Chapter 10 - Precision and Accuracy of Chemical Analyses

Question 1

Measurements always contain ____ and ______

Answer 1

• errors
• uncertainties

Question 2

Measurement data can only give us an _______ of the “true” value

Answer 2

estimate

Question 3

Why is estimating the reliability of experimental data is extremely important whenever we collect laboratory results?

Answer 3

because data of unknown quality are worthless

Question 4

Results that might not seem especially accurate may be of considerable value…

Answer 4

if the limits of uncertainty are known

Question 5

It is the best estimate of the true value

Answer 5

central value for the set

Question 6

It is done in order to improve the reliability and to obtain information about the variability of results

Answer 6

Replicates

Question 7

The most widely used measure of central value

Answer 7

mean

Question 8

mean is also called as

Answer 8

arithmetic mean or average

Question 9

Mean is obtained by

Answer 9

by dividing the sum of replicate measurements by the number of measurements in the set

Question 10

∑xi means

Answer 10

to add all of the values xi for the replicates

Question 11

Formula of Mean

Answer 11

x̄ = add all values for replicates / number of replicates

Question 12

It is the middle result when replicate data are arranged in increasing or decreasing order

Answer 12

Median

Question 13

How can you determine the median for an odd number of results?

Answer 13

by arranging the results in order and locating the middle result

Question 14

How can you determine the median for an even number of results?

Answer 14

the average value of the middle pair is used

Question 15

It indicates the closeness of the measurement to the true or accepted value

Answer 15

Accuracy

Question 16

Accuracy is expressed by the ____

Answer 16

error

Question 17

It is a measure of how close a series of measurements are to one another

Answer 17

Precision

Question 18

Precision describes the ________ of measurements - in other words, …

Answer 18

• reproducibility
• the closeness of resuts that have been obtained in exactly the same way

Question 19

Three terms used to describe the precision of a set of replicate data:

Answer 19

1. Standard deviation
2. Variance
3. Coeffcient of variation

Question 20

These three are functions of
how much an individual result xi differs from the mean, called the ________

Answer 20

deviation from the mean di

Question 21

Deviation from the mean di formula

Answer 21

di = |xi - x̄|

Question 22

What must be comapred to evaluate the accuracy of a measurement?

Answer 22

The measured value must be compared to the correct value

Question 23

What must be comapred to evaluate the precision of a measurement?

Answer 23

You must compare the values of two or more repeated measurements

Question 24

The difference between the experimental value and the accepted value

Answer 24

Error

Question 25

Formula of Error

Answer 25

Error = experimental value - accepted value

Question 26

It is the correct value based on reliable references

Answer 26

Accepted value

Question 27

It is the value measured in the lab

Answer 27

Experimental value

Question 28

How can we determine precision?

Answer 28

just by measuring replicate samples

Question 29

Why is accuracy often more difficult to determine?

Answer 29

because the true value is usually unknown

Question 30

What must be used instead to determone accuracy?

Answer 30

Accepted value

Question 31

Accuracy is expressen in term of, what?

Answer 31

either absolute or relatice error

Question 32

Formula for Absolute Error

Answer 32

E = xi - xt

Question 33

xi in Absolute error is...

Answer 33

a measurement of the quantity

Question 34

xt in Absolute error is...

Answer 34

the true or accepted value of the quantity

Question 35

often a more useful quantity than the absolute error

Answer 35

Relative Error

Question 36

Formula of Relative Error

Answer 36

Er = xi - xt / xt x 100%

Question 37

Relative error is also expressed in

Answer 37

parts per thousands (ppt)

Question 38

Results can be _____ without being _____ and ____ without being _____

Answer 38

- precise, accurate - accurate, precise

Question 39

Types of Errors in Experimental Data

Answer 39

1. Random (or indeterminate) 2. Systematic (or determinate) 3. Gross Error

Question 40

It causes data to be scattered more or less symmetrically around a mean value; usually small in values and not avoidable

Answer 40

Random (or indeterminate) error

Question 41

Random error

Answer 41

- observational error - environmental error

Question 42

It causes the mean of a data set to differ from the accepted value

Answer 42

Systematic (or determinate) error

Question 43

In general, a systematic error in a series of replicate measurements...

Answer 43

causes all the results to be too high or too low

Question 44

Example of Systematic error

Answer 44

The loss of a volatile analyte while heating a sample

Question 45

Systematic error

Answer 45

- Instrumental error - Environmental error - Observational error

Question 46

Systematic error other examples

Answer 46

- Miscalibration of pH meter - Miscalibration of analytical balance - Evaporation of liquids - Spattering chemical

Question 47

- third tye of error - usually occur only occasionally, are often large, and may cause a result to be either high or low

Answer 47

Gross error

Question 48

Gross error is often a product of ____ ___

Answer 48

Human error

Question 49

Gross errors lead to ____

Answer 49

outliers

Question 50

Systematic errors have a ____ ___

Answer 50

definite value

Question 51

Systematic errors have an ____ ___

Answer 51

assignable cause

Question 52

Systematic errors are of the same magnitude for replicate measurements made in the same way, leading to _____ in measurement results

Answer 52

bias

Question 53

Bias in measurement results affects, what?

Answer 53

- all the data in a set in the same way - it carries a sign

Question 54

Systematic errors may be either ________ or ________

Answer 54

- constant - proportional

Question 55

Magnitude of a _____ ___ stays essentially the same as the size of the quantity measured is varied

Answer 55

Constant error

Question 56

With constant errors, the ____ ____ is _____ with sample size, but the ____ ____ ____ when the sample size is changed

Answer 56

- absolute error is constant - relative error varies

Question 57

It increase or decrease according to the size of the sample taken for analysis

Answer 57

Proportional errors

Question 57

With proportional errors, the ______ ____ ____ with sample size, but the _____ ____ stays _____ when the sample size is changed

Answer 57

- absolute error varies - relative error stays constant

Question 57

It is always desirable because the response of most instruments changes with time as a result of component aging, corrosion, or mistreatment

Answer 57

Periodic calibration of equipment

Question 58

Most personal errors can be minimized by

Answer 58

- careful - disciplined laboratory work

Question 59

materials that contain one or more analytes at known concentration levels

Answer 59

Analysis of Standard Samples (or Standard Reference Materials, SRMs)

Question 60

SRMs can be prepared by

Answer 60

- synthesis - purchased from a number of governmental and industrial sources

Question 61

It contains the reagents and solvents used in a determination but no analyte

Answer 61

Blank

Question 62

Many of the sample constituents are added to simulate the analyte environment, which is called the _____ ____

Answer 62

Sample matrix

Question 63

As the size of a measurement ______, the effect of a constant error ______

Answer 63

- increases - decreases