__
Sampling__

**Census**

-Sample everything

The disadvantages are;
not practical and sometimes
destructive testing
has to be used.

**Sample**

-Usually, samples take data from around 10% of
the population of interest

In exam answers you will get marks for
using the words in red... "the sample
must be representative
of the whole population
to avoid being biased".

__1) Simple random sampling__

-The idea behind this method is that each
member of the population has an equal chance of being questioned.

__2) Stratified sampling__

-The population is subdivided into
categories and the sample has the same PROPORTION of each category as the
population.

For example; in a school with 200 girls
and 100 boys a 10% sample would include 20 girls and 10 boys.

Once you know how many of each category
are included, use __simple random sampling__ to choose which girls and boys are asked.

The advantage of this is that it is more
representative of the population than a simple random sample

__3) Systematic sampling__

A systematic sample questions people/tests
products etc. AT REGULAR INTERVALS.

For example; for a 10% systematic sample
(i.e. questions every tenth person)...

*Generate a random number between 1 and
10 and then go up in tens.*

For a 20% sample (i.e. question
every fifth person)...

*Generate a random number between 1 and
5 and then go up in fives.*

The advantage of this is that it is quick

The disadvantage is that subdivisions can
seriously affect results e.g. you could get the third-best pupil in every class

__4) Cluster sampling__

A group is chosen at random from the
populations and a __simple random sample__ is conducted within that group.
For example if I want to find out about patients in the UK I might choose just
one hospital and just sample patients from that hospital.

The advantage of this is that it is quick
& convenient

The disadvantage is that one group in a
particular location may not be representative of the whole population. For
example if I chose my hospital in London, their diseases might not be the same
as in Scotland.

__5) Quota sampling (also called
convenience sampling)__

This is exactly the same as cluster
sampling except for once the group is chosen, no precautions are carried out to
ensure that people are chosen totally randomly from the within group. For
example, people would be stopped on the hospital corridors instead of random
numbers to select particular patients being generated by a computer. This
obviously has problems, for example, very ill people who do not leave their beds
will not be asked.

The disadvantage is that one group in a
particular location may not be representative of the whole population (as with
cluster) AND that there is is no RANDOM selection from the cluster. EACH MEMBER
OF THE GROUP DOES **NOT** HAVE AN EQUAL CHANCE OF BEING CHOSEN.

The advantage of this is that it is very
quick & convenient.