§ 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.

§ 91.03 What do the data-collection terms mean?

Arriving load.  The number of passengers onboard a transit vehicle as it arrives at a stop.

Automatic passenger counter.  An automated means of counting passengers as they board or alight transit vehicles with treadle mats, infrared beams, or other devices placed by the doors of a transit vehicle.

Calculated load.  The number of passengers onboard a transit vehicle as it arrives at or leaves a stop that you calculate from data on boardings and alightings at individual stops.  It should equal the observed load.

Commuter vanpool.  A common form of vanpool service (VP) that comprises vans, small buses and other vehicles operating as a ridesharing arrangement, providing transportation to a group of workers commuting directly between their homes and their regular work sites within the same geographical area.  The vehicles would not be in revenue service during the working hours of the participating workers.

Distance-based approach.  A method to obtain PMT that keeps track of the distance traveled by every passenger.    

Leaving load.  The number of passengers onboard a transit vehicle as it leaves a stop.

Load-based approach.  A method to obtain PMT that is based on the boardings and alightings at individual stops and on the distance between consecutive stops.

NTD sample.  The sample of service units you select at random according to your sampling plan that meets FTA’s 95% confidence and 10% precision levels for reporting to the NTD.

Observed load.  The number of passengers onboard a transit vehicle as observed directly by a ride-checker while onboard that transit vehicle.

Ride check.  A method of collecting sample data with one or more persons observing and recording passenger boarding and alighting activities while riding in a transit vehicle. 

Stop.  Any spatial location at which a transit vehicle allows passengers to board or alight from the vehicle. 

Vehicle trip length.  The total distance traveled by a transit vehicle during a one-way trip for scheduled services.  For example, the cumulative distance traveled from the beginning point to the end point of a particular alignment of a route is the vehicle trip length for this trip.  The vehicle trip length may vary by direction and alignment for a given route.

§ 91.05 What do the estimation-related terms mean?

100% count.  A method of obtaining service-consumed data.  This term also often refers to the results of a 100% count.  For UPT, it involves counting passengers each time they board a transit vehicle in revenue service, such as through a registering farebox.  For PMT, it involves recording the distance traveled by all passengers.  A 100% count of PMT is typically only possible for systems that have only two stops, for rail systems that record entry and exit from the system, or for rail systems that rely upon destination-based tickets. 

Average passenger trip length (APTL).  The average distance traveled for an unlinked passenger trip. It is calculated as PMT divided by UPT.

Average route length.  The average length of a route actually traveled by vehicles in scheduled services.   It is calculated by dividing the annual vehicle revenue miles by the number of annual vehicle revenue one-way trips for that route. 

Expansion factor.  A measure of actual services operated or consumed during a given duration of time.  It is used to convert a sample average to the total of service-consumed data for that duration.  It varies with sampling plans.  The total number of one-way bus trips operated during an entire report year is an example of an expansion factor; when multiplied by the sample average PMT per one-way bus trip derived from annual NTD sample, it yields a measure of annual total PMT.

Passenger miles traveled (PMT).  The total distance traveled by all passengers during a given period.

Potential passenger miles traveled.  The maximum number of passenger miles that could have been traveled by all passengers along a given fixed route during a year (or some other time duration.)  It is calculated by multiplying a 100% count of UPT times the average route length of that route during that duration.

Sample average.  The sample total divided by the number of service units in the sample.  It may be calculated for the entire annual sample, or by the type of service day, or for specific weekday time periods.  For example, dividing the total PMT in an annual NTD sample by the total number of one-way bus trips in the annual NTD sample gives a sample average PMT.

Sample data.  The data collected from a sample of service units according to a sampling plan that meets FTA’s 95% confidence and 10% precision levels.

Sample ratio.  The ratio of the sample total for one measure of service-consumed over the sample total for another measure of service-consumed.  For example, the ratio of the sample data for PMT over the sample total for UPT gives the sample APTL.  It may be calculated for the entire annual sample, or by the type of service days, or for individual service group if your sampling plan is based on service grouping.

Sample total.  The sum total of all data across the service units in a random sample.  For example, if you are sampling for PMT, the PMT sample total is the sum of the PMT collected for each of the one-way bus trip in the sample.  It may be calculated for the entire annual sample, or by the type of service day, or for specific weekday time periods.

Service-consumed data.  Passenger miles traveled and unlinked passenger trips.

Type of service days.  Weekdays, Saturdays, or Sundays.  For scheduled services, service days in a report year are classified according to the schedule operated on that day.  If a weekday that is a holiday is served with a Sunday schedule, that weekday is considered to be a Sunday.  For non-scheduled services, service days are the actual days of a week regardless of whether they are a holiday or not.

Typical day.  For your scheduled services, it is a day on which you operate your normal, regular schedule and there are no anomalies such as extra service added for a special event or reduced service as a result of weather or interruption.  For your non-scheduled services, it is any day of operation.

Unlinked passenger trips (UPT).  The number of passengers who board transit vehicles in revenue service.  Passengers are counted each time they board a vehicle, no matter how many vehicles they use to travel from their origin to their destination.

Weekday time periods.  Weekday AM Peak, Weekday Midday, Weekday PM Peak, and Weekday Other.  The Reporting Manual instructs how you should define the start and end points of each period.

§ 91.07 What do the reporting terms mean?

Auditable record.  Documentation of information collected and processes used in collecting that information that demonstrates your compliance with NTD requirements. Such documentation may also help quality control within your agency when your NTD staff change over time.

First-time reporting.  Reporting of a particular service to the NTD for the first time.

§ 91.09 What abbreviations for general terms are used in this Manual?

APTL.  Average passenger trip length in miles

APC.  Automatic passenger counter

FTA.  Federal Transit Administration

NTD.  National Transit Database

PMT.  Passenger miles traveled

PPMT.  Potential passenger miles traveled

UPT.  Unlinked passenger trips

§ 91.11 What abbreviations for NTD modes are used in this Manual?

CB.  Commuter bus – Fixed-route bus systems that are primarily connecting outlying areas with a central city. Service typically uses over-the-road buses with service predominantly in one direction during peak periods, limited stops, and routes of extended length.

CC.  Cable car – A railway propelled by moving cables located beneath the street. While popular at the turn of the last century, the only surviving system is operated in San Francisco.

CR.  Commuter rail – Rail service operating on either old freight railways, or on tracks that are shared with freight railways, Amtrak, or both. The service is characterized by relatively long distances between stops, for service primarily connecting a central city with outlying suburbs and cities. The service may be either diesel or electric-powered and usually has grade-crossings with roadways.

DR.  Demand response – Shared-ride demand response service is scheduled in response to calls from passengers. Many transit systems operate demand response (DR) service to meet the requirements of the Americans with Disabilities Act (ADA).

DT.  Demand response – Taxi – A special form of the demand response mode operated through taxicab providers. The mode is always purchased transportation type of service.

HR.  Heavy rail – An electric railway that operates local service in exclusive right-of-way. The service is characterized by long trains of six to eight cars or more and by relatively short distances between stops for local service within a city and the immediate suburbs. The Nation’s traditional subway systems are classified as heavy rail.

JT.  Jitney – A transit mode comprising of owner-operated passenger cars or vans operating on fixed routes (sometimes with minor deviations) as demand warrants without fixed schedules or fixed stops.

LR.  Light rail – An electric railway that operates local service in mixed traffic with road vehicles, or has grade crossings with roadways. The service is characterized by short trains of one to four cars and by relatively short distances between stops for local service within a city and the immediate suburbs.

MB.  Bus – Fixed-route bus service is the most-prevalent mode in the country. MB service is powered by a motor and fuel contained within a vehicle. Deviated fixed-route service is also reported as MB.

MG.  Monorail and automated guideway – An electric railway that straddles a single guideway. It may have vehicle operators or may use computers to guide the vehicles.

PB.  Público – Publicos are jitney services operated in Puerto Rico. A transit mode comprising of passenger vans or small buses operating with fixed routes but no fixed schedules. Publicos (PB) are a privately owned and operated public transit service which is market oriented and unsubsidized, but regulated through a public service commission, state or local government. Publicos (PB) are operated under franchise agreements, fares are regulated by route and there are special insurance requirements. Vehicle capacity varies from eight to 24, and the vehicles may be owned or leased by the operator.

RB.  Bus rapid transit – Fixed-route bus systems that either (1) operate their routes predominantly on fixed-guideways (other than on highway high occupancy vehicle (HOV) or shoulder lanes, such as for commuter bus service) or (2) that operate routes of high-frequency service with the following elements: Substantial transit stations, traffic signal priority or preemption, low-floor vehicles or level-platform boarding, and separate branding of the service. High-frequency service is defined as 10-minute peak and 15-minute off-peak headways for at least 14 hours of service operations per day. This mode may include portions of service that are fixed-guideway and non-fixed-guideway.

SR.  Streetcar rail – Rail systems operating routes predominantly on streets in mixed-traffic. This service typically operates with single-car trains powered by overhead catenaries and with frequent stops.

TB.  Trolleybus – Fixed-route service using rubber tire buses powered by electric current from overhead wires using trolley poles. Service using rubber tire replica trolleys or historic trolleys, powered by an on-board motor are not included in this mode.

TR.  Arial tramway – A system of aerial cables with suspended vehicles.

VP.  Vanpool – A commuting service operating under prearranged schedules for previously formed groups of riders in vans.

YR.  Hybrid rail – Rail systems primarily operating routes on the National system of railroads, but not operating with the characteristics of commuter rail. This service typically operates light rail-type vehicles as diesel multiple-unit trains (DMU’s).