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Sample Size Determination to Evaluate the Impact of Highway Improvements

Fowkes, A.S. and Watson, S.M. (1989) Sample Size Determination to Evaluate the Impact of Highway Improvements. Working Paper. Institute of Transport Studies, University of Leeds , Leeds, UK.

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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.

Item Type: Monograph (Working Paper)
Copyright, Publisher and Additional Information: Copyright of the Institute of Transport Studies, University Of Leeds
Institution: The University of Leeds
Academic Units: The University of Leeds > Faculty of Environment (Leeds) > Institute for Transport Studies (Leeds)
Depositing User: Adrian May
Date Deposited: 26 Mar 2007
Last Modified: 06 Jun 2014 08:22
Published Version: http://www.its.leeds.ac.uk/
Status: Published
Publisher: Institute of Transport Studies, University of Leeds
Identification Number: Working Paper 282
URI: http://eprints.whiterose.ac.uk/id/eprint/2274

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