PS和SOP有什么区别吗

如题，总觉得两者好像区别不大的样子。。。。。是不是把PS改一改就是SOP了呢？

补充内容 (2013-10-10 15:31):
我的理解是，PS写的是为什么选这个专业和这个program，还有这个program选择我的理由，而SOP只要写为什么我要选择这个program就好了

以下内容摘自Berkeley IEOR admission页面：
原文在此

"The Statement of Purpose:
Please describe your aptitude and motivation for graduate study in your area of specialization, including your preparation for this field of study, your academic plans or research interests in your chosen area of study, and your future career goals. Please be specific about why UC Berkeley would be a good intellectual fit for you.

The Personal History Statement:
In an essay, discuss how your personal background informs your decision to pursue a graduate degree. Please include any educational, familial, cultural, economic, or social experiences, challenges, or opportunities relevant to your academic journey; how you might contribute to social or cultural diversity within your chosen field; and/or how you might serve educationally underrepresented segments of society with your degree."

我的个人理解是，SoP要更加学术，主要写自己为什么选择某个program，以及自己的职业规划。而PS可以写一些非学术的东西，比如人生经历等等，目的是说明你为什么想读graduate school。