Signal detection theory (Part - 1) Video Lecture - NEET

Voiceover: In this video,
I'm going to be talking about
something known as
Signal Detection Theory.
Signal Detection Theory,
basically, looks to see
how we make decisions, so decision making,
under conditions of uncertainty,
so with uncertainty.
Let me give you an example of what
Signal Detection Theory is trying to do.
I want you to look at the screen
and tell me if there is any change.
It was pretty obvious that I put up
this bright green circle. Okay?
Keep looking at the screen
and think to yourself
and try and notice any changes. Okay?
This time around, I put
up this fainter green dot
as compared to this bright green dot.
The bright green dot is
a fairly strong signal
and this faint green dot
is a fairly weak signal.
Signal Detection Theory is, basically,
trying to figure out at
what point is a signal
strong enough that we
are able to notice it,
in the first place, and also in order to…
Signal Detection Theory is,
basically, trying to decide
at what point are we
able to detect a signal,
and it had its origins in radar.
Back when radar was being developed,
they had to figure out a way to determine
whether a strong signal is a ship
or a large whale or a school of fish,
and that's where it had its origins.
Signal Detection Theory also plays a role
in psychology and, in psychology,
imagine that we show a list of words
to an individual, and then we show them
a second list of words,
and we ask them to recall
which words from the second
list were on the first list.
The decision that they
have to make is to decide
which word on the second
list was also present
on the first list, and the
uncertainty is their ability
to memorize all the
words on the first list,
so they're not sure, 100%, whether a word
is exactly the same as
the one on the first list,
or very similar, and I can give you
a real-world example of
Signal Detection Theory.
Imagine that you're
driving to work or school
and you're waiting at a traffic light.
It's a foggy day, and you have to decide
when to start driving,
so you have to decide
when the light turns green
and you have to start driving.
Now it's really hard
to see the green light,
so it might be, kind of, faint,
kind of, like this green light,
and you have to decide,
at what point in time,
how strong does the signal have to be
in order for you to say, "Yes,
"the light is definitely
green. Let me start driving."
In that case, there are
a few different options.
Let me just draw a quick table here.
There are a few different options.
Either the signal is present,
so the light is green,
so signal can be present,
or the signal can be
absent, so the light is red,
or it's not green. You can either say,
"Yes, the light is green," or you can say,
"No, the light is not green."
There are a few different possibilities.
If you say, "Yes, the
light is definitely green,"
you're 100% sure, maybe
it's something like this,
then that would be a hit.
However, if the light is present,
or maybe it's really
faint, and it's present,
but you're not 100% sure
whether it's green or not,
you might say, "No," and,
since the signal is present,
and you're saying, "No,"
that's a miss, so it's incorrect.
Another possibility would
be the signal being absent,
so maybe the signal's
absent, but you say, "Yes,"
and that would be a false
alarm, so false alarm.
The final possibility is
that the signal is absent
and you say, "No," and that's correct,
so that would be a correct rejection,
a correct rejection. If the signal
is really, really strong, so if it's this,
you might always get it right.
Whenever the signal is
present, you'll always say,
"Yes," and when the signal's gone,
you'll always say, "No."
In that case, it's pretty easy to decide
whether a signal is present or not.
On the other hand, if
it's a fairly weak signal,
maybe this faint green dot,
you might get some false alarms.
You might say, "Yes,"
when the signal's actually present,
or you might say, "No,"
because you don't see
this faint green dot, and
you might get some misses.
An easy signal, like the
really bright green dot,
would create more hits than misses,
whereas a weak signal
would create less hits than misses.
Now, the strength of a signal,
which is what we were just talking about,
is a variable known as D Prime,
so this is the strength of a signal.
Another variable is C,
and that is strategy.
Let me just talk about this.
Strategy would be…
Let's look at the example
of you driving to work
and you're waiting at the traffic light.
One strategy could be,
if you see any light,
you're going to say,
"Yes," and start driving.
Another strategy would be,
if you see any green light,
you're going to say, "Yes,"
and start driving, or,
third strategy would be,
if it's a green light
and it's elevated up off the floor,
and it was presented
immediately after a red light,
then you're going to start driving,
so you have different strategies.
There are two big strategies.
You could either have a
conservative strategy,
so conservative, or you can
have a liberal strategy.
If you have a conservative strategy,
you would always say, "No,"
unless you're 100% sure
that the signal is present.
You're always going to be saying, "No,"
unless you're 100% sure
the signal is present,
and the bad thing about that is that,
even though you'll get all
the correct rejections,
you might also get some misses.
On the other hand, you can
have a liberal strategy,
where you always say,
"Yes," and, in that case,
you'll always get all the hits;
however, you might get a few false alarms.
These are the two different strategies
that you can use, so
this would be strategy
and this would be the
strength, so D Prime and C.