Sample Size Determination to Evaluate the Impact of Highway Improvements

This paper was prepared for the Department of Transport, as a support document to a main report on the feasibility of measuring responses to highway improvements. The paper discusses the statistical issues involved, particularly as regards the determination of suitable sample sizes. Worked examples are provided, using such data on ambient variability and adjustment factors as were available to us. Some of the data is included as an appendix where it was felt to be otherwise not easily available. The note asks two sort of questions. Firstly, what is the minimum sample size to take to be a certain percent confident that a given quantity lies in a range of a given width. Secondly, what sample sizes should be taken in Before and After studies so as to be a certain percent confident that a change in a quantity by a given amount will be detected as a statistically significant difference at some chosen significance level. Three sorts of quantities are discussed: - total flows past a point, which may be counted by loops, tubes or manually; - partial flows, such as a particular 0-D flow, which require roadside interviews; - journey times over particular links

Size determination of the Centaur Chariklo from millimeter-wavelength bolometer observations

Using the Max-Planck Millimeter Bolometer Array (MAMBO) at the IRAM 30m telescope we detected emission at 250 GHz from the Centaur Chariklo (1997 CU26). The observed continuum flux density implies a photometric diameter of 273 km. The resulting geometric albedo is 0.055, somewhat higher than expected from a comparison with most of the other few Centaurs and cometary nuclei for which such data are available.Comment: 4 pages, 1 Postscript figure, to appear in Astronomy & Astrophysic

Soil particle-size analysis: A comparison of two methods

Knowing the proportion of particle sizes in soil is important to soil scientists and agronomists. The mixture of sand, silt, and clay influences water movement, solute transport, nutrient retention, and many other properties and processes in soil. The standard method for particle size determination is a somewhat time-consuming process. An equally accurate but shorter method would be appealing for many reasons. The objective of this study was to compare a standard method of particle-size analysis using a hydrometer to an abbreviated hydrometer method, which, instead of 12 h for the standard method, requires about 3 h to complete. Twenty-four soil samples of varying textural classes determined by the standard method were reprocessed for particle-size and textural-class determination using an abbreviated hydrometer method. Results of the methods comparison showed that the textural class from the abbreviated method matched that of the standard method in only 10 of 24 samples and that the abbreviated method over-estimated the amount of total sand in the soil sample. The abbreviated method was reasonably accurate in comparison to the standard method with respect to percentages of clay and silt. Based on this comparison, the time savings gained with the abbreviated method do not outweigh the lack of accuracy of particle-size determination with coarsetextured soils, but may be justifiable for fine-textured soils without a large fraction of sand-sized material