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Current time:0:00Total duration:6:32

Signal detection theory - part 1

Video transcript

in this video I'm going to be talking about something known as signal detection theory now signal detection theory basically looks to see how we make decisions so decision-making under conditions of uncertainty so with uncertainty now let me give you an example of what signal detection theory is trying to do so I want you to look at the screen and tell me if there is any change so it was pretty obvious that I put up this bright green circle okay so now keep looking at the screen and love and think to yourself and try and notice any changes okay so this time around I put up this fainter green dot as compared to this bright green dot now the bright green dot is a fairly strong signal and this faint green dot is a fairly weak signal so 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 I so signal detection theory is basically trying to decide okay at what point are we able to detect a signal and it had its origins in radar so 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 so 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 or on the first list now the decision that they have to make is to decide which word on the second list was press also press on the first list and the uncertainty is them is their ability to memorize all the words on the so they're not sure 100% whether a word is exactly the same as the one on the first lists are very similar and I can give you a real world example of signal detection theory so imagine that you're driving to work or school and you're waiting at a traffic light and 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 at what how strong does the signal have to be in order for you to say yes okay the light is definitely green and let me start driving so in that case there are a few different options so let me just draw a quick table here so there are few options so there are a few different options so 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 and you can either say yes so you can say yes the light is green or you can say no the light is not green so there are a few different possibilities so if you say yes the light is definitely green you're 100% sure so 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 you're saying no that's a miss so it's incorrect another to posit another possibility would be the signal being absent so maybe the signal is absent but you say yes and that would be a false alarm so false alarm and the final possibility is that the signal is absent and you say no and that's correct so that would be a correct rejection correct rejection so if the signal is really really strong so if it's this you might always get a right you whenever the signal is present you'll always say yes and when the signal is gone you'll always say no so in that case it's a pretty easy it's pretty easy to decide whether a signal is present or not on the other hand if it's a faint it's a fair really weak signals so maybe this faint green dot you might get some false alarms you might say yes when you're not when the signals actually present or you might say no because you don't see the pain current you might get some misses so an easy signal like a like the really bright green dot would create more hits than misses where as 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 know known as D prime so this is the strength of a signal and another variable is C and that is strategy so let me just kind of talk about this so strategy would be let's let's let's look at the example of you driving to work and you're waiting at this stop sign at the traffic light so 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 and there are two big strategies so you could either have a conservative strategy so conservative or you can have a liberal strategy so if you have a conservative strategy you would always say no unless you're 100% sure that the signal is present so you're always going to be saying no how much you're a hundred percent sure the signals 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 so 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