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WHAT IS SAMPLING?
1. Sampling is a shortcut method for investigating a whole population.
2. Data is gathered from a small part of the whole parent population or sampling frame, and used to
inform what the whole picture is like.
WHY DO WE NEED TO SAMPLE? WHY NOT USE DATA FROM THE WHOLE POPULATION?
1. In reality, especially for a business that needs to make a profit, there is simply not enough time,
energy, equipment, labour and money to measure every single item/ site within the parent
population or whole sampling frame.
2. This means that we have to use an appropriate sampling technique to obtain a representative,
statistically valid, sample of the whole.
WHAT CONSIDERATIONS DO WE NEED TO MAKE BEFORE WE TAKE A SAMPLE?
1. The larger the sample size, the more accurate the representation of the whole population/ site.
2. The sample size chosen needs to be a balance between obtaining a statistically valid representation,
and the time, energy, money, labour, equipment and access available.
3. A sample with the least bias is the most statistically valid.
4. Most sampling approaches assume the parent population has a normal distribution where many
items are clustered close to the mean, with few extremes.
5. A 95% probability or confidence level is usually assumed, for example 95% of the population lie
within two standard deviations of the mean.
6. This also means that up to 5% lie outside this. Therefore, sampling however good it claims to be, can
only be a very close estimate.
There are three main types of sampling strategy...
You may then need to decide on a point, line or area method.
This is the least bias of all the sampling techniques, are there is no subjectivity. Each member of the
total population has an equal chance of being selected.
They sample can be obtained using a random number table Microsoft Excel has a function enabling
this to be carried out.
RANDOM POINT SAMPLING
1. A grid is drawn over a map of the study area.
2. Random number tables are used to obtain coordinates or grid references for the points.
3. Sampling takes places as feasibly close to these points as possible.
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RANDOM LINE SAMPLING
1. Pairs of coordinates or grid references are obtained using random number tables, and marked on a
map of the study area.
2. These lines are joined to form lines to be sampled.
RANDOM AREA SAMPLING
1. Random number tables generate co-ordinates or grid references which are used to mark the bottom
left (south west corner) of the quadrates or grid references to be samples.
POINT, LINE AND AREA SAMPLING DIAGRAM
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SYSTEMATIC POINT SAMPLING
1. A grid is used and points can be at the intersections of a grid line, or in the middle of each square.
2. Sampling is done at the nearest feasible place.
3. Along a transect line, sampling points for, say vegetation day collection, could be identified, for
example every 4m along a sand dune transect.
SYSTEMATIC LINE SAMPLING
1. The east or north facing lines on a grid over a map (e.g.…read more
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A systematic approach can still be used, say by asking every fifth person.
STATIFIED RANDOM SAMPLING
1. A wide range of data and fieldwork situation can lend themselves to this approach wherever there
are two study areas being compared e.g. two woodlands with distinctly different habitats.
2. Random point, line or area techniques can be used as long as the number of measurements taken is
proportional to the size of the whole.