American Education

本文详细介绍了美国高等教育体系,包括不同年级学生的分类、课程选择、学分制度、转学流程及评分标准等内容,并概述了本科及研究生教育的具体情况。

The "junior" and "senior," or third and fourth years, are the "upper classes." Students in these years are known as "juniors" and "seniors"- "upperclassmen." When they enter their junior year, they must choose a "major" field of study. They must take a certain number of courses in this department, or field. In some schools, students also choose a "minor" field. There is usually time for students to choose several other "elective" (extra) courses in other subjects.

Each student is assigned a "faculty advisor" who teaches courses in the student's major field. This advisor helps the student select a program of study.

An international student will also have an "International Student Advisor." This person helps the international students adjust to U.S. life, handles visa and other paperwork problems, and organizes activities for international students.

Classroom learning: Classes range from large lectures for several hundred students to smaller classes and "seminars" (discussion classes) with only a few students. Students enrolled in lecture courses are often divided into smaller groups, or "sections." The sections meet separately to discuss the lecture topics and other material.

Professors usually assign textbook and other readings each week. They also require several written reports each semester (term). You will be expected to keep up to date with the required readings in order to join in class discussions and to understand the lectures. Science students are also expected to spend time in the laboratory.

Academic year: The school calendar usually begins in August or September and continues through May or June. It is a good idea for international students to enter U.S. universities in autumn. Most new students enter at this time, so they can adjust together. Also, many courses are designed so students will take them in sequence, starting in autumn and continuing through the year.

The academic year at many schools is composed of two terms or semesters. Other schools use a three-term calendar known as the "trimester" system. Still others divide the year into the "quarter" system of four terms, including a summer session which is optional.

Credits: Each course is considered to be worth a number of "credits" or "credit hours." This number is roughly the same as the number of hours a student spends in class for that course each week. A course is typically worth three to five credits.

A full program at most schools is twelve or fifteen credit-hours (four or five courses per term). International students are expected to enroll in a full program during each term.

Transfers: If a student enrolls in a new university before finishing a degree, usually most credits earned at the first school can be used to complete a degree at the new university. This means a student can transfer to another university and still graduate within a reasonable time.

Marks: Professors give each student a mark or "grade" for each course. The marks are based upon:


Classroom participation.
Discussion, questions, conversation; Students are expected to participate in class discussions, especially in seminar classes. This is often a very important factor in determining a student's grade.


A midterm examination.
Usually given during class time.


One or more research or term papers, or laboratory reports.

Possible short exams or "quizzes."
Sometimes the professor will give an unannounced "surprise quiz." This doesn't count heavily toward the grade but is intended to inspire students to keep up with their assignments and attendance.


Final examination.
Held some time after the final class meeting.

Advanced placement: Some colleges and universities give college credit to students for work they have done in high school. Some schools also give advanced standing to students who prove that they have achieved college level proficiency in a certain subject.

This means that a student who is just entering college, a freshman, can take courses normally only open to sophomores.

The school will probably ask the student to take a test to prove that he or she can do sophomore-level work, or the school will give advanced placement to a student who has scored exceptionally high on college admission exams. Students who have completed "A" levels at home often receive advanced placement standing.

Postgraduate (Graduate) Education
In order to find a professional job, a college graduate with a Bachelor's Degree today usually will want to consider graduate study.
Students from some countries are only permitted to study overseas at the graduate level. Because requirements are different in many countries, you should inquire about the credentials you will need to get a job in your country before you apply to a postgraduate school in the U.S.A.

Master's Degree: Many international students are able to qualify for the jobs they want after they have earned a Master's Degree. This degree is usually required in fields such as library science, engineering, or social work. The M.B.A., or Master of Business Administration, is an extremely popular degree that usually takes two years. Some Master's programs, such as journalism, only take one year.

In an academic Master's program, students study such fields as history and philosophy. These degrees are considered stepping-stones toward a doctorate (Ph.D.).

Most time in a Master's program is spent in classroom study. A Master's Degree candidate usually must prepare a long research paper called a "master's thesis."

Doctorate (Ph.D.): Many graduate schools consider the Master's Degree as the first step towards attaining the Ph.D. (doctorate). But at other schools, students may prepare directly for the doctorate without also earning a Master's Degree. It may take three years or more to earn the Ph.D. Degree. For international students, this time may be as long as five or six years.

For the first two years, most doctoral candidates enroll in classes and seminars. For at least another year, students will conduct firsthand research and write a thesis or dissertation. This paper must contain views, designs, or research that have not been previously published.

A doctoral dissertation is a discussion and summary of the current scholarship on a given topic. Most universities awarding doctorates also require their candidates to have a reading knowledge of two foreign languages, to spend some required length of time "in residence" attending class regularly, to pass a qualifying examination that officially admits candidates to the Ph.D. program, and to pass an oral examination on the same topic as the dissertation.

 

 

 

 

<think>好的,我需要帮助用户在R中实现限制三次样条分析并对分类变量进行因子化处理。首先,用户提到了使用read.csv加载数据文件an_data.csv,所以第一步应该是数据导入。确保文件路径正确,可能需要使用read.csv("an_data.csv")或者包含完整路径。 接下来是因子化处理分类变量。用户要求指定参考水平,这意味着在将变量转换为因子时,需要使用factor()函数,并通过levels参数来设定参考水平。例如,如果变量"gender"有水平"male"和"female",想以"female"为参考,可以使用factor(data$gender, levels=c("female", "male"))。这一步需要注意检查数据中的分类变量名称和可能的水平,避免拼写错误。 然后是限制三次样条分析。用户提到的是基于Cox回归模型,所以可能需要使用survival包中的coxph函数,以及rms包中的rcs函数。但需要确认用户是否已经安装了这些包,如果没有,需要先安装并加载。例如,使用install.packages("rms")和library(rms)。此外,限制三次样条通常需要指定节点位置,这里可能需要使用ns()函数来自splines包,或者rcs()函数,具体取决于包的使用方法。用户可能需要确定节点的数量和位置,通常默认是基于数据分位数。 在代码编写部分,需要先处理因子变量,然后构建模型。例如,假设分类变量是“gender”和“stage”,而连续变量是“age”,时间变量是“time”,状态变量是“status”。模型可能类似于coxph(Surv(time, status) ~ rcs(age, 3) + factor(gender) + factor(stage), data=data)。这里需要注意rcs的节点数,比如3个节点,但用户可能需要根据实际情况调整。 另外,指定参考水平的时候,因子化步骤中必须正确设置levels参数,确保第一个水平是参考水平。例如,如果stage的参考水平是“stage1”,则需要factor(stage, levels=c("stage1", "stage2", "stage3"))。 还需要检查数据中是否存在缺失值,处理缺失值的方法可能需要用户决定,比如na.omit()或者插补,但用户的数据处理步骤中没有提到,可能需要提醒用户注意。 最后,绘制图形部分可能需要使用rms包中的相关函数,或者ggplot2来可视化样条曲线。但用户的主要请求是代码实现,可能不需要图形部分,但可以提及。 总结步骤: 1. 导入数据 2. 因子化分类变量并指定参考水平 3. 加载必要的包(survival, rms, splines等) 4. 构建Cox模型,使用rcs处理连续变量 5. 输出模型结果 需要验证代码是否正确,例如检查是否正确定义了因子和样条项。同时,可能需要处理可能的错误,比如包未安装,变量名错误等。最后,生成相关问题,可能涉及模型解释、节点选择、因子化注意事项等。</think>以下是分步实现的R代码解决方案: ```r # 1. 加载必要程序包 library(survival) library(rms) library(splines) # 2. 数据读取与预处理 data <- read.csv("an_data.csv", stringsAsFactors = FALSE) # 3. 分类变量因子化(示例变量:gender, stage) # 假设gender包含"male"和"female",指定参考水平为"female" data$gender <- factor(data$gender, levels = c("female", "male")) # 第一个水平为参考 # 假设stage包含"I","II","III",指定参考水平为"I" data$stage <- factor(data$stage, levels = c("I", "II", "III")) # 4. 构建限制三次样条模型(示例变量:age) # 先验证数据完整性 dd <- datadist(data) options(datadist = "dd") # 建立Cox比例风险模型,对age变量应用3节点限制三次样条 model <- cph(Surv(time, status) ~ rcs(age, 3) + gender + stage, data = data, x = TRUE, y = TRUE) # 5. 模型结果输出 print(model) summary(model) ``` 代码说明: 1. 因子化处理使用`factor()`函数,通过`levels`参数明确指定参考水平[^1] 2. `rcs()`函数来自rms包,用于创建限制三次样条项,3表示节点数 3. 数据预分布设置`datadist()`可确保预测函数正常工作 4. 模型结果包含样条项的系数和非线性检验
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