This command turns the manual sample mode on or off. The manual sample mode allows samples to be added to the map by left-clicking on the map with the mouse. Samples can also be added by typing the coordinates (x, y) on the keyboard. Samples added with judgment sampling may be removed by using Edit / Undo from the menu.
IMPORTANT NOTE: Manually adding samples changes the underlying statistical assumptions made about existing sampling designs.
Visual Sample Plan provides this command as a convenience and cannot be held responsible for its misuse.
VSP allows the user to manually add sampling locations to the study area. This is not a recommended way to sample because it injects the bias of the sampler into the project, with the result being the probability structure that underlies probability sampling is lost. However, if the user has justification for the intervention, VSP allows this option. Please consult EPA's guidance documents, Guidance for the Data Quality Objectives Process (EPA 2000a) and Guidance for Data Quality Assessment Practical Methods for Data Analysis (EPA 2000b) for information on the limitations of judgment sampling. These documents are currently available at: http://www.epa.gov/quality/qa_docs.html
The user of this option will want to keep in mind Deming's statement, "Judgment-samples, so far as I know, are not amendable to statistical analysis" (Deming 1950, p. 11).
The user chooses both the number of samples and the sampling locations.
Deming, WE. 1950. Some theory of sampling. Dover Publications, Inc. New York.
EPA. 2000a. Guidance for the data quality objectives process. EPA QA/G-4, EPA/600/R-96/055, U.S. Environmental Protection Agency, Office of Research and Development, Quality Assurance Division, Washington DC.
EPA, February 2006. Data Quality Assessment: Statistical Methods for Practitioners. EPA QA/G-9S, Office of Environmental Information, U.S. Environmental Protection Agency, Washington DC.