Analyzing visual evidence (and vampires) | Reading | Khan Academy

- [David] Raiders, hello.

This video is about
analyzing visual evidence,

but let's cut right to it.

It's also about vampires.

(spooky music)
(thunder booming)

And we'll get to them,

those cape wearing, widows peaked wonders

of the mountains of Transylvania.

But first, what do we even
mean by visual evidence?

Like textual evidence, sure,

we know what textual evidence is.

It's written language.

How do we know there's
potassium hydroxide in my soap?

It's on the label.

Ah, but how much potassium hydroxide?

Visual evidence can tell us.

Visual evidence includes the
graphs, charts, tables, images,

and illustrations that
authors use in their texts.

Authors include visual evidence

to help readers better understand ideas

because sometimes words just don't cut it.

Visual evidence can help
strengthen a written argument

and help express complicated ideas.

So KOH, the potassium hydroxide,

makes up 13.4% by weight of the soap.

And sure it could list that
percentage on the label,

but to my eye, seeing the number

and the pie chart is more helpful.

I can see at a glance

that potassium hydroxide,
labeled here in red,

only makes up a small part of my soap.

This is useful because if
there were a lot more KOH

than there already is,

the soap would burn my
skin when I used it.

Soap chemistry, visual evidence at work.

Oftentimes, you will be asked
to evaluate visual evidence

to see whether it supports

or challenges an argument in a text.

This is something you'll
have to do in many classes,

history and science
classes as well as English.

And questions about this
particular skill are common

on standardized exams.

So this brings us to vampires

and specifically a very
complicated idea about vampires

that some Norwegian researchers
wanted to test in 1994.

Does garlic scare vampires away?

This is a real study.

They said something like,
according to folklore,

garlic repels vampires.

We wished to verify this,

but owing to the lack of
vampires, we used leeches instead.

Leeches, the real life
blood sucking worms.

So like vampires, kinda.

Based on the data,

does garlic actually keep vampires away?

Remembering that leches here
are our stand-ins for vampires.

In a research setting,
the leches were presented

with a hand smeared with garlic

and a clean hand with the expectation

that they would attach
themselves to one or the other.

From our data, we conclude
that the relationship

between vampires and garlic is blank.

So here's the graph.

What conclusions can you make

about the traditional relationship

between garlic and vampires?

What does this graph say happens?

What does the visual evidence tell you?

I'm going to put a little music on.

Feel free to pause the video and discuss

or make your own guesses.

Alright, see you in a bit.

(soft music)

So the title of this graph is:

How Long Do Leeches Take to
Attach Themselves to a Hand?

Right, and we have two bars
on the horizontal axis,

the X axis.

One is with garlic, the
other is without garlic.

And over here on the Y
axis, the vertical axis,

we can see that the legend reads seconds.

That's how many seconds
the leches took to attach.

This graph is potentially confusing

because it might seem

like the bigger bar is
the better one, right?

But this is response time,

how long do they take?

So 14.9 seconds over here
versus 44.9 seconds over here.

The shorter the response time,

the faster the leach latches on.

What can we conclude from
this study therefore,

what was the main conclusion of the study?

Well, basically the leeches went

for the garlic covered hand,
the seasoned hand much faster,

30 seconds faster on
average than they went

for the unseasoned hand.

That means they like
the garlicky hand more

than the ungarlicky hand.

And remember, tradition holds

that garlic repels vampires,
it sends them away.

So from the result of this
study, we can conclude

that their tradition is wrong
and vampires love garlic,

if vampires and leeches are the same.

Yeah, it's a bit of a leap.

Maybe this particular case feels silly.

Fine, but this is a useful skill.

You will be asked to
evaluate visual evidence

to see whether it supports

or challenges an argument in a text.

A friend will send you an infographic

about a political topic.

You might look up stats
for your favorite athlete.

You might have to puzzle
through a misleading graph

on TV news.

Understanding what data is trying

to tell us is a core reading skill.

Information, now it's got
numbers in it, trademark.

And now you've got numbers in you.

Nice work.

Watch out for leeches.
You can learn anything.

David out.