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Model:
A model is an example of a physical, mathematical, program, tool etc which allows one to test how a system or a process work and is very similar to the real life system.

The main hazards of computer modelling can be summarised under a few headings: (Taken from http://www.numberwatch.co.uk/computer_modelling.htm) //Assumptions:// At almost every stage in the development of a model it is necessary to make assumptions, perhaps hundreds of them. These might or might not be considered reasonable by others in the field, but they rapidly become hidden.

//Auditability//**:** In olden times, if a scientist published a theory, all the stages of reasoning that led to it could be critically examined by other scientists. With a computer model, it is possible, within a few days of development, for it to become so complex that it is a virtual impossibility for an outsider to understand it fully.

//Omissions:// Often vital elements can be left out of a model and the effect of the omissions is only realised if and when it is tested against reality. //Subconscious//: Even in relatively simple physical measurements it has been shown that the results can be affected by the desires and expectations of the experimenter.

//Sophistication:// The structure simply becomes too large and complex for the inputs that support it.

//Testability:// If a model gives a reasonably accurate prediction on a simple system then we have reasonable, but not irrefutable, grounds for believing it to be accurate in other circumstances. Unfortunately, this is one of the truisms that have been lost in the enthusiasms of the new age.

//Chaos:// Large models are often chaotic, which means that very small changes in the input variables produce very large changes in the output variables. Some very simple processes can amplify errors, taking the difference between numbers of a similar magnitude for example. The errors (or noise) are then propagated through the system. If there are feedback mechanisms present, it is quite possible for systems to operate on the noise alone.



Simulation:
Simulation is when the model of the system is tested/operated and check if there are any errors that need to be reconfigured and experimented with. It is similar to a science experiment to see how something works and if there are any improvements which could be made to it. For example, design of complexed systems such as aeroplanes.

Feedback Loop:
A system that feeds back some of the output to the input of a system, i.e. control system usually has input and output to the system; when the output of the system is fed back into the system as part of its input, it is referred to as feedback.

=Social impact of reliance on simulations to examine issues of public policy:=

Basically, public policy unlike goods usually have a much deeper social impacton people. Moreover the effects from public policies on people are significantly harder to simulate using technology. For example if a company was designing a product, a designer can simulate its behavior using a computer model and understand how the product will act in the real world. However the most fundamental issue with relying on simulations to examine the effects of public policy is that it is almost impossible to simulate human behavior in any situation. This doesnt mean that simulations that model human behavior don’t work or are unreliable. Fundamentally, the effectiveness of a simulation comes down to how sure and accurate is the acutal person, who creates the simulation of the behavior of the object he is modeling. At present, product designers break down their object's behavior into numbers and create simulations from there. By doign this the simulations tend to be very accurate, e.g. 10 plus 10 always equals 20. On the otherhand simulating human behavior, however, becomes more difficult due to the subjectivity involved.

Therefore the accuracy of simulations comes down to objectivity and subjectivity. An example of a hypothetical issue regarding public policy would be:

A decrease in interest rates. A decrease in interest rates means that saving money and depositing it in banks is no longer as attractive to people as it was before. Also, with low interest rates, businesses can afford to borrow and invest more. Hence, investment in the economy increases, leading to economic growth. Although no such model ever comes to being 100% accurate at simulating the behavior of the economy, the mathematical objectivity involved allows models simulating the economy to be fairly accurate.

=Reliability of predictions based on computer models, for example, weather, global warming=

There are a lot of issues that concern the reliability of predictions based on computer models, such as the weather predictions where there are fields of expertise all specializing on it. This paper will focus on weather about its effect and importance. The importance of the weather and the need to predict it accurately is illustrated by the fact that every local news show includes weather forecasts. People need to know what the weather will be like–either where they are or where they are going–so that they can plan their activities accordingly. One of the problems with weather forecasting is that weather changes quickly in time and space. A weather forecast at 8:00 in the morning may not be reliable at 3:00 that same afternoon. A weather forecast in one city is not the same for neighboring regions. For this reason, local weather people never say what will happen, only what is likely to happen in the local area.

The weather has such widespread impact on people’s personal lives, including their jobs, their recreation, their safety, and their property. When the weather is bad, many activities become more difficult to perform. Commercial transportation slows down on the roads, on the waterways, and in the air. Businesses of all kinds are interrupted by bad weather. Power plants and energy traders rely on knowledge of the weather to operate their equipment and to deliver power to consumers, government and business. Knowledge of the weather and associated winds allows commercial and general aviation to operate safely through hazardous weather avoidance, and to operate more efficiently through fuel savings. When all of these factors are taken together, it is easy to see that extreme cases of bad weather are responsible for the loss of tens of billions of dollars each year.

Weather also affects everyone in a sort of way as seen in the huge hurricane Katrina where trade was affected as New Orleans had one of the sites that tourists visited so a lot was lost in lives both American and foreigners and economically the city was devastated and cost the govt. about $81.2 billion.

In an effort to protect lives and property, there is a wide-range network of radars that help forecasters accurately predict severe storms and hourly weather trends. Ex: Another new system that is used by National Weather Service Forecast offices is the AWIPS computer system. AWIPS (or Advanced Weather Interactive Processing System) forecasts the process by combining a number of separate data sets and communication systems into one. Using the latest technology, this system allows meteorologists to manipulate these data sets in a myriad of combinations for use in the analysis of the weather, resulting in more timely and accurate warnings and forecasts. But it is new and costs a lot of money which some countries or organizations can’t afford and may still use the old system plus there isn’t much adaptability to it yet so a lot is still to be learned and its potential abilities.