PPT - Data sampling Flashcards
Sampling strategies + Calculating sample size (24 cards)
The sample strategy determines what?
Where in the study area must I take my samples?
What is a sample size?
How many samples are required?
What is a T-question?
Testing hypotheses.
(searching for relations, comparing groups, or detecting changes) using statistacl tests.
What is an E-question?
Estimating population parameters.
(means, percentages, or fractions) using confidence intervals.
What is a sampling strategy?
The strategy is the plan you set forth to be sure that the sample you use in your research study represents the population from which you drew your sample.
What is the description of Random sampling?
Samples that are taken randomly from the whole study area;
This ensures that each of the population of sampling units has an equal chance of being selected.
How do you use random sampling with the coordinate system?
- Random number between 0 and the length.
- Random number between 0 and the width.
- together results in a coordinate which is the sampling location.
- Repeat for amount of samples.
How do you use random sampling with the grid system?
- Numbered grid lay-over on the study area.
- Random number generator for a sampling location.
- repeat for amount of samples.
What are the advantages of random sampling?
- Selecting is quick and easy (compared to other random sampling designs)
- Statistical analysis of data is straight foreward.
What are the disadvantages of random sampling?
- Estimates are less precise on heterogenous sites (compared to stratified random sampling).
- Travel time between samples can be high.
What is the description of stratified random sampling?
Study area are divided into strata (habitats, underground, etc.) and random samples are taken in each strata.
What variation is ideal for stratified random sampling?
Low variation of the variable within different strata
and a high variation of the variable between strata.
What are the advantages of stratified random sampling?
- All strata will be sampled
- Comparison between strata can be made
- Greater precision for each stata and overall mean estimates.
What are the disadvantages of stratified random sampling?
- Data preparation can be time consuming
- Most appropriate stratification may change when repeat surveys are carried out; Monitoring efficiency may also change.
What is the description of Systematic sampling
Samples are taken at regular intervals.
What do you need to preselect for systematic sampling?
sampling interval (how big/much between the samples)
What are the advantages of systematic sampling?
- Provide efficient means of mapping distribution and calculating abundance at the same time.
- Determining sample locations is easy.
What are the disadvantages of systematic sampling?
- If sampling interval matches a periodic feature in the habitat, significant bias may be introduced.
- Strictly speaking, statiscal tests are not valid, although conclusions are unlikely to be substancially effected.
What is the description of multistage sampling
Refers to sampling plans where sampling is carried out in stages using smaller, and smaller unints at each stage.
What is the description of two-stage random sampling
Random minor units are sampled within random major units.
What is clustered sampling strategy?
If ALL minor units are sampled within the major units.
What are the advantages of two-stage sampling?
- Can reduce sampling time, increasing efficiency
- Useful for sites that are heterogeneous at small spatial scales and stying gradients along transects.
What are the disadvantages of two-stage sampling?
- When calculating overal means, etc. large errors are obtained if sample units within major units are highly correlated.
