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Sensitivity vs Specificity Explained (Medical Biostatistics)

Sensitivity vs Specificity Explained (Medical Biostatistics)


okay so welcome to another MedCram
lecture we’re going to talk about what may be considered a dry subject but I
think it’s actually very important it’s the topic of sensitivity and specificity
in relation to statistics now sometimes we see this appear on test and we’ve got
to know what sensitivity and specificity means I think it’s also very helpful in
interpreting some of the data that we see in medical journals and it’s
actually very helpful in our everyday life in terms of understanding the test
that we’re ordering on patients so I think it deserves a little bit of a of
an introduction and an analysis and how to actually do the questions that you
need to do for tests so imagine if you will with me that you’ve got a nice car
and you want to protect your car so you install a car alarm
he’s got a little blinking light there and you’ve got a knob in that car that
allows you to change the sensitivity on that car alarm well you’ve got a couple
of choices you can either have the sensitivity high and what does that mean
that means that well if a thief tries to break in the car alarm is probably going
to go off but it also means that since you’re parked on a busy street if a
motorcycle goes by that could also set it off so if you have a thief and the
car alarm went off then that would be a true positive result you would hear that
car alarm go off and sure enough there’s a thief trying to break into your car
that would be a true positive so the disease here is is your car being broken
into or is it not being broken into and the test is is the car alarm going off
or as the car alarm not going off and you can actually break this up into a
square and have the four possibility so you could in terms of our disease you
could have a positive disease or negative disease and in terms of a test
you can either have the alarm going off or the alarm not going off so in this
box here this would be if there was a thief trying to steal your car and the
car alarm was going off that would be a true positive okay if there wasn’t a
thief trying to steal your car but your car alarm went off that would be a false
positive that would be like a motorcycle going by
or somebody touching the car as it as it went by and the car alarm goes off you
go out there the alarms going off but no one’s tried to break in and obviously
that’s not an optimal result you don’t want to have false positives because
every single time the alarm goes off you’ve got to go out there you’ve got to
check the car you got to make sure no one’s getting into it also the more
times this car alarm goes off the less likely you are of taking it seriously so
not good the other possibility is which is also
not a good possibility is that your car is being broken into but the alarm
doesn’t go off that would be a false negative that’s probably the worst
result that you could have you go out to your parking lot the car’s gone okay
the alarm never went off it didn’t disengage the engine that’s a problem
and then finally the thing that happens you know all the time is a true negative
car alarm is not going off and your car is not being broken into so that’s
that’s a good result so we like true positives we like true negatives we want
to minimize these as much as possible and you can see how that would happen
depending on how you sent the sensitivity of your car alarm if you
sent the sensitivity of the car alarm up very high what you would see is you
would start to have a lot of these false positives but the chances of having
false negatives would go down okay conversely if you set the sensitivity of
your car alarm very low you probably wouldn’t be getting motorcycles setting
off the car alarm but on the other hand it might be easier for a thief to break
into your car and to steal it without the car alarm going off so the point
here is I want to show you that there’s a trade-off in terms of where you set
the sensitivity for a test it’s very important because depending on where you
set the sensitivity for a test you’re either going to get more false positives
or false negatives and depending on what you’re testing for and what kind of
situation you’re in you know you may want to set the sensitivity of that
alarm to different areas for instance if you’re living in a very high crime area
where you know cars are getting stolen every day and there’s not a lot of
motorcycles sure set that sensitivity up pretty high if on the other hand you’re
parking your car in the area where there’s not a lot of thieves and and
there’s really been no reported thieves but there’s a lot of motorcycles going
by you may want to set that sensitivity low so it just sort of depends on where
you’re living and same thing applies to medicine now we’re
going to talk a little bit about these false positive true positives false
negatives in this box a little bit but I wanted to introduce to you the concept
of what sensitivity is and what specificity is sensitivity
mathematically speaking ok is the true positives divided by the true positives
plus the false negatives and you can see here that sensitivity is the true
positives divided by the true positives plus the false negatives and so the
sensitivity as you can see mathematically would be very high if the
false negative rate was very low in fact if something is very sensitive and it’s
negative you can practically rule out that disease okay on the flip side
specificity which takes of this column specificity is the true negative rates
down here divided by the true negative plus the false positive and clearly you
can see there that if the specificity is very high this false positive has to be
very low and so if something is very specific and it’s positive the chances
of it being falsely positive are extremely low we’ll get into that more
another way they’ll sometimes test you is they’ll look at all of the different
populations of things okay so this is what we’re dealing with if we can go
back to the car example here is a population of all the things that could
falsely set off an alarm and here is the population of things that could truly
set off alarm so this is what thieves could do okay and so what we’re trying
to do is we’re trying to devise a test that would separate these things out so
what I mean to say here is that this would be the disease negative state okay
so this is the disease here is your car being stolen and here this is all the
stuff that could set off a car alarm but it’s not really your car being stolen so
this would be like motorcycles boomboxes things of that nature and this is all of
the things that thieves could do to steal your car so this would be the
disease positive state the problem here is that there’s some overlap and
no matter where you set your threshold for your sensitivity you’re going to
have a problem in that you’re going to be including one or the other so there’s
there’s a couple of areas here where you could set the sensitivity you could set
the sensitivity right here at Point a you could set the sensitivity here at
point B or you could set the sensitivity here at Point C or any of these points
and now I want you to notice here what what’s going on so if we were to set set
it here then everything to the right would be positive and everything to the
left would be negative everything to the right of B would be positive to the left
would be negative everything to the right of C would be positive and to the
left to C would be negative so the question is and this is the question
that you’ll sometimes get on tests is which of the following points represents
the most sensitive test and which of the following points represents the most
specific test so specificity is the opposite of sensitivity in this case so
of course if you are anything that is on the right side of a test is going to be
a positive test anything that is on the left side of this point is going to be a
negative test depending on which population they belong to will tell you
whether it’s a true or a false negative or positive let’s take for example this
point here at C okay so here at C if we were to set the alarm at C in terms of
specificity and sensitivity then all of this area right here would be a positive
now because it belongs to the actual disease it would be a true positive but
all of this back here would be a negative now some of these would be
false negatives because they still belong to the disease but then some of
these would be true negatives because they don’t belong to the disease and so
what you notice here is that if something is positive it is definitely
part of the disease but you’re missing out on a lot of falsely negative but
notice here that we’ve moved this C all the way out here so that there is no
more false positives if you have a very low false positive rate that means
you’re very specific okay just for review remember that the sensitivity is
the true positive rate do positive rate divided by the true
positive rate plus the false negative rate and the
specificity is the true negative rate divided by the true negative rate plus
the false positive false positive rate so because of that if we were to look at
this point here a notice what we have here at a at a there is no more false
negatives and because there is no false negatives negative would be on this side
of the line okay so negative there are no disease or
robbers of the car if you will that are showing up on this line because we’ve
moved it so far to this side so this is the most sensitive setting that we could
set it to so imagine that imagine this in terms of our car if we turn the
sensitivity of the car alarm up very very high
we’re gonna catch every single thief that’s gonna try to steal our car which
is represented by this curve but we’re also going to catch a lot of false
positives here which represent motorcycles and you know very loud cars
or loudspeakers that are driving by on the converse if we set a very specific
test at right here notice here that we’re no longer gonna pick up it’ll be
negative now because negative is to the left of the line all of these things
that could make the car alarm go off will be treated appropriately as a true
negative but we’ve made it so specific now for the car alarm to go off that
there’s things that thieves can do to break into your car okay so long as you
set this up with the negative disease on the left and the positive disease on the
right always remember that this direction is the most sensitive and this
direction is the most specific which actually is going to help us later that
to remember that sensitivity is on the left and specificity is on the right now
hope this clears things up in terms of sensitivity and specificity the next
lecture is going to talk about the actual mathematics and this lecture I
want to talk about concepts let’s talk next about the actual mathematics and
how to calculate things so you can calculate things quickly
on it test exams thanks for joining us

2 thoughts on “Sensitivity vs Specificity Explained (Medical Biostatistics)

  1. Do you have a memory aid to help remember the key differences between sensitivity and specificity? Please comment below!

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