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{{Infobox Systems Thinking for Safety}}
==Summary==
'''Continual adjustments are necessary to cope with variability in demands and conditions. Performance of the same task or activity will vary'''

'''''Understand the variability of system conditions and behaviour. Identify wanted and unwanted variability in light of the system’s need and tolerance for variability'''''

[[File:Performance variability.jpg|none|thumb|500px|"“Performance variability is both normal and necessary, and it is mostly deliberate. Without performance variability, success would not be possible." Image: [https://flic.kr/p/dA481J NATS Press Office] CC BY-NC-ND 2.0]]

In organisations, demand is at least partly unpredictable, resources fluctuate, and goals and norms shift. System conditions and preconditions for performance are not completely knowable and they vary over time. This means the work cannot be specified precisely in procedures and so people must make continuous approximate adjustments in order to adapt to the system conditions. Performance variability, at the level of the organisation, airspace, team and individual, is both normal and necessary, and it is mostly deliberate. Without performance variability, success would not be possible. Variability is always there, even if the procedures do not account for it and if those at the blunt end are not aware of it. In order to understand work, one must understand how and why performance varies.

To respond to varying demand, we adjust and adapt. For operational staff, this involves moment-to-moment adjustment. Obvious examples are adjustments to spacing on final approach to reduce delay and optimise runway utilisation. Further away from the front-line, the time-scales for adjustments are longer.

Variability of any function does not exist in isolation – it is affected by the variability of other functions and the system as a whole. Therefore, the variability of all relevant functions needs to be considered.

The predictability, variability and adequacy of the various preconditions and conditions of performance relating to people, procedures, equipment and organisation affects the variability of these functions. These include system conditions or states (e.g. runway clear, aircraft at position, upload complete), previous task steps or activities (e.g. landing clearance, coordination, system input), and resources (information, staffing, procedures, working environment, equipment, etc).

Variability may be fairly predictable, or may be irregular, but with an historical experience base. Or it may be inherently unpredictable, and outside the historical experience base, including new, unanticipated, emergent variation, perhaps associated with abnormal or previously neglected issues within the system.

Performance variability has many reasons, and attempting to reduce variability without first understanding it may limit the degrees of freedom to select different options to deal with a situation. Hardening constraints, by introducing stricter rules and more procedures, may not be sustainable strategies. But by understanding variability, you increase your knowledge of the system.

To get this understanding, you cannot only ask why something goes wrong. You need to ask why things normally go right. For example, take a routine scenario in ATC: an aircraft gets airborne and is transferred from tower to approach. A situation like this is very likely to be handled in different ways, for lots of reasons. People will find ways to fill in the gaps in the system, with various adjustments to balance various goals.

From a higher level perspective, there are a few crucial questions: 1) Is performance variability within acceptable limits? Variability leads to success within a certain range of tolerance. But this tolerance is not fixed, and will itself vary over time with the system conditions (e.g. demand and constraints). 2) Is the system operating within the desired boundaries? Performance variability of various functions and flows of work will combine and interact at a system level and may approach certain boundaries. 3) Are adaptations and adjustments leading to drift into an unstable or unwanted system state? Drift happens slowly, and can be difficult to identify from the inside without appropriate measures. System level data on normal performance are needed to answer these questions. Where unwanted variability is identified, this will mean acting on the system (e.g. demand, resources and flows of work), not the person.

== Practical advice ==
* '''Understand variability past and present.''' Try to get a picture of historical variation in system performance. Consider what kind of variation can be expected given the experience base, how performance varies in unusual ways, and what is wanted and unwanted in light of the system’s need and tolerance for variability.
* '''Be mindful of drift.''' Variability over the longer term can result in drift into an unwanted state. Consider what kind of measurements might detect such drift.
* '''Understand necessary adjustments.''' Operators must make continuous adjustments to meet demand in variable conditions. The nature of these adjustments and adaptations needs to be understood in normal operations, as well as in unusual situations.

== View from the field ==
'''Marc Baumgartner''' ATCO, Skyguide, Switzerland; Former President and CEO, IFATCA

<blockquote>''“Air traffic management can be compared to the story of ‘Beauty and the Beast’. Front-line staff love to perform well. It is the nature of operational work that, in amongst the more routine work, we must respond to high demand situations that stretch the system’s capability. In some cases, the beauty appears; demand is high but resources are good and the work flows. As a controller, this could mean working 90 movements on an airport where the declared capacity is 75 per hour. In such cases, operational staff feel very dynamic, flexible, and creative. But in other cases, the beast rears its ugly head. By surprise, unknown system features or behaviours emerge, turning our job into a real struggle. In both cases, it is necessary to make constant adjustments to developing situations. The fascinating thing is that the system can oscillate rapidly from beauty to beast to beauty again. The ATM system is intrinsically unstable and things only go right because we make them go right via our ability to vary our performance. It is the nature of our ordinary everyday work to transform the ‘beast’ into a more stable and safe system.”''</blockquote>


Source: [http://skybrary.aero/bookshelf/books/2882.pdf Systems Thinking for Safety: Ten Principles. A White Paper. Moving towards Safety-II], EUROCONTROL, 2014.

The following [http://www.skybrary.aero/bookshelf/books/3380.pdf Systems Thinking Learning Cards: Moving towards Safety-II] can be used in workshops, to discuss the principles and interactions between them for specific systems, situations or cases.