Download Sampling - Integrated College Dungannon

SAMPLING
Pages 14 to 20
SAMPLING
• What is it?
- A sample is a small part or a
fragment of a whole used to
represent what the whole
picture is really like.
- The whole picture is called the
‘sampling frame’ and
comprises the total ‘population’
of items.
- For example an area of
woodland is the sampling
frame and the total number of
trees in it is the population.
• Why use sampling?
• Impossible, impractical and
unnecessary to study the
entire population.
• Sampling is an accepted short
cut, it is easier, quicker and
cheaper.
• For example if looking at how
a river changes with distance
downstream it is impossible to
study the entire course of the
river, so a number of carefully
selected sites will be used
which forms a sample.
How do you select a sample?
•
There are 4 main things to
consider:
1.
Sample size
2.
Elimination of bias
3.
Type of sampling procedure
4.
Type of sampling method
1. Sample Size
The larger the size of the
sample the more accurately it
will reflect the total population.
Standard deviation can be
used to calculate the minimum
sample size required,
depending on how reliable you
want your data to be.
http://geographyfieldwork.com/Mi
nimumSampleSize.htm
The Normal Distribution Curve
Normal Distribution
Curve
( Bell Curve)
Large no.
of values
lie close
to the
mean
MEAN
Frequency
Fewer values
lie at the
extremes
-3 -2 -1
1
2
Standard Deviations
3
•
Key to graph
•
RED = One standard deviation away
from the mean in either direction on
the horizontal axis (68% of population
included)
•
RED + GREEN = Two standard
deviations away from the mean
(95% of population included)
•
RED + GREEN + BLUE = Three
standard deviations away from the
mean (99% of total population
included)
•
When using a sample you want to
aim for the mean of the sample to
closely equate with the mean of the
total population.
2. Elimination of bias
• Bias needs to be
eliminated if data
collected is to be
accurate and
representative.
• To do this you need to
carefully consider the
characteristics of your
population.
• For example if carrying out a
survey on opinions regarding a
new leisure centre you need to
question people of all ages
and economic groups.
• If you just did a sample in the
most accessible areas, or the
area you feel most comfortable
with, your sample would be
biased.
• To overcome the risk of bias
different types of sampling
procedures and methods
must be used.
3. Type of Sampling Procedures
• A) Point Sampling
Individual points, or sites, are
selected for investigation.
E.g. sites along a river, or a
vegetation sampling quadrat
which uses pins
to locate points.
• B) Grid or Area
Sampling
Involves delimiting an area for study.
The size of area can vary e.g. a grid
quadrat for vegetation sampling.
Continued…..
• C) Line or Transect
Sampling
A line is plotted along which data
collection will occur. Generally
used when the variable under
investigation are expected to show
progressive variation spatially.
E.g. a housing survey along a road,
or succession of vegetation
long a line.
• D) Belt Sampling
An area along a line is defined for
investigation. This could be used
for vegetation or soil studies where
there is a wider areal focus.
4. Type of sampling method
• A) RANDOM
• Used when area very
large or limited time
available.
• A numbered grid can be
laid over a map of the
area.
• A computer generated
random number table is
then used to select which
squares to sample in.
Continued…..
•
•
•
•
B) SYSTEMATIC
Samples are taken at fixed
intervals, usually along a line.
Line or transect sampling and
belt sampling fall into this
category.
Could be used to show how price
buildings changes with distance
from the CBD.
•
•
•
•
•
•
C) STRATIFIED
Used where there are small areas
within a larger study location
which are clearly different.
E.g. an area with more elderly and
very young people.
The sample must include a
representative proportion of the
elderly and very young.
Sampling could still be carried out
randomly or systematically within
each separate sub-group .
Stratified sampling recognises
major differences within
communities before sampling
begins.
Questions see pages 14 to 23
• 1. Identify 3 factors which need to be considered by a geographer
when selecting a sample population for study. (June 2004)
• 2. Explain why systematic sampling would be used if investigating
the changes in a river channel downstream.
• 3. I want to carry out an investigation in Dungannon about attitudes
towards the builiding of a new gaelic pitch at a particular site in the
town. The population of the town is 40% Protestant, 50% Catholic,
& 10% other. What type of sampling would be best and why?
• 4. A mixture of Stratified and Random sampling was used to carry
out a vegetation survey at Peatlands Park comparing the damage
done by drainage and peat extraction. Explain how this may have
been carried out and why this may have been the most appropriate
method.
Questions continued…..
•
5. What does the normal distribution curve show?
•
6. In tabular format briefly outline the advantage and disadvantages of the 3
sampling methods.
•
7. What is a quadrat?
•
8. Give 3 reasons for using a quadrat in a vegetation study.
•
9. Explain the differences between the 3 types of quadrats and draw each.
•
10. Why might you want to alter the size of your quadrat?
•
11. What is a questionnaire?
•
12. Explain the 3 types of questions which can be used when designing a
questionnaire.
•
13. Draw a spider to show the things you should consider when designing a
questionnaire.