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We're surrounded by evidence these days, and in criticalthinking we're often looking at the status of evidence.We're all asking what evidence means, and this couldinvolve looking at its credibility. So what is credibility?How can critical thinking help us judge the credibility ofevidence, and how can it help us adjudicate betweenopposing sides?

Credibility is about believability. Is what we're being toldlikely to be true? But how can we judge credibility?

We would use certain criteria to judge this, some ofwhich might not be used, but the whole range of thecriteria can apply to a number of situations.

Criteria are benchmarks against which we judgesomething. A general criterion for judging credibility ismotive. Has the person or organization got a motive or areason that might affect the credibility of that report?

It might well be that the motive is all to the good, thatthe organization or the individual who gives us thatreport just wants to report what they sincerely believe tobe the case.

So let's take the news. The BBCs charter requires it totake a balanced view in all it's news reports. Therefore ithas a motive for trying to be fair and unbiased and not,for example, favoring one political party over another.But this isn't always the case with news reporting.

Privately owned newspapers usually do have a particularpoint of view. They may, thus, have a motive forreporting news in a way that may be biased. Bias is aspecific criterion within motivation.

Historical evidence might well be biased in the directionof, as they often say, the people who win the wars. Itmight well be that advertisers present things in a biasedway.

So for example, if you take a pride in your appearance,you may well be interested in the evidence about beautyproducts given in the ads. But there may be credibilityissues here. Although the beauty ads may well quotescientific findings, they may well be biased, as the aim isto get us to buy the product.

For example in this ad, we're not told what is clinicallyproven, nor are we told how many independent testswere done. For example, was improvement found in 10tests out of 10, or 10 out of 100? However, you mightthink that evidence for medical treatments wouldn't bebiased, but that's not necessarily the case.

Well, I heard of one pharmaceutical company that wastesting a new drug for carcinogenic properties, that iswhether it causes cancer or not. And they decided to testthis on a batch of chickens. In fact, none of the chickenscontracted cancer and this was duly reported. However,what wasn't reported was that a significant number ofthe chickens in this sample contracted heart disease.And that's what we call selective bias.

Another criterion is vested interest. Does the person ororganization providing the evidence have something togain?

This is often used, but we need to be careful when we'reusing it because we need to be sure that we're clear whatan individual or a group of individuals or an organizationhas a vested interest in. So it's not enough to say thisperson or this organization has a vested interest,because what is that interest that they have vested in aparticular claim being accepted?

A vested interest was raised in the controversy over MMR,a vaccine to inoculate babies against measles, mumpsand rubella. In an article in the top medical journal, TheLancet, Dr. Andrew Wakefield suggested that the MMRvaccine increased the risk of autism, and this alarmedmany parents who refused to allow their children to bevaccinated.

Well, it generated real concern amongst parents. Andeven Tony Blair, who was prime minister at the time,didn't have his baby son, Leo, vaccinated against MMR.However, it then came to light that Dr. Wakefield hadreceived money from lawyers fighting alleged MMRdamage cases. He was thus seen to have a vestedinterest, and suddenly, his evidence was less credible.And even The Lancet said that they wouldn't havepublished his report if they'd known about thisconnection with lawyers.

So another criterion of credibility is neutrality.

Here we have evidence from a source where there is noobvious vested interest, no obvious bias, no obviousmotive to distort. And so in that way, neutrality is thepositive, the obvious positive of these other criteria.

It could also be said that doctors are neutral. They seekin their work to be objective. That means that they haveto set aside their personal interests, feelings, andprejudices, and in their diagnosis and recommendationsput the patient's best interest first.

Another and rather different criterion is ability toperceive. It means actually being there, seeing or hearingfor yourself rather than relying on what you're told.

Here we have the classic example of evidence which isbased on someone who was there, someone who saw orheard something.

The police, for example, put great store on evidencegiven by eyewitnesses. However, even this evidence canbe flawed.

Had a long face.

Had a chubbyish face.

I think he was wearing a balaclava over his face.

He had blonde hair. And he had a woolly hat as well. I think he had blonde hair underneath.

And bold blonde hair.

I think it was brown.

He was tall. He was slim, and he had black hair.

He had kind of gingerish-colored hair.

We know from many experiments that psychologistshave done that people will say that they have seensomething and it was not there.

Again, we tried this with our witnesses. There wasnothing to show our robber was armed, and when simplyasked to describe the incident, none of the studentsmentioned a weapon. However, we then asked anothergroup of students if they could describe the weapon therobber was carrying.

You know the attacker, he was carrying something in hishands, some sort of weapon. Can you describe it, what itwas?

Looked like a knife.

I didn't see a knife.

It was quite a big knife.

I didn't see it.