Chapter 3

Question 1

▸ Sample type
▸ Size of sample
▸ Sample preparation needed

Answer 1

SELECTING A METHOD

Question 2

▸ Sample type/ homogeneity/ size
▸ Sampling statistics/ errors

Answer 2

OBTAINING A REPRESENTATIVE SAMPLE

Question 3

a ‘portion of material selected
from a larger quantity of material

Answer 3

SAMPLE

Question 4

▪ Homogeneous
▪ Heterogeneous
▪ Static (contained) system
▪ Dynamic system

Answer 4

Representative Sample

Question 5

▪ Simple Random Sampling
▪ Stratifies Random
Sampling
▪ Systematic Sampling

Answer 5

Random Sample

Question 6

a sample that is typical of the
parent material for the
characteristic under
inspection

Answer 6

Representative Sample

Question 7

▸ the parent material is changing with respect to time
▸ removal of a portion at any instant represents only a ‘snapshot’ of
that moment in time and in that particular location.
* The fact that it can never be reproduced presents difficulties in
applying statistical control and consequently cannot be the subject
of conventional statistical sampling plans

Answer 7

Dynamic system

Question 7

▸ the composition of the parent material is permanent with
respect to position in space and stable over the time of
sampling and testing
▸ e.g. oil in a drum; tins of fruit in a warehouse

Answer 7

Static (contained) system

Question 8

a sample deliberately chosen by
using a sampling plan that
screens- out materials with certain
characteristics and/or selects only
material with other relevant
characteristics

Answer 8

Selective Sample
(directed/ focused sampling)

Question 9

a sample is selected by a
random process to eliminate
problems of bias in selection
and/or to provide a basis for statistical interpretation of measurement data.

Answer 9

Random Sample

Question 10

3 Types of Random Sampling

Answer 10

Simple random
sampling, Stratified random
sampling, Systematic sampling

Question 11

Any sample has an
equal chance of
selection

Answer 11

. Simple random
sampling

Question 12

the lot is subdivided/
stratified and a simple
random sample selected
from each stratum.

Answer 12

Stratified random
sampling

Question 13

The first sample is
selected at random and
then the subsequent
samples are taken
according to a previously
arranged interval, e.g.
every 5th, 10th or
whatever is appropriate

Answer 13

Systematic sampling

Question 14

▸ consists of two or more portions of
materials (collected at the same
time) selected so as to represent
the material being investigated
▸ taken in proportion to the amount
of the material that they represent

Answer 14

Composite Sample

Question 15

can take a number of forms (a single pile of
grain, soil present in an area of contaminated land or a
shipload of coal.
- not in the form of separate permanently
identifiable units.

Answer 15

Bulk materials

Question 15

comprised of identifiable units, which may
be assigned number.

Answer 15

Packaged goods

Question 15

bulk materials that are partially packaged into
smaller units, such as in bags or drums.

Answer 15

Segments

Question 16

(for both bulk materials and packaged goods)
- a quantity of material transferred on one occasion
and covered by a single set of shipping documents.
- may be made up of one or more lots

Answer 16

Consignment

Question 17

a quantity of material which is assumed to represent a
single population for sampling purposes
- may consist of one or more batches

Answer 17

Lot

Question 18

a quantity of material which is known or assumed to
have been produced under uniform conditions

Answer 18

Batch

Question 19

portions of the material obtained from the lot/batch
by using a sampling device
- often combined to produce a primary or gross sample

Answer 19

Increments

Question 19

portion of material delivered to the
laboratory for analysis
- often obtained from the primary or composite
sample by a series of division and reduction processes (e.g. coning
and quartering, riffling

Answer 19

Laboratory sample

Question 19

intermediate sample that the analyst
obtains from the laboratory sample

Answer 19

Test/ analytical sample-

Question 20

the quantity of material that is actually submitted for analysis, taken from the test sample - has to go through a number of treatment steps before the final aliquot is obtained

Answer 20

Test portion/ analytical portion

Question 21

❖ describes when, where and how samples are to be taken. ❖ IUPAC: A predetermined procedure for the selection, withdrawal, preservation, transportation and preparation of the portions to be removed from a population as samples

Answer 21

SAMPLING PLAN

Question 21

▸ should specify the number and size of primary samples to be obtained from the lot/batch ▸ should include the number, location and size of the portions to be taken, and instructions for the extent of compositing and reduction of these portions to produce the laboratory sample ▸ should address whether the process of sampling should be a ‘oneoff’ or whether it should be repeated and if so, how often ▹ sampling scheme/ sampling schedule- sampling plan with regular requirement for analysis ▹ sampling programme - describes a combination of procedures where several related sampling schemes are combined

Question 22

TYPES OF SAMPLING

Answer 22

Probability Sampling Bulk Sampling Acceptance Sampling Non-Probability Sampling

Question 22

allows a statistical evaluation to be applied to the data. ▸ used when a representative sample is required

Answer 22

Probability Sampling (random sampling)

Question 23

involves taking increments from the bulk material in such a way that any portion of The bulk has an equal probability of being sampled. ▸ used when little information is available about the material that is being sampled, or ▸ sampling from a batch or consignment of manufactured product for quality control purposes

Answer 23

Simple random sampling

Question 23

requires the consignment to be subdivided into groups (strata) according to predefined criteria. A simple random sampling plan is then applied to each stratum. The number of samples taken from each stratum is proportional to its size (e.g. weight or volume). ▸ aims to produce a more representative sample than would be obtained by simple random sampling.

Answer 23

Stratified sampling

Question 24

most commonly used sampling technique. ▸ involves taking increments from the bulk material at predetermined intervals, as defined by the sampling plan.

Answer 24

Systematic sampling

Question 25

▸ used when a representative sample cannot be collected or is not appropriate ▸ the correct sampling approach to use to produce a selective sample

Answer 25

Non- Probability Sampling

Question 26

use of knowledge about the material to be sampled, and the reason for sampling, to select specific samples

Answer 26

. Judgement sampling

Question 27

requires the consignment to be subdivided into groups (as for stratified sampling) then judgement sampling is used to select samples from each group

Answer 27

Quota sampling

Question 28

involves selecting samples on the basis of availability and/or accessibility

Answer 28

Convenience sampling

Question 29

involves the taking of a sample from material which does not consist of discrete, identifiable or constant units ▸ bulk material may be gaseous, liquid or solid.

Answer 29

Bulk Sampling

Question 30

involves the application of a predetermined sampling plan to decide whether a batch of goods meets the defined criteria for acceptance

Answer 30

Acceptance Sampling

Question 31

Examples of measurement objectives ✓ Control of limits, such as product specifications, environmental quality standards ✓ (EQS), maximum contaminant levels (MCL) or maximum residue limits (MRL) ✓ Investigation of trends (variation in time) or distributions (variation in space) The quality requirements are in many cases set as the analytical quality ✓ HORRATR –value (requirement on between laboratory precision) of less than 1.5 in interlaboratory method study ✓ For environmental control in Denmark: maximum standard deviation at low concentrations, maximum relative standard deviation at higher concentrations, maximum bias and maximum error, values set individually and for quality classes Conventionally, arbitrary quality requirements have been applied ✓ Limit of detection (LOD) below 10% of limit, repeatability standard deviation better than 5%, bias less than 20% and all measurements within linear range

Question 32

How can knowledge on the type of sampling error help in designing the sampling? The major source of uncertainty in contaminant measurements for a row of soil piles from the previous example was shown to be varying contaminant concentration with depth, i.e. a systematic effect. Sampling or sub-sampling of an increased number of replicates would not have reduced uncertainty because the effect of doing so is mostly limited for systematic errors. Instead, a more suitable sampling method was designed, as described