Sample Area Weights

Area weights are used in the generation of the cumulative distribution function (CDF) plots and the summary statistics. The total area sampled in the five Estuary regions is adjusted for regional area weights using the spsurvey package which is available for the R software environment for statistical computing and graphics. The CDF plots and summary statistics for the individual regions are not adjusted.

In the initial sampling design, area weights were calculated for 100 water and 140 sediment sites per region. Each region’s original sample frame area is shown in the table below. No random samples were allocated to the Rivers region; therefore, that region was not included in the total sample frame.

The area weights, however, must be re-calculated each year according to the actual number of sites sampled; by dividing the product of the total sample frame area used for sample selection and the original area weights, by the sum of the original weights for the targeted sites. The targeted sites include sites that could not be sampled for any reason (as one needs to adjust for the area that could not be sampled; for example any site that was not accessible during the sampling cruise), and replacement sites. Therefore, the area weight assigned to each sample will decrease, and the power of analysis will increase as the spatial coverage of the Estuary increases and more samples are collected over time. The re-calculated area weights for each region for the period 2002-2012 are shown in this excel file.

For more information, please contact John Ross at [email protected]