The term survey in the current context implies the use of a randomization rule in the selection of locations across the domain of interest and the caveat that all locations have a positive chance of being selected. Approaches available to achieve these criteria for monitoring natural resources include: simple random sampling, systematic sampling, and grts (Stevens and Olsen, 2004) based sampling. A restricted survey design implies that part of the domain will be sampled by application of a survey, and other parts by application of one of the other spatial designs. There are six sub-categories of survey-based designs:
- Non-stratified Independant Random Survey - a sampling technique where a group of subjects (a sample) for study is selected from a larger group (a population). Each individual is chosen entirely by chance and each member of the population has an equal chance of being included in the sample. A non-stratified sample does not take separate samples from strata or sub-groups of a population.
- Stratified or Variable Probability Independant Random Survey - a sampling technique where a group of subjects (a sample) for study is selected from a larger group (a population). Each individual is chosen entirely by chance and each member of the population has a known, but possibly non-equal, chance of being included in the sample. A stratified sample is obtained by taking samples from each stratum or sub-group of a population.
- Non-stratified Generalized Random-Tessellation Stratified (GRTS) Survey - produces a probability sample with design-based variance estimators. It provides a spatially balanced, random sample, allows for unequal probability sampling, and can provide an over-sample of sample sites to accomdate field implementation issues. A non-stratified sample does not take separate samples from strata or sub-groups of a population.
- Stratified or Variable Probability GRTS Survey - produces a probability sample with design-based variance estimators. It provides a spatially balanced, random sample. Allows for unequal probability sampling, and can provide an over-sample of sample sites to accomdate field implementation issues. A stratified sample is obtained by taking samples from each stratum or sub-group of a population.
- Non-stratified Systematic Survey - a method of selecting sample members from a larger population according to a random starting point and a fixed, periodic interval. Typically, every "nth" member is selected from the total population for inclusion in the sample population. Systematic sampling is still thought of as being random, as long as the periodic interval is determined beforehand and the starting point is random. A non-stratified sample does not take separate samples from strata or sub-groups of a population.
- Stratified Systematic - a method of selecting sample members from a larger population according to a random starting point and a fixed, periodic interval. Typically, every "nth" member is selected from the total population for inclusion in the sample population. Systematic sampling is still thought of as being random, as long as the periodic interval is determined beforehand and the starting point is random. A stratified sample is obtained by taking samples from each stratum or sub-group of a population.
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