.35 m
Calculating Precision - Practice
Distance 1 Distance 2 Distance 3 Distance 4 Distance 5 Distance 6
2.5 m 2.0 m 2.5 m 2.7 m 2.6 m 2.3 m
-- In groups of 2, calculate precision of the following-- Round to 2 decimal places-- Raise your hand when your team is done.
Precision of measurement is ____ m + _____ m
Temp 1 Temp 2 Temp 3 Temp 4 Temp 5 Temp 6
98.2 °C 98.0 °C 98.2 °C 98.1 °C 98.1 °C 98.3 °C
Precision of measurement is ____ °C + _____ °C
Which set of measurements is more precise? Distance or Temp?
B. Percent Error
Indicates accuracy of a measurement
100literature
literaturealexperimenterror %
your value
accepted value
B. Percent Error A student determines the density of a
substance to be 1.40 g/mL. Find the % error if the accepted value of the density is 1.36 g/mL.
100g/mL 1.36
g/mL 1.36g/mL 1.40error %
% error = 2.9 %
B. Percent Error
In groups of 2 calculate the percent error Round to 2 decimal places Raise your hand when your team is done
1. Experimental Value = 5.75 g
Actual Value = 6.00 g
2. Experimental Value = 107 ml
Actual Value = 105 ml
3. Experimental Value = 1.54 g/ml
Actual Value = 2.35 g/ml
Let’s Experiment…
1. Measure the wooden block with the formula chart ruler
2. Measure the level in the graduated cylinder
3. Measure the level in the beaker 4. Write your name on the chart at the
front of the room and record the above measurements in the columns indicated
Lab Results
Did we all come up with exactly the same numbers? Why or Why not?
Which are most precise measurements? Why?
Which are most accurate measurements? Why?
What’s the precision of the measurements? Perform the calculations
What is the percent error? Perform the calculations
Check for Understanding
Accuracy – Correctness of data
Precision – Consistency of results
Precision of Measurement – Determining the spread from average value (tolerance)
Percent Error – Comparison of experimental data to published data
Check for Understanding
How can you ensure accuracy and precision when performing a lab?
What is the percent error when lab data indicates the density of molasses is 1.45 g/ml and Perry’s Handbook for Chemical Engineering shows 1.47 g/ml?
Independent Practice
Accuracy and Precision Worksheet 1
Accuracy and Precision Pre-test
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