I II III Accuracy, Precision, Percent Error 101 MEASUREMENT.

I

II

III

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

I

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III

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