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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

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 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 if…?

A

limits of uncertainty are known

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5
Q

in order to improve reliability to obtain information about the variability of results, what is carried

A

replicates

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6
Q

best estimate of true value

A

central value for the set

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7
Q

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 formula

A

x̄ = add all values for replicates / number of replicates

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10
Q

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

A

median

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11
Q

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

A

accuracy

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12
Q

accuracy is expressed by what

A

error

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13
Q

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

A

precision

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14
Q

what does precision describe

A

closeness of results that have been obtained in exactly the same way

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15
Q

Three terms used to describe the precision

A
  1. standard deviation
  2. variance
  3. coefficient of variation
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16
Q

how much an individual results xi differs from the mean

A

deviation from the mean

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17
Q

deviation from the mean formula

A

di = |xi - x̄ |

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18
Q

Evaluate accuracy

A

measured value compared to correct value

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19
Q

Evaluate precision

A

compare values of two or more repeated measurements

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20
Q

difference between experimental value and the accepted value

A

error

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21
Q

correct value based on reliable references

A

accepted value

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22
Q

value measured in the lab

A

experimental value

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23
Q

often more difficult to determine because the true value is usually unknown

A

accuracy

24
Q

Terms used to express accuracy

A
  • absolute error or
  • relative error
25
Q

xi

A

measurement of the quantity

26
Q

xt

A

true or accepted value of the quantity

27
Q

absolute error formula

A

E = xi - xt

28
Q
  • foten a more useful quantity than the absolute error
  • also expressed in parts per thousand (ppt)
A

relative error

29
Q

relative error formula

A

Er = (xi - xt/xt) x 100%

30
Q

Types of Errors in Experimental Data

A
  1. Random error
  2. Systematic error
  3. Gross error
31
Q

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

A

random error

32
Q

random error or __

A

indeterminate error

33
Q

Example of Random Error

A
  1. observational error
  2. environmental error
34
Q

student repeatedly weights a beaker with solution and gets different measurement every time

A

observational error (random)

35
Q

when getting a pressure reading, students were entering and exiting the lab, causing the pressure to be lower

A

environmental error (random)

36
Q

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

A

systematic error

37
Q

systematic error or __

A

determinate error

38
Q

in general, a systematic error in a series of replicate measurements causes what?

A

all results to be too high or too low

39
Q

Examples of Systematic Error

A
  1. Instrumental error
  2. Environmental error
  3. Observational error
40
Q

scale is improperly calibrated so it reads 2mg even with nothing on it

A

Instrumental error (systematic)

41
Q

The Geiger counter shows the radiation, however, when a cell phone is near the Geiger counter, the radiation is read as greater than what is actually is because the cell pone is radiating RF waves which disrupts the reading

A

Environmental error (systematic)

42
Q

observer looks at the beaker from above eye level and write down the volume as 40 mL, when in reality when looked at eye level you can see the liquid has a volume of about 42 mL

A

Observational error (systematic)

43
Q
  • often the product of human errors
  • usually occur only occasionally
  • may cause a result to be either too high or low
A

Gross error

44
Q

Gross errors lead to __

A

outliers

45
Q

results that appear to differ markedly from all other adta in a set of replicate measurements

A

outliers

46
Q
  • systematic error leads to __ in measurement results
  • affects all the data in a set in the same way and that it carries a sign
A

bias

47
Q

Systematic errors may be either __ or __

A
  • Constant
  • Proportional
48
Q

magnitude stays essentially the same as the size of the quantity measured is varied

A

constant error

49
Q

With constant errors, the __ error is constant with sample size, but the __ error varies when sample is changed

A
  • absolute error (constant)
  • relative error (varies)
50
Q

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

A

Proportional errors

51
Q

With proportional errors, the __ error varies with sample size, but the __ stays constant when the sample size is changed

A
  • absolute error (varies)
  • relative error (constant)
52
Q

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

53
Q

Examples of methods used to recognize and adjust for a systematic error in an analytical method

A
  1. Analysis of Standard Samples (or Standard Reference Materials, SRMs)
  2. Blank Determinations
  3. Variation of Sample Size
54
Q

materials that contain one or more analytes at known concentration levels

A

Standard Reference Materials (SRMs)

55
Q

contains reagents and solvents used in a determination but no analyte

A

blank

56
Q

sample constituents to simulate the analyte environment

A

sample matrix

57
Q

as the size of measurement __, the effect of a constant error __

A
  • increases (size)
  • decreases (constant error)

CHM020 - Analytical Methods for Biology (22 decks)

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