Introduction to Sampling
Sampling represents the problem of accurately acquiring the
necessary data in order to form a representative view of
- State the objectives of the survey
- Define the target population
- Define the data to be collected
- Define the variables to be determined
- Define the required precision and accuracy
- Define the measurement `instrument'
- Define the sample frame, sample size and sampling method, then select the sample
The sample frame is the list of people (`objects' for inanimate populations) that make up the target
population; it is a list of the individuals who meet the `requirements' to be a member of that population.
The sample is selected from the sample frame by specifying the sample size (either as a finite number, or
as a proportion of the population) and the sampling method (the process by which we choose the
members of the sample).
- Sample is of insufficient size: --> means that you weren't
very clever when you defined the sample
- The sample is biased: --> often biases can be subtle and can take
time to find and correct. Control samples are usually not an adequate
substitute as they be biased as well
- The wrong variables were measured: --> the collected data are
measuring secondary effects not primary effects
- The sample is censored: --> there exists a population which
is below the threshold of your measuring technique or apparatus
- The data precision is low: --> you have only low signal-to-noise
- Decline in Salmon Runs: What constitutes a representative
- How to measure deforestation/clear cutting? From the ground?
From the Air? From satellites?
- How to characterize this distribution
- How to measure how much old growth is needed for spotted owl
environment? 500 Acres? --> What about predators?
- What is the characteristic annual rainfall in Eugene?
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