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WFM Glossary

Workforce Management Terminologies, definitions, formulas and Erlang calculators!

Erlang Calculator click here

Two-Skill Calculator click here

Calculators click here

Browse the glossary using this index

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ALL


Abandoned Call

A call that is canceled by the customer before contact with an agent was made. See also redial.


Automatic Call Distributor, a part of a PABX that can distribute calls that arrive on one or more numbers to extensions which are part of one or more groups that are assigned to that number.


Average Excess, alternative service level definition, defined as the average time calls wait beyond the AWT.

After Call Work


An employee who works in a Call Center, also called (Call Center) representative ('rep'), or CSR (customers sales representative).


Average Handling Time, the time agents spend on average on a service request, usually a call, an email or a chat. The wrap-up time is also included. See also handling time.


A step-by-step method to solve a certain, often computational, problem.


Also known as business analytics, a fact-based approach to business process and product improvement using advanced mathematics and ICT.


Automatic Number Identification, a technique used to identify customers by their telephone number. Used in combination with CTI to show right away customer information on the agent's computer screen.


Average Speed of Answer, the average time a call waits before being connected to an agent.


Acceptable Waiting Time, the target upper bound to the waiting time, often equal to 20 seconds, used in the definition of the SL. Also called Time to Answer (TTA) and Service Time (ST).



Business-to-business, relative to commercial activities between businesses.


Business-to-consumer, relative to commercial activities between business and end-consumers.


Business process outsourcer, a company offering call center services, often to multiple organizations at the same time, regularly from a cheaper off-shored location such as India or the Philippines.


Call Blending

A way of handling different call queues, for example inbound and outbound calls, at the same time by assigning them in a dynamic way to one or multiple agent groups. Call blending is often implemented as a way of maximizing occupancy.

Call Center

A collection of resources (typically agents and ICT equipment) capable of delivering services by telephone.

Call Center Arithmetics

Basic WFM calculations based on totals and averages, without taking fluctuations and flexibility into account. See also call center mathematics.

Call Center Mathematics

Advanced WFM calculations going beyond call center arithmetics, taking fluctuations and flexibility into account.


In the context of contact centers, a means to have contact with customers. Examples are telephony, fax, and internet.

Contact Center

A collection of resources (typically agents and ICT equipment) capable of delivering services through multiple communication channels.

Cost Center

From Wikipedia: ”In business, a cost centre is a division that adds to the cost of an organization, but only indirectly adds to its profit. Typical examples include Research and Development, Marketing and Customer service.”


Customer Relationship Management, mainly used to denote computer systems that allow to record and retrieve interactions with the customer. CRM systems made the advent of call centers possible.


Denotes an agent who has more than one skill, who can therefore handle more than one type of call. A generalist is fully cross- trained. Also denoted as X-trained.


Computer-Telephony Integration, the process that enables communication between and integration of telephone equipment and computer systems.


Data Mining

A field of science aimed at discovering relations in data. It has a big overlap with statistics, but it is less mathematical, having its origins in computer science.

Decimal Separator

The Decimal Separator is the charactar used to indicate where the fractional part of a number begins and where the integer part ends.

The symbol used for the ecimal separator differs per country and is either a dot '.' or a comma ','. Because of this, the decimal separator also differs between versions of excel.

On this website we use the English version of excel, therefore we use the dot as the decimal separator.


Dialed Number Identification Service, a part of the ACD that recognizes the calling number.



Danish mathematician who created the safety staffing models. Also the dimensionless unit of load.


A safety staffing model that takes fluctuations in the number of arrivals and handling times into account.


A safety staffing model that, in addition to the Erlang C model, takes abandonments and retrials into account.



First Come First Served, refers to the orders in which queued calls are served: in the order of arrival.


First Call Resolution applies when a call is handled right the first time, not necessitating a second call. The FCR rate is the fraction of first calls for which this is the case. See also reconnect.


The prediction of (exogenous) variables such as call volume, AHT and patience.



An agent who has all skills and/or can handle all types of calls.

Gross Workforce

Measure of the workforce, which is equal to net workforce plus shrinkage.

Gross Workload

Measure of the workload, which is equal to net workload plus safety staffing. Equal to net workforce.


Handling Time

The time spent to handle a service request, which can be calculated over the following PABX metrics: Handling time = Ring time + Talk time + Hold time + Transfer time + Wrap-up time. See also AHT.



Information and Communication Technology, technology relative to computers and technology-assisted communication.

Intra-Day Management



Interactive Voice Response, part of an ACD that allows a customer to enter information by responding to instructions through the keypad of the telephone or the VRU.



Longest idle agent, an agent selection rule used in an ACD mechanism. The agent with the longest idle time since his/her last phone activity (after call work), will be offered the next call.


Amount of work offered to the system, defined by the product of the forecast and the AHT, measured in Erlang. For example: if the forecast of the arrival rate is 4/min and the AHT is 3 minutes, then the load is 4 * 3 = 12 Erlang.


Longest waiting call, a call selection rule used in multi-skill call centers.



The mean (or average) of a series of numbers of length n is defined as the sum of the numbers in the series divided by n. In formula form this is shown as follows:

\( AV = \frac{1}{n} \sum_{i=1}^{n} a_i = \frac{1}{n}(a_1+a_2+a_3+\cdots+a_n) \)

For example: if our series of numbers is 1 2 3 4 5, then the mean is equal to \( \frac{1}{5} \times (1+2+3+4+5) = \frac{1}{5} \times 15 = 3 \).

The excel function for the mean is AVERAGE()


Quantitative measures, commonly used for assessing, comparing, and tracking performance of a system. In contact center, a group of metrics is often displayed in a dashboard for analysis and decision supporting. For example, Handling time, Occupancy, Shrinkage, Longest Idle Agent, etc.


Used throughout the fellowship course as mathematical model, a description in mathematics terms of part of a system, that allows an analysis of certain aspects of that system. For example, a simulation model.


Net Workforce

Equal to gross workload.

Net Workload

The amount of agent time needed to handle a certain amount of work, without accounting for safety staffing. For example, during a day, there are 1000 calls with an AHT of 4 minutes. Then the net workload is 4000 minutes.


Informal statistical term meaning unexplained variability (as in Poisson noise).



The time that an agent is handling calls (talk time plus wrap-up) divided by the total time that the agent is available for handling calls. It can be calculated by; net workload / gross workload or net workload / net workforce. Not to be confused with utilization or productivity.

Offered Load

See load.


Operations Research, the science that uses mathematical models to improve business operations. Also known as Management Science, therefore sometimes called OR/MS. See also

Order of Operations

When we speak about operations mathematically, it means things like add, subtract, multiply, divide, squaring, etc. Remember: If it isn't a number it is probably an operation.

When you see something like ...

7 + (6 × 52 + 3)

... what part should you calculate first? 

Start at the left and go to the right? 

Or go from right to left?

Warning: Calculate them in the wrong order, and you will get a wrong answer !

So, long ago people agreed to follow rules when doing calculations, and they are:
Do things in Parentheses First. Example:
yes 6 × (5 + 3)=6 × 8=
no 6 × (5 + 3)=30 + 3=
Exponents (Powers, Roots) before Multiply, Divide, Add or Subtract. Example:
yes 5 × 22=5 × 4=
no 5 × 22=102=
Multiply or Divide before you Add or Subtract. Example:
yes 2 + 5 × 3=2 + 15=
no 2 + 5 × 3=7 × 3=
Otherwise just go left to right. Example:
yes 30 ÷ 5 × 3=6 × 3=
no 30 ÷ 5 × 3=30 ÷ 15=
How Do I Remember It All ... ? PEMDAS !
Parentheses first
Exponents (ie Powers and Square Roots, etc.)
Multiplication and Division (left-to-right)
Addition and Subtraction (left-to-right)




Private Automatic Branch eXchange, the telephone switch local to the company.


The amount of time a customer is willing to wait before abandoning the queue.


The organizational process of creating and maintaining a plan.


French mathematician (1781-1840) who discovered the Poisson distribution which mathematically describes arrivals to a call center. Also used as Poisson process, focusing on the times of arrivals, and Poisson noise, focusing on the error with respect to the forecast.

Predictive Dialer

Functionality of an ACD that allows outbound calls to be automatically initiated, anticipating future availability of agents.


The time that an agent is available for handling calls divided by the total time that the agent is paid. It can be calculated by; gross workload / gross workforce, or net workforce / gross workforce. Not to be confused with utilization or occupancy.


Quality of Service

See SL.


Real-Time Performance Management



The fact that a caller, after having been served, calls back after some time for the same service. See also FCR.


The fact that a caller, after having abandoned or being blocked, calls back after some time for the same service. See also abandonment.


The fact that a caller, after having been served or not, calls back for the same service. Comprises of redials and reconnects.


Real-Time Performance Management, the activity having as goal on the day of execution to take actions as to obtain the required SL as good as possible.


Safety Staffing

The additional number of agents needed to reach a certain service level. The safety staffing level is often calculated by means of the Erlang-C or Erlang-X formulas.


Skill(s)-based routing, the fact that different types of calls are routed to different agent groups based on the type of the call and the skills of the agents.


The process of deciding how to commit resources between a variety of possible tasks.


A forecasting term meaning that a pattern repeats itself, in call centers typically at intra-day, intra-week and intra-year level.

Short Abandon

A call that abandoned very quickly making it unlikely that the caller was already tired of waiting.


The fraction of paid time that an agent is not available for taking calls because of holidays, training, paid breaks, etc.

Below you can see an example of a shrinkage table. It is important to note that this is merely an example to illustrate how shrinkage is calculated. There can be many more activities that can be added and not all activities are relevant for all companies.


The activity of mimicking a real-world system over time, usually performed with a computer to evaluate the performance of such a system.

Skill Group

A group of agents all having the same skill set.

Skill Set

The set of skills that an agent or group of agents have.


Service Level, a somewhat ambiguous term that in the broad sense can refer to all aspects of service (waiting time, abandonments, and so forth). In the narrow sense, it is defined as the percentage of calls answered within the AWT. The industry standard is 80/20: 80% of the calls have to be answered within 20 seconds. Quality of Service is a synonym of the former broader meaning.


Service Level Agreement, the contract between a business unit and higher management or an outsourcer with its client company concerning the required service levels.


An agent who can only handle one type of call. It can also mean an agent who can do even the most difficult calls, in contrast with a generalist. To avoid ambiguity it is better to use the term single-skilled agent.


Single skill first, an agent selection rule used in multi-skill call centers, that assigns calls to agents with the least number of skills.


Service Time, see AWT.


Traffic Management

Synonym to RTPM.


Telephone Service Factor, see SL.


Time To Answer, see AWT.



The time that an agent is handling calls (talk time plus wrap-up time) divided by the total time that the agent is paid. It can be calculated by: net workload / gross workforce. Utilization = occupancy * productivity. Not to be confused with productivity or occupancy.



Voice Response Unit, synonym to IVR.


Waiting Time

The time a call spends between entering the queue (often after a recorded message, or after having made a choice in a VRU) and an agent being connected to the call.

Weighted Mean

The weighted mean is similar to the general mean, except that some points contribute more to the mean than some others. Mathematically if we have a set of points \(a_n\) and a set of weights \(w_n\) that belong to the points, then the weighted average is equal to

\( WAV=\sum_{i=1}^{n}\frac{a_iw_i}{w_i} = \frac{a_1w_1 + a_2w_2 + \cdots + a_nw_n}{w_1+w_2+\cdots+w_n} \)

for example, if we have two sets of points, 1 2 with weight 1 and 3 4 5 with weight 2, then the weighted average of all points is equal to

\( \frac{1 \times 1+2 \times1+3 \times2+ 4 \times2+5 \times2}{1+1+2+2+2}=\frac{1+2+6+8+10}{1+1+2+2+2}=\frac{27}{8}=3.375 \)

In excel there are two ways to calculate the weighted average. say we have the values in column A and the weights in column B.

In the first method we set the product of A and B in column C. Then, the weighted average is given by WAV = SUM(C:C)/SUM(B:B)

The second method uses the sumproduct function. This method doesn't require an additional column to be used. WAV = SUMPRODUCT(A:A,B:B)/SUM(B:B)


Workforce Management consists of all activities from forecasting and planning to online control that have to do with the employment of agents in call center.

WFM Tool

A computer tool that assists planners with their WFM tasks. It minimally consists of forecasting, Erlang C, and agent scheduling modules.


Workforce Optimization includes WFM, call monitoring and agent performance management.


The amount of available or required agent time when used in capacity calculations. Can be the net or gross workforce. Should match the workload.


The amount of work on incoming call volume during a certain amount of time, usually measured in minutes. Can be measured without safety staffing (net workload) or with safety staffing (gross workload). Not to be confused with offered load.

Wrap-up Time

Time after the end of a call that the agent spends on the call. Consist usually of entering call-related data in a computer system.



Synonym to cross-trained.