Chapter 14: Survey and Sampling Strategies

Data are an essential component of transport modelling. Data collection is therefore a significant activity of the transport planner. It is also an expensive activity, so that careful design and planning of survey instruments and procedures is essential to reduce costs and increase the effectiveness of the data. Furthermore, survey data affect the models that transport planners develop. Errors in the data will create errors in the models, and often these errors can be far more serious for the model than they are in the data. Because sample data always have error, it is important to understand how to minimise survey and data error so that data of adequate quality are produced for modelling purposes. At the outset, it is useful to distinguish between a census and a survey. A census involves measurement or enumeration of every member of a population of interest. (The word population is used here to define the universe of the units of interest to the study, which may be people, vehicles, buildings, etc.) A survey involves a sample from the universe. This sample may be small or large, depending on many factors. However, the intent is always to draw a sample from the population that can be considered to be representative of the entire population, no matter how small or large the sample.