§ 91.01 What do the sampling-related terms mean?

Alternative sampling plan.  A sampling plan that reflects the conditions of your service, and is independently developed and certified by a qualified statistician to meet FTA’s 95% confidence and 10% precision levels.  It is one of two forms of customized sampling plans.  The other form is template sampling plans.  It is equivalent to an alternative sampling technique as defined in the NTD Reporting Manual.

Confidence level.  The chance of an estimate of service-consumed data obtained through random sampling falling within a particular range of the true value. FTA requires a minimum level of 95% confidence for estimates of annual UPT and annual PMT reported to the NTD.  A particular confidence level is only meaningful when it is stated with a particular precision level.

Customized sampling plan.  A sampling plan that reflects the conditions of your service and meets FTA’s 95% confidence and 10% precision levels.  It is either a template sampling plan or an alternative sampling plan.  It differs from a ready-to-use sampling plan in that it takes account of the specific characteristics of your service.

Efficiency option.  A characteristic of a sampling plan that affects its sampling efficiency.

Initial annual sample size.  The annual necessary sample size of a period-based template sampling plan that is determined from the companion spreadsheet template before it is allocated to each quarter, month, or week.  This can differ from the realized annual sample size for a given set of sample data.

Mandatory revising year.  A report year for which you must consider whether you need to revise your template sampling plan. 

Margin of safety.  A percent increase in the statistical variation of your sample data in developing a template sampling plan.  For example, if the statistical variation of your sample is S and the margin of safety is 25%, you must use 1.25S as the statistical variation in developing your template sampling plan.  A margin of safety of 25% is used automatically for all period-based template sampling plans.  If you develop alternative sampling plans, you should also use this margin of safety.  The objective is to counter the potential fluctuations in the statistical variation in a sample from one year to another due to sampling and other reasons. 

Major change to a service.  Any change to your service that is likely to lead to major changes in how your customers use your service.  Examples of major changes include making transfers fare free; adding or cutting express routes; expanding or contracting your service by more than 25% in vehicle revenue miles; or restructuring your service affecting more than 25% of your service in vehicle revenue miles.

Necessary sample size.  The sample size that meets FTA’s minimum 95% confidence and 10% precision levels and uses a 25% margin of safety.

Precision index.  A number that reflects the level of precision that your current NTD sample achieves in the resulting annual total PMT.  It is used for you to determine whether you must revise your current template sampling plan after you have made major changes to your service since you started using the current template sampling plan.  Once you have entered your current sample data into the template for this Sampling Manual as if you are going to develop new template sampling plans, this precision index is made available in the PeriodPrecision Worksheet for period- based sampling and in the IntervalPrecision Worksheet for interval-based sampling. 

Precision level.  The degree of errors in an estimate of service-consumed data obtained through random sampling that is stated in percentage terms relative to the true value.  FTA requires a minimum of 10% precision for estimates of annual service-consumed data reported to the NTD.  A particular precision level is only meaningful if it is stated with a particular confidence level.

Random sampling.  Selection of one or more service units at random from a list of service units to be operated.

Ready-to-use sampling plan.  A sampling plan that has been developed specifically for this Sampling Manual with sample data from a variety of transit agencies.  It does not necessarily reflect the conditions of your service.  Ready-to-use sampling plans have limited applicability.

Realized annual sample size.  The annual necessary sample size of a period-based template sampling plan that is based on quarterly, monthly, or weekly sampling.  For example, if you choose weekly sampling and your template sampling plan requires 3 one-way trips per week, the realized annual sample size would be 156 one-way trips.

Sample size.  The number of service units that are sampled, and for which unlinked passenger trips and passenger miles traveled are measured.

Sampling efficiency.  The degree to which a sampling plan minimizes the necessary sample size for meeting FTA’s confidence and precision levels.  Sampling plans that take advantage of certain characteristics of your service can sometimes require a smaller necessary sample size.  A smaller necessary sample size reduces the time and cost of sampling, data collection, and data processing.

Sampling frequency.  The number of times per year that a sample is drawn; in this Manual, sampling frequency is quarterly, monthly, or weekly for period-based sampling, and is every day, every 2nd day, every 3rd day, every 4th day, every 5th day, and every 6th day for interval-based sampling.  For example, if your period-based sampling plan requires 10 service units per month, before the current month ends you must select at least 10 at random from the full list of all service units to be operated during the next month.

Sampling plan.  A plan for selecting service units at random, for collecting sample data, and for estimating annual service-consumed data that meets FTA’s 95% confidence and 10% precision levels.  Each sampling plan consists of four elements: a unit of sampling and measurement, a set of efficiency options, a sampling frequency, and a necessary sample size.

Sampling without replacement.  Selection of a sample of service units at random without the chance of a single service unit being selected more than once.

Sampling year.  Any report year for which you obtained annual UPT, annual PMT, or both through random sampling that meet FTA’s 95% confidence and 10% precision levels.  It can be a mandatory sampling year or an intermediate report year for which you choose to sample.

Service grouping.  One efficiency option for which you divide your service into two or more groups with the objectives of reducing within-group differences and increasing between-group differences.  For example, separating your bus routes into express routes and local routes is likely to reduce differences in average passenger trip length across one-way bus trips within each group.

Service unit.  An amount of revenue travel by a single transit vehicle, a set of transit vehicles, or a component of a transit vehicle.  For non-scheduled services, it is typically one vehicle day.  For scheduled bus services it is typically either a one-way bus run or else a round-trip bus run.  For rail services, it is either a one-way car run, a one-way train run, or a round-trip car run or a round-trip train run.

Statistical variation.  The degree of differences in a quantity across the full list of service units operated during a given duration of time, such as differences in PMT across all one-way trips of a bus service in a full report year.  A larger variation requires a greater sample size to meet given confidence and precision levels.

Table of random numbers.  A list of integers whose frequency and order of appearance in the list have been determined entirely by chance.  It is the basis of a commonly used method of random sampling.

Template sampling plan.  A sampling plan that is developed with the companion template of this Sampling Manual.

Unit of sampling and measurement.  A service unit you choose for your sampling plan.