Sampling

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  • Created by: SB
  • Created on: 13-05-13 12:50

Sampling

A sample needs to be representative of its population so you need to avoid SAMPLING BIAS. 

A BIASED SAMPLE IS ONE WHICH DOESNT FAIRLY REPRESENT THE POPULATION.

To avoid sampling bias:

1. Select from the correct population and make sure none of the population is excluded- this means drawing up an accurate sampling frame and sticking to it. 

2. Select your sample at random:

  • Every person or item in the population has an equal chance of being in the sample. 
  • Each selection is independent of every other selection. 
  • To get a truly random sample, you need a complete list of the population- an accurate sampling frame.
  • To choose the sample, give every sampling unit a number. Then generate a list of random numbers (using a random number generator) and match them to the sampling units to select your sample. 

3. Make sure all your sample members respond- otherwise the results could be biased

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Advantages and Disadvantages of Sampling

Sampling - Advantages

  • Quicker and cheaper than a census, and easier to get hold of all the required information.
  • Its the only option when surveyed items are used up or damaged.

Sampling - Disadvantages

  • Samples can be easily affected by sampling bias
  • Variability betwen samples- each possible sample will give different results, so you could just happen to select one which doesnt accurately reflect the population. 

(To reduce variability use a large sample size. The more sampling units that are surveyed, the more reliable the information should be). 

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