How many monitors should a CCTV operator view?

December 2004 CCTV, Surveillance & Remote Monitoring

How many monitors can a CCTV operator review effectively? It is a question I get asked repeatedly and it occurs in just about any country where there is CCTV.

Given the cost considerations of CCTV, management is often keen on putting as many monitors in front of an operator as possible. From an engineering point of view, the surveying task rarely incorporates an analysis of monitor to operator ratios on the basis of any security risk factor. If anything, getting more monitors up in front of operators looks technically impressive. However, by putting too many monitors up to be watched by one person, are we defeating the objective of having somebody detect incidents on a reliable basis?

Getting an answer to this question is just about impossible at the present time. Little research has been done on the question. What has been done shows a simple common sense result - as we add more monitors, detection rates go down.

A research project by Jim Aldridge at the UK Police Scientific and Development Branch (PSDB) some years ago looked at how well operators could detect somebody with an umbrella in the main street using different numbers of monitors. They found that observers viewing one, four, six and nine monitors showed accuracy detection scores of 85%, 74%, 58% and 53% respectively in picking up the person with the umbrella. The PSDB research also indicated that observers were significantly less likely to detect targets at greater depths (or in the background area) within the image when there were more monitors. Research at the TNO-FEL research organisation in Holland is currently underway looking at the impact of increasing the number of monitors for CCTV and I am sure there will be a similar trend.

Walking into various control rooms I have encountered anything from three to about 35 monitors per operator. Interestingly enough, in an environment such as casinos where the detection of theft is critical and things are fast moving, it is rare to have more than five monitors per operator, including control screens. Similarly, most diamond monitoring operations have between three and six screens.

Teaming up a couple of operators to watch a common monitor wall as well as their own specific spot monitors is less common but one way of approaching this numbers issue. This kind of approach is used both in Cape Town and Johannesburg city centre operations for example. Something that comes through for certain is that a single screen is the ideal solution, but even that does not guarantee detection, and it is likely to be impractical to run any CCTV scheme with such an arrangement.

Points to consider

Given that we have no formulae at this stage to calculate the right kind of ratio, I have outlined some critical things to consider below. Remember these have to be taken in combination and they are purely to a guide to put things in perspective.

Is the behaviour obvious and easy to detect?

If the behaviour stands out on screen and is easily noticeable, more monitors can be used. Common assaults and gang fights in a city centre, for example, are easier to detect than a subtle disguised movement at a roulette table. However, there are also many street crimes such as pickpockets and drug dealing that are more difficult to pick up. If the incident causes a disruption when it has happened, then it is easy to recognise something has occurred. If nobody notices because the action has been so subtle, all the responsibility falls on the operator.

How complex are the scenes and the backgrounds - ie, to what extent can behaviour be identified against complex visual backgrounds?

The more that is going on within a scene, the more difficult it is to detect something in particular. Seeing a person walking down an empty street on Sunday morning is a lot more difficult than monitoring the movement of people in the walkways of a major shopping centre.

How easy is it to tell the difference between normal and incident behaviour?

Where criminals attempt to camouflage incident behaviour by blending in with the activities around them, it requires far more attention by the operator to what is going on.

How many types of incidents can occur?

The more that can occur, the more focus and decision-making needs to go into it. One finds selective attention works with a limited number of things to look out for. You screen out the things that do not matter. X-ray screeners, for example, who get told to focus on bomb material, spot more of these images during testing. But they also spot less of everything else. If you are looking for incidents that can happen in any number of ways, you really have to pay attention to the screen.

How long do the incidents take to develop?

In some cases, criminals have to set up the scene for the incident, position themselves, wait for the right moment and then strike. This gives the operator more time to spot the incident in process than a quick opportunistic grab at something.

Do the incidents take place in the foreground, middle ground, or background or a combination of these?

If it is a combination, then operators have to look through the picture rather than just at it. It has been proven time and again that people can miss incidents happening in the background because they are focusing on something more prominent in the foreground part of the picture.

How long are monitor displays maintained unchanged?

The longer a particular picture is on screen, the more the operator gets a feel for it and can 'tune in to the scene'. It is called situational awareness and allows operators to spot changes in the conditions more easily. If monitor views are cycled on a continual basis, the operator has to familiarise themselves with the situation each time.

What is being monitored?

Is behaviour relatively structured and predictable in the scene being monitored, or can things change rapidly and without warning, even when an incident has not occurred? Stable environments are easier to keep track of than changing environments.

What is the lighting and brightness of the scene displayed by the monitors?

Dark and shadowy areas are more difficult to view than brightly light scenes. However, putting bright and dark scenes on monitors close together is a major problem. Things like rain can also change the viewing dynamics.

What is the chance of incident behaviour occurring simultaneously on more than one monitor?

Generally, certain scenes or locations tend to drop off and become quieter at certain times. Operators often use this to 'rest' from paying attention to those areas and paying more attention to areas where things are happening. However, if there is an equal chance of things happening across all monitors and they all have to have equal attention, dividing your time between them has a direct impact on the likelihood of detection.

There are a number of other factors that will influence the number of monitors. If operators are required to perform different tasks (phone, logging, access control etc), the attention to the CCTV task as a whole goes down and consequently attention to screens becomes even more diluted.

The size of the screens, clarity of the picture, and viewing distance from the operators are also factors to consider. Assisted viewing such as screens being triggered by alarm events can both improve detection and, where extensive false alarms are encountered, reduce it.

A correct answer to how many screens are appropriate could be available at some stage in the future through the use of a formula that considers the above factors. In the meantime, we have to use the common sense solution. Also to recognise there will be busy periods and less active periods.

The bottom line is how important is it for your operation to detect a security risk - that way you can quantify the cost of failure against the cost of manpower and facilities that go into making your 'operator to monitor' ratio as effective as possible.

For more information contact Craig Donald, Leaderware, 011 787 7811, craig.Donald@leaderware.com, www.leaderware.com


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