Sampling is the process of gaining information about a population from a portion of that population called a sample. A key goal of sampling design is to specify the sample size (number of samples) and sampling locations that will provide reliable information for a specific objective (called the Sampling Goal ) at the least cost. VSP does the required calculations for sample size and sample location and outputs a sampling design that can be displayed in multiple formats. VSP does not address sample collection methods. It assumes the sample support (amount of material in the sample) is sufficient and the sample is representative of the population. A few designs in VSP assume the sample is taken across an entire grid (say a 10 x 10 cm swipe), but most designs assume the sample is a point sample taken at an X, Y coordinate location, and has sufficient volume for measurement and testing.
VSP automates the mechanical details of calculating sample size, specifying random sampling locations, and comparing sample costs with decision error rates. These activities can be accomplished in the context of your own site map displayed onscreen with various sampling plans overlain on sample areas that you select.
The first thing you will do after opening the program is to import or construct a visual map of the study site. Next, you select the area or areas to be sampled. The Sample Area may be only a portion of the study site.
Then, for the Sampling Goal that you select, VSP will lead you through the quantitative steps of the DQO process (Steps 6 and 7) so that the program has the information needed to compute the recommended minimum number of samples (sample size).
The locations of the samples over the Sample Area are determined by the specific sampling design (pattern) that you select. For buildings and rooms, a CAD drawing or floor plan are loaded. The Map View displays samples placed on maps and room drawings. On a site map, VSP displays the sample locations for easy visualization. VSP also lists the geographical coordinates of the sample locations, which can be saved and exported as a Drawing Exchange Format (DXF) or SHP file for use in a geographical information system (GIS) or saved as a text file for use in global positioning system (GPS) software.
Two additional output formats for the design created in VSP are available: a Graph View of the design, and a Report View. The Graph View displays either a Decision Performance Goal Diagram for Sampling Goals that involved selecting between alternative actions, or a performance graph comparing number of samples to a design parameter for the other classes of sampling goals. The Report View is a text report that describes the sampling design in detail. The report contains the input values, the assumptions, the cost of the design, VSP output, technical description of the sample size formula and algorithms used, and a sensitivity analysis table to assess what would happen if more or fewer samples are collected than the optimal number calculated by VSP. The analytical value of lab results for samples can be input into VSP for Data Analysis – summary statistics of results, statistical tests applied to results for making decisions, and graphical displays of results.
For an illustration on how to use these features in VSP, please refer to chapter 4 of the VSP User’s Guide.