Introduction to Sampling

Sampling represents the problem of accurately acquiring the necessary data in order to form a representative view of the problem.

Methodology:

• 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).

Typical Problems:

• 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 results

Examples:

• Decline in Salmon Runs: What constitutes a representative measure?
• 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?