1. Evidence

2. Types of Evidence:
When a writer attempts to persuade, he or she often uses evidence to support the claim(s) brought up in the essay.
However, there are different types of evidence. Some of the more common ones include the following:
Personal Experience
Testimonials
Appeals to Authority
Polls and Surveys
Scientific Studies
Statistics

3. Types of Evidence:
Each of these types of evidence has its uses and abuses. Also, some forms of evidence tend to be stronger and more convincing in formal argument than others.

4. Personal Experience
This type of evidence can be powerful for narrative essays, often engaging the readers’ emotions or feelings of solidarity with the writer.
However, in formal argument, this form of evidence is often less reliable simply because broader claims are often made in essays of this type and this sort of evidence is less suitable for those claims largely because it’s only representative of that individual’s experience.

5. Personal Experience: Look at the following argument:
I always feel better after having a slice of chocolate cake, so I think that everyone who is depressed needs to eat more chocolate cake.
While the writer may always feel better after consuming chocolate cake, this may not be true for all people, especially those whose depression is caused by a medical condition that chocolate cake would do nothing to alleviate.

6. Testimonials:
Testimonials are largely individuals who have agreed to speak in favor of an idea, product, or political position because of their positive experience with it.
Frequently, this type of evidence is used in commercials and advertisements.
Unfortunately, for formal argument, this sort of evidence is weak and often flawed.
Testimonials are often not representative. They are frequently selected because of their glowing praise and may not be typical of others’ experience.
Testimonials often set up a conflict of interest. The people who are used for testimonials frequently have something to gain by doing so.
Testimonials often omit information to make the idea or product appear more appealing.

7. Appeals to Authority
This sort of evidence is a statement by someone recognized as some sort of expert. This type of evidence can be very effective in formal arguments. However, it is often misused as well.
Examine the following argument:
Are Botox injections a safe alternative to face-lifts? According to an interview with Dr. N.O. Worries published in Cosmo, there are no dangerous side-effects associated with Botox injections. Dr. Worries performs hundreds of Botox injections each month, is well-established as a physician in New York City, and has her own private practice. She claims she never had a serious problem with any of her injections, and her patients have never reported any side-effects. Furthermore, Hollywood’s Association for Cosmetic Surgeons officially stated in a press release that Botox has never been shown to cause any negative effects, despite what other physicians might argue.

8. Appeals to Authority
Note that both authorities in the argument are likely biased. They stand to gain financially by making these statements. Dr. Worries’s testimony is also problematic because it is based on her experience alone. It is likely that she has not sought out the Botox failures. The claims of the organizations are suspect also because these organizations are probably made up of those who perform Botox injections.
Note that this critique doesn’t mean that appeals to authority are wrong. It just means that they need to be examined for bias and conflict of interest.

9. Polls and Surveys
Polls and Surveys are also often used as evidence. However, we do need to be careful and examine how they were conducted. Biased survey questions might well render the evidence skewed and useless.
Note that the questions
Do you support people’s freedom of speech to burn the US flag in protest?
And
Do you support the desecration of the US flag through burning?
Would produce very different answers.

10. Scientific Studies
Scientific studies are often useful as evidence, but we need to be careful about what we can claim based on the results.
We need to look at what exactly was tested. If it was a group of people, what was the demographic? If it was a narrow group, we would only be able to say that the study speaks to that group.
We also need to look at how the study was performed, how well it was able to control for factors that may skew the results.

11. Statistics
These can be helpful forms of evidence, but they can also be deceptive or used deceptively.
For example, an advertisement may claim that the average salary for a company is over $100,o00 a year.
That sounds impressive, but note that there are 3 types of averages and the ad doesn’t tell us which is used. It may well be that the company has one very highly paid executive, but very poorly paid workers in all other areas. The over $100,000 average salary is simply gotten from calculating the mean. The large executive salary skews the results and makes the company sound like a great place to earn money when, in fact, it may not be.

12. What are the Best Types of Sources?
Those that are scholarly and peer-reviewed (books and journals for example). These are going to be the least likely to be problematic.
Those published by reputable organizations or by reputable individuals that have no conflict of interest and who are experts in the fields on which they write.
The least reliable are often internet sites, blogs, or publications known for sensationalism.
Note that you should examine all sources you are considering using closely.