### 经济代写|微观经济学代写Microeconomics代考|Theories and Models

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 经济代写|微观经济学代写Microeconomics代考|Theories and Models

In mainstream economics, at least, theories have models as their “logical backbones.” A model is a collection of assumptions and hypotheses that are linked by the rules of logic and mathematics. A model makes several assumptions about an aspect of reality and derives hypotheses from these assumptions in a logically consistent way. To understand the difference between theories and models, look at the following example.

Assume one wants to develop a theory about the functioning of the price mechanism on markets. In order to do so, one thinks about, for example, the way individuals sell and buy their stuff and how these buying and selling decisions explain the formation of prices. This structured way of thinking is one of the models underlying one’s theory.

The crucial function of a model, in the context of theory formation, is to make sure that the key causal mechanisms underlying a theory are made explicit and logically consistent. Look at the following model to understand why:
Model 1
Assumption 1: All human beings are in the streets.
Assumption 2: Peter is a human being.
Hypothesis: Peter is sitting at my home.
“Model 1 ” is a model because it has a set of assumptions and a hypothesis, but the hypothesis does not follow logically from the assumptions. In this case, the model is logically inconsistent, even though the hypothesis might be correct empirically (Peter is sitting right next to me). The point is that the assumptions cannot explain my observation, which makes the model useless for any theory. A consistent model is therefore a necessary condition for a good theory.

## 经济代写|微观经济学代写Microeconomics代考|The Virtue of Thriftiness

An important criterion for good models is simplicity, frugality, or thriftiness. The idea is often referred to as Ockham’s razor (named for an English Franciscan Friar in the fourteenth century), which states that, among competing models, the one with the fewest assumptions should be selected. However, this concept is much older. Aristotle $(2004)$, in his Posterior Analytics, stated that, “we may assume the superiority ceteris paribus [all things being equal] of the demonstration which derives from fewer postulates or hypotheses.” Ockham’s razor is widely accepted among economists. (Solow, 1997, p. 43) summarizes the self-image of the profession in a very concise way: “Today, if you ask a mainstream economist a question about almost any aspect of economic life, the response will be: suppose we model that situation and see what happens. [… ] A model is a deliberately simplified representation of a much more complicated situation. […] The idea is to focus on one or two causal or conditioning factors, exclude everything else, and hope to understand how just these aspects of reality work and interact.”

Ockham’s razor necessarily implies that the assumptions of a model should not be realistic in the naïve sense that the assumptions shall fit reality. Scientific theory building necessarily reduces complexity to make a situation comprehensible for the human mind. Robinson (1962) found a nice expression for the problems implied by models built on “realistic” assumptions: “[a] model which took account of all the variegation of reality would be of no more use than a map at the scale of one to one.” However, the epistemic problem goes even deeper, as illustrated by the novel Tristram Shandy by Sterne (2003). The book is the autobiography of the protagonist, which is so detailed that it takes the author 1 year to write down a single day of his life. From this perspective, the map is even more detailed than the territory and the level of detail one considers adequate must be based on a subjective value judgment.
Maps have to simplify in order to be useful. On the other hand, is there a “right” way to simplify? The answer to this question must also be “no,” because it depends on what one wants to do with the map. If one is driving a car, contour lines are not essential and may easily distract attention from other more important information. However, if one is planning to hike in the mountains, contour lines are crucial. Therefore, a good simplification depends on its purpose.

## 经济代写|微观经济学代写Microeconomics代考|Do Assumptions Matter

If assumptions shall not be realistic, then maybe one can conclude that assumptions do not matter at all. This position has, in fact, been put forward by (Friedman, 1953 , p. 14), one of the most influential economists of his time. He proposed that “Truly important and significant hypotheses will be found to have ‘assumptions’ that are wildly inaccurate descriptive representations of reality, and, in general, the more significant the theory, the more unrealistic the assumptions (in this sense).”

There is some debate as to whether Friedman adheres to the extreme position that assumptions do not matter at all (called instrumentalism) or not but, for the sake of argument, consider this position and see where it leads. According to an instrumentalist’s view, one should judge a theory according to the validity and usefulness of the hypotheses, whereas the assumptions are irrelevant. Does this position make sense? Look at the following model.
Model 3
Assumption: Seatbelts reduce the likelihood of fatal accidents.
Hypothesis: Seatbelts reduce the likelihood of fatal accidents.
Model 3 looks like a pretty nonsensical waste of time and is an example of a circular argument, but why does one find it intuitively unconvincing? The hypothesis can be empirically tested and it has been confirmed by the data. Therefore, according to an instrumentalist’s view, a theory that is built on this model passes the test of usefulness. The idea that assumptions are completely irrelevant is, of course, flawed because it prevents one from learning anything about the causal mechanisms that drive the hypotheses, if one cannot rule out the trivial model where hypotheses and assumptions coincide. Even if one’s mind can never grasp the true causal mechanism, and thus one has to be satisfied with crude narratives and heuristics, declaring the assumptions irrelevant leaves one with only cookbooks.

Instrumentalism is an extreme position and there are reasons to assume that Friedman’s own position is more balanced. He argues that the role of a positive science “is the development of a ‘theory’ or ‘hypothesis’ that yields valid and meaningful (i.e., not truistic) predictions about phenomena not yet observed.” It can be argued that the term “truistic” refers to models of the above type that are only uninteresting tautologies. In the end, scientific theory building has a subjective component, because the balance between, on the one hand, meaningful simplifications of the assumptions and of the supposed causal mechanisms, and, on the other hand, the explanatory power of the hypotheses cannot be precisely nailed down. It is the art and craft of experienced scientists to see if a theory is “in balance” in this sense.

## 经济代写|微观经济学代写Microeconomics代考|Theories and Models

“模型1”是一个模型，因为它有一组假设和一个假设，但假设不是从假设逻辑上得出的。在这种情况下，模型在逻辑上是不一致的，即使假设在经验上可能是正确的（彼得坐在我旁边）。关键是这些假设无法解释我的观察结果，这使得该模型对任何理论都无用。因此，一个一致的模型是一个好的理论的必要条件。

## 经济代写|微观经济学代写Microeconomics代考|Do Assumptions Matter

Model 3 看起来是一种非常荒谬的浪费时间，是循环论证的一个例子，但为什么人们会觉得它在直觉上没有说服力呢？该假设可以通过经验检验，并已被数据证实。因此，根据工具主义者的观点，建立在这个模型上的理论通过了有用性的检验。当然，假设完全不相关的想法是有缺陷的，因为如果不能排除假设和假设重合的琐碎模型，它会阻止人们了解驱动假设的因果机制。即使一个人的头脑永远无法掌握真正的因果机制，因此必须满足于粗糙的叙述和启发式方法，但宣布这些假设无关紧要，只会留下食谱。

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## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。