CHEM: Errors in Chemical Analysis

Question 1

(TRUE OR FALSE)
Data of unknown quality are useless!

Answer 1

true

Question 2

(true or false)
All laboratory measurements contain
experimental error.

Answer 2

true

Question 3

true or false

It is necessary to determine the magnitude of the accuracy and reliability in your measurements.

Answer 3

true

Question 4

true or false

No measurement is perfectly accurate or exact. Many instrumental, physical and human limitations cause measurements to deviate from the “true” values of the quantities being measured. These deviations are called “experimental uncertainties,” but more commonly the shorter word “error” is used.

Answer 4

true

Question 5

two or more determinations on the same sample
Example: One student measures Fe (III) concentrations six times.

Answer 5

Replicates

Question 6

average or arithmetic mean

Answer 6

Mean

Question 7

the middle value of replicate data

Answer 7

median

Question 8

(true or false)

If an odd number of replicates, the middle value of replicate data

Answer 8

true

Question 9

(true or false)

If an even number of replicates, the middle two values are averaged to obtain the median

Answer 9

true

Question 10

describes the reproducibility of measurements.

Answer 10

Precision

Question 11

How close are results which have been obtained in exactly the same way?

Answer 11

Precision

Question 12

derived from the deviation from the Mean

Answer 12

reproducibility

Question 13

of an element of a data set is the absolute difference between that element and a given point. Typically the point from which the deviation is measured is the value of either the median or the mean of the data set

Answer 13

absolute deviation (D)

Question 14

is the average of the absolute deviations and is a summary statistic of statistical dispersion or variability.

Answer 14

Average absolute deviation or Average deviation or mean absolute deviation

Question 15

Average absolute deviation or Average deviation or mean absolute deviation

Answer 15

Standard Deviation

Question 16

is a measure of the dispersion of a collection of number

Answer 16

Standard Deviation

Question 17

measures the spread of the data about the mean value. It is useful in comparing sets of data which may have the same mean but a different range

Answer 17

Standard Deviation

Question 18

is a measure of statistical dispersion

Answer 18

Variance

Question 19

is the average squared deviations about the mean. Thus, variance is the square of the standard deviation.

Answer 19

variance

Question 20

is a normalized measure of dispersion of a probability distribution. It is defined as the ratio of the standard deviation to the mean

Answer 20

Coefficient of Variation (CV)

Question 21

the closeness of the measurement to the true or accepted value

Answer 21

Accuracy

Question 22

called as the error:
absolute or relative error of a result to its true value.

Answer 22

closeness

Question 23

Occasional error that obviously differs significantly from the rest of the results

Answer 23

Outlier

Question 24

Described by the standard deviation, the variance and the
coefficient of variation (all are function of the deviation from the mean).

Answer 24

•Precision

Question 25

Described by the absolute and relative error.

Answer 25

Accuracy

Question 26

the difference between the experimental value and the true value. Has a sign and experimental units

Answer 26

Absolute Error (E)

Question 27

the absolute error corrected for the size of the measurement or expressed as the fraction, %, or parts-per-thousand (ppt) of the true value. Er has a sign, but no units.

Answer 27

Relative Error (Er)

Question 28

true or false Systematic or determinate errors affect accuracy

Answer 28

true

Question 29

( true or false) Random or indeterminate errors affect precision!

Answer 29

true

Question 30

(true or false) Gross errors or blunders lead to outlier’s and require statistical techniques to be rejected.

Answer 30

true

Question 31

failure to calibrate, degradation of parts in the instrument, power fluctuations, etc

Answer 31

Instrument errors

Question 32

errors due to no ideal physical or chemical behavior - completeness and speed of reaction, interfering side reactions, sampling problems

Answer 32

Method errors

Question 33

occur where measurements require judgment, result from prejudice, color acuity problems

Answer 33

Personal errors

Question 34

Potential Instrument Errors

Answer 34

Variation in temperature •Contamination of the equipment •Power fluctuations •Component failure All of these can be corrected by calibration or proper instrumentation maintenance.

Question 35

method errors

Answer 35

Slow or incomplete reactions •Unstable species •Nonspecific reagents •Side reactions These can be corrected with proper method development

Question 36

Personal Errors

Answer 36

Misreading of data •Improper calibration •Poor technique/sample preparation •Personal bias •Improper calculation of results These are blunders that can be minimized or eliminated with proper training and experience

Question 37

Es is of the same magnitude, regardless of the size of the measurement. •This error can be minimized when larger samples are used. In other words, the relative error decreases with increasing amount of analyte.

Answer 37

Constant errors

Question 38

Es increases or decreases with increasing or decreasing sample size, respectively. In other words, the relative error remains constant.

Answer 38

Proportional errors

Question 39

true or false 1. Analysis of standard samples •2. Independent Analysis: Analysis using a "Reference Method" or "Reference Lab" •3. Blank determinations •4. Variation in sample size: detects constant error only

Answer 39

true

Question 40

true or false caused by uncontrollable variables which normally cannot be defined •The accumulated effect causes replicate measurements to fluctuate randomly around the mean. • Random errors give rise to a normal or Gaussian curve. • Results can be evaluated using statistics • Usually statistical analysis assumes a normal distribution

Answer 40

true

Question 41

named after Carl Friedrich Gauss •characteristic symmetric "bell shape curve" that quickly falls off towards plus/minus infinity

Answer 41

Normal or Gaussian Curve

Question 42

(true or false) gross errors cause an experimental value to be discarded. • Lead to outlier’s and require statistical techniques to be rejected.

Answer 42

true

Question 43

Examples of _________ are an obviously "overrun end point“ (titration), instrument breakdown, loss of a crucial sample, and discovery that a "pure" reagent was actually contaminated.

Answer 43

gross error

Question 44

true or false We do NOT use data obtained when gross error has occurred during collection.

Answer 44

true