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CHM020 - ANALYTICAL METHODS FOR BIOLOGY > Chapter 10 - Precision and Accuracy of Chemical Analyses > Flashcards

Chapter 10 - Precision and Accuracy of Chemical Analyses Flashcards

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1
Q

Measurements always contain ____ and ______

A
  • errors
  • uncertainties
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2
Q

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

A

estimate

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3
Q

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

A

because data of unknown quality are worthless

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4
Q

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

A

if the limits of uncertainty are known

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5
Q

It is the best estimate of the true value

A

central value for the set

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6
Q

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

A

Replicates

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7
Q

The most widely used measure of central value

A

mean

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8
Q

mean is also called as

A

arithmetic mean or average

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9
Q

Mean is obtained by

A

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

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10
Q

∑xi means

A

to add all of the values xi for the replicates

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11
Q

Formula of Mean

A

x̄ = add all values for replicates / number of replicates

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12
Q

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

A

Median

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13
Q

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

A

by arranging the results in order and locating the middle result

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14
Q

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

A

the average value of the middle pair is used

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15
Q

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

A

Accuracy

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16
Q

Accuracy is expressed by the ____

A

error

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17
Q

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

A

Precision

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18
Q

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

A
  • reproducibility
  • the closeness of resuts that have been obtained in exactly the same way
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19
Q

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

A
  1. Standard deviation
  2. Variance
  3. Coeffcient of variation
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20
Q

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

A

deviation from the mean di

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21
Q

Deviation from the mean di formula

A

di = |xi - x̄|

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22
Q

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

A

The measured value must be compared to the correct value

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23
Q

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

A

You must compare the values of two or more repeated measurements

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24
Q

The difference between the experimental value and the accepted value

A

Error

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25
Q

Formula of Error

A

Error = experimental value - accepted value

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26
Q

It is the correct value based on reliable references

A

Accepted value

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27
Q

It is the value measured in the lab

A

Experimental value

28
Q

How can we determine precision?

A

just by measuring replicate samples

29
Q

Why is accuracy often more difficult to determine?

A

because the true value is usually unknown

30
Q

What must be used instead to determone accuracy?

A

Accepted value

31
Q

Accuracy is expressen in term of, what?

A

either absolute or relatice error

32
Q

Formula for Absolute Error

A

E = xi - xt

33
Q

xi in Absolute error is…

A

a measurement of the quantity

34
Q

xt in Absolute error is…

A

the true or accepted value of the quantity

35
Q

often a more useful quantity than the absolute error

A

Relative Error

36
Q

Formula of Relative Error

A

Er = xi - xt / xt x 100%

37
Q

Relative error is also expressed in

A

parts per thousands (ppt)

38
Q

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

A
  • precise, accurate
  • accurate, precise
39
Q

Types of Errors in Experimental Data

A
  1. Random (or indeterminate)
  2. Systematic (or determinate)
  3. Gross Error
40
Q

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

A

Random (or indeterminate) error

41
Q

Random error

A
  • observational error
  • environmental error
42
Q

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

A

Systematic (or determinate) error

43
Q

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

A

causes all the results to be too high or too low

44
Q

Example of Systematic error

A

The loss of a volatile analyte while heating a sample

45
Q

Systematic error

A
  • Instrumental error
  • Environmental error
  • Observational error
46
Q

Systematic error other examples

A
  • Miscalibration of pH meter
  • Miscalibration of analytical balance
  • Evaporation of liquids
  • Spattering chemical
47
Q
  • third tye of error
  • usually occur only occasionally, are often large, and may cause a result to be either high or low
A

Gross error

48
Q

Gross error is often a product of ____ ___

A

Human error

49
Q

Gross errors lead to ____

A

outliers

50
Q

Systematic errors have a ____ ___

A

definite value

51
Q

Systematic errors have an ____ ___

A

assignable cause

52
Q

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

A

bias

53
Q

Bias in measurement results affects, what?

A
  • all the data in a set in the same way
  • it carries a sign
54
Q

Systematic errors may be either ________ or ________

A
  • constant
  • proportional
55
Q

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

A

Constant error

56
Q

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

A
  • absolute error is constant
  • relative error varies
57
Q

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

A

Proportional errors

57
Q

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

A
  • absolute error varies
  • relative error stays constant
57
Q

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

A

Periodic calibration of equipment

58
Q

Most personal errors can be minimized by

A
  • careful
  • disciplined laboratory work
59
Q

materials that contain one or more analytes at known concentration levels

A

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

60
Q

SRMs can be prepared by

A
  • synthesis
  • purchased from a number of governmental and industrial sources
61
Q

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

A

Blank

62
Q

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

A

Sample matrix

63
Q

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

A
  • increases
  • decreases

CHM020 - ANALYTICAL METHODS FOR BIOLOGY (5 decks)

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