Procuring, Managing, and Evaluating the Performance of Contracted TMC Services
representative of the results of the entire system. Typical sampling methods are described below and Table 7 shows example areas where different types of sampling may be used in conjunction with the evaluation of contracted services.
Typical sampling methods can include:
Random sampling – While the statistical methods for performing a random sample are beyond the scope of this text, the process involves taking a number of independent observations from the entire population and using the performance of that group as a measure of the performance of the whole.
Periodic (systematic) sampling – in contrast to the technique above, where only a subset of the entire population is examined, systematic sampling selects elements at regular intervals. These data are then used in the evaluation.
Strata sampling – another kind of sampling that is useful is strata, where individual samples are taken within classifications. An example would be speed readings from individual classes of vehicles, such as passenger cars, buses, and trucks. In this type of sampling, it is very important to ensure that the properties of the individual strata are the same, which may mean that more samples have to be taken in one stratum than another.
Trend analysis – trends are changes over space or time. This type of sampling utilizes repeated observations of the same subset of the population to identify changes that may occur. Changes may be spatial if the data producers are moving (such as vehicles) or temporal if the data producers are stationary (such as traffic recorders).
Customer feedback – the use of surveys is a standard technique in sampling customers to obtain their feedback, score, or opinions on various services that a contractor might be performing, such as courtesy patrol.
Third party audits – third party audits are conducted by hiring an outside company to examine records of the contractor and assess their adherence to a performance metric. These could be human resource related items, such as pay and promotion, operational aspects such as data collection, record keeping, and TMC floor operations, or satisfaction measures, such as courtesy patrol complaints.
Table 7. Typical application areas for various sampling techniques.
Types of contracted services(s) where sampling could be used
Hours of operation
Equipment health and availability
Web site availability
Changes in response times
Changes in queues from non-geometric conditions
Changes in usage statistics (web, 511, courtesy patrol)
Changes in TMC floor operations (responsiveness, accuracy, etc)
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