本篇就广告商对不同层次的电视观众采用不同的广告策略作为文章的分类标准，文章的结构清楚，层次分明。由此可见写分类说明文时，确定好分类标准后，可采用总分总的写法， 即段首句点明主题→分别列举内容→得出结论。
(5)对比、对照法（contrast，comparison）。
比较对照型说明文是指通过对两种或两种以上事物进行比较分析，找出他们之间的相似点和差异。通过比较，事物的特征可以得到清楚的显现，便于读者获得对事物的崭新认识。无论是比较法还是对照法，首先只有同一范畴的事物才能进行比较；第二对比主体应平等；第三对比方法上可以采用点对点（当对比的情形比较多时）或段对段(对比点不多时)的方式（具体如下）。如话题X company is better than Y company， 可以采用段对段的方式，可以从员工的工资待遇、公司的管理方式及公司产品的质量用一段话进行对比，也可以分开逐项对比。
范文：
Different Attitude toward Life
All human beings pass the following stages in life： birth， growth， development and death. But due to people"s different attitudes towards life， they may live many different particular kinds of lives. Life attitude is the way people think and feel about life or the way people behave towards life. Generally speaking， there are two main attitudes toward life， namely： optimism and pessimism.
Optimism means facing all the situations in life， even the adverse circumstances， with an optimistic attitude. It is not only a sanguine state of mind but also a precious character. People with this attitude are always hopeful and always believe life is good. What"s more， they are capable of dealing with any kind of life difficulties occurring in their life. Looking back into human history， it is not difficult to note the great achievements made by he disabled but optimistic persons， such as the well-known British Beethoven. Of course， determined and noble people are not the only kind of people who are optimistic. Anybody who has a strong belief and never easily yield to the challenges and hardships in life will hold an optimistic attitude toward life.
By contrast， there is the other completely opposite attitude toward life： pessimism. Pessimism is negative attitude toward life. In contrast to the hopeful lifestyle of the optimistic people， pessimistic people regard life as a dull and dark thing， they feel depressed and hopeless most of the time. As lots of researches have shown， most suicide and crimes are committed by pessimistic people. The reasons causing this harmful way of thinking can be different， low social status， high job pressure， past experience of failure and dysfuncitonal family environment can all result in a pessimistic toward life. But unlike the optimistic people the pessimistic ones regard these setbacks in life insurmountable and don"t make a second effort to succeed.
Life is equal to everyone. Whatever it is glorious or depressing depends on your attitude. So， every person should hold an optimistic attitude and take each opportunity to find and enjoy the beauty in life， and then， life will also reward your for your optimism.
本篇文章的对比方法很清楚，作者采用了段对段的比方式。同时从文章结构也可以看成总分总的方法。
从以上范文可以看出，在实际写作过程中，很少只限定用某一钟方法，往往都是综合使用各种不同的方法，但是动笔之前，最好确定以某一种方法为主，其余为辅，灵活综合使用。
思考与练习：
I．分析下段描写文字，指出人物描写中存在的问题并改正。
She is a pretty girl of eighteen. She has long black hair and big black eyes. She is always wearing a pair of glasses. Her nose is straight but her mouth is very small like a cherry. She is neither far nor thin， neither tall nor short. When she is smiling， you can see her white regular teeth. She always wears jeans and sports shoes. Every time， when she talks with the other people， she always speaks in a loud voice with smiles. In our class， not only teachers like her very much， but also the students.
II. 分析下文，指出文章中存在的问题并尝试修改。
A year， as one hundredth of life， has no more of surprising， but except my freshman year. First in my life I have met so many interesting people from different places.
The first several weeks we called a “nobleman time”. We didn"t know each other and everyone wanted to be noticed and leave a fine impression on the others. So the walls were kept white and the room clean. If one had some trouble， may warm hands of helpers appeared at once.
Then the time passed， step by step， noblemen returned to their old years. Some short stories from the west which made us laugh and stomachache were produced； sometimes we were too excited to fall asleep. There were so many songs we sang and listened to. With the development of football skills， the wall was no longer white. So the second period could be called a “happy time.”
Until the final exam was at hand， the happy time stopped. Instead， a “hard working time”came， the rooms were empty. At night， all emergency lights were produced.
During the second term， it seemed that no one dares to play too much. But in fact， there were no differences from the last term. In spite of this， forms of spare time were changed. Instead of short stories， many noblemen began to have long talks after several cups of drinks and good meals. Some sentimental songs were often sung. We were often confused by many kinds of things. Some people still had “go for self-study”. This was a new balance point.
From “the noble time”to “the happy time” then to “the hard work time”， at last “the new balance time”. The freshman year was thus spent.
III． 根据要求练习写作。
1. Life on College Campus is Interesting or Monotonous (to write an Exposition)
2. There Is a Generation Gap (to write an Exposition)
3. One person Who Influences Me Most (to write a Narration)
4. Should Women Be Encouraged to Go out to Work (to write an Argumentation)
5. Is Failure a Bad Thing？(to write an Argumentation)