Mapping indoor radon-222 in Denmark, design and test of the statistical model used in the second nationwide survey

In Denmark, a new survey of indoor radon-222 has been carried out. 1-year a lpha track measurements (CR-39) have been made in 3019 single-family houses . There are from 3 to 23 house measurements in each of the 275 municipaliti es. Within each municipality, houses have been selected randomly. One impor tant outcome of the survey is the prediction of the fraction of houses in e ach municipality with an annual average radon concentration above 200 Bq m( -3). To obtain the most accurate estimate and to assess the associated unce rtainties, a statistical model has been developed. The purpose of this pape r is to describe the design of this model, and to report results of model t ests. The model is based on a transformation of the data to normality and o n analytical (conditionally) unbiased estimators of the quantities of inter est. Bayesian statistics are used to minimize the effect of small sample si ze. In each municipality, the correction is dependent on the fraction of ar ea where sand and gravel is a dominating surface geology. The uncertainty a nalysis is done with a Monte-Carlo technique. It is demonstrated that the w eighted sum of all municipality model estimates of fractions above 200 Bq m (-3) (3.9% with 95%-confidence interval = [3.4,4.5]) is consistent with the weighted sum of the observations for Denmark taken as a whole (4.6% with 9 5%-confidence interval = [3.8,5.6]). The total number of single-family hous es within each municipality is used as weight. Model estimates are also fou nd to be consistent with observations at the level of individual counties. These typically include a few hundred house measurements. These tests indic ate that the model is well suited for its purpose. (C) 2001 Elsevier Scienc e B.V. All rights reserved.