Chapter 3 Flashcards
▸ Sample type
▸ Size of sample
▸ Sample preparation needed
SELECTING A METHOD
▸ Sample type/ homogeneity/ size
▸ Sampling statistics/ errors
OBTAINING A REPRESENTATIVE SAMPLE
a ‘portion of material selected
from a larger quantity of material
SAMPLE
▪ Homogeneous
▪ Heterogeneous
▪ Static (contained) system
▪ Dynamic system
Representative Sample
▪ Simple Random Sampling
▪ Stratifies Random
Sampling
▪ Systematic Sampling
Random Sample
a sample that is typical of the
parent material for the
characteristic under
inspection
Representative Sample
▸ 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
Dynamic system
▸ 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
Static (contained) system
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
Selective Sample
(directed/ focused sampling)
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.
Random Sample
3 Types of Random Sampling
Simple random
sampling, Stratified random
sampling, Systematic sampling
Any sample has an
equal chance of
selection
. Simple random
sampling
the lot is subdivided/
stratified and a simple
random sample selected
from each stratum.
Stratified random
sampling
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
Systematic sampling
▸ 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
Composite Sample
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.
Bulk materials