Critical Thinking — The Use of Evidence



this is the second in a sequence of videos to help with critical thinking and this video is going to be about the use of evidence and in particular evaluating how evidence is used now when you find evidence in an argument one thing that you should be interested in is whether the evidence is actually right whether it's true but in critical thinking we're not particularly interested in this question it's an important question but it's not a critical thinking question in critical thinking we're going to assume that the evidence is true and the question that we want to answer is could the conclusion still be wrong so in other words were interested in the link between the evidence and the conclusion and we're trying to decide whether the conclusion actually follows from the evidence okay so let's have a look at a couple of examples here's the first example we have an argument which says in 2010 the prison population of Great Britain was about 84,000 whereas the corresponding figure for Spain was only 74,000 this suggests that there are more criminals here in the UK and therefore that you're more likely to be a victim of crime than in Europe okay at this point you might like to pause the video and have a think for yourself about the use of the evidence in this example – these figures have 84,000 and 74,000 support the conclusion that you're more likely to be a victim of crime here in the UK than in Europe so pause the video and have a think okay well the first thing you might have noticed is that the evidence is about the prison population of Great Britain and that's used to support a statement about the number of criminals in the UK and a further conclusion that you're more likely to be a victim of crime here than in Europe now this doesn't work just because the prison population is higher in Great Britain than in Spain doesn't mean there are more criminals here in the UK it could be that in the UK and more effective at catching a criminals and so a higher proportion of people who commit crime end up in prison or it could be that we have a different criminal justice system which means that judges are more likely to hand down a custodial sentence to somebody who's convicted of a crime so a higher proportion of the people who are convicted of a crime end up in prison here in the UK than in Spain the fact that the prison population is higher in Britain certainly doesn't mean that you're more likely to be a victim of crime because even if there are more criminals there doesn't mean you're more likely to be a victim of crime it could be that there are more criminals but individually they commit fewer crimes something else you might have spotted is that the initial evidence is comparing Great Britain with Spain but the conclusion is about Great Britain and Europe and we certainly can't conclude that you're more likely to a victim of crime here in Britain than in Europe just because of some figures comparing Great Britain with Spain and there's another problem which is to do with the statistics the numbers themselves because the evidence here is about the number of people in prison but the conclusion is about their likelihood of being a victim of crime now actually the number of people in Britain is much higher than the number of people in Spain so if you work it out the proportion of Britons that are imprisoned is lower than the proportion of who are in prison if anything that means you're less likely to encounter a criminal in Britain than you are in Spain okay I hope you thought to some of those problems you may have thought of some others as well for example you might have thought that the argument was extremely bizarre because if so many prisoners are locked up in Britain surely they're not on the streets to commit crime and if anything the fact that a higher number of people in Britain are in prison suggests that you're safer here there are lots of things wrong with the evidence in this argument but the important thing is that in critical thinking what we're trying to explain is how the conclusion can be wrong even if the evidence is right you'll notice that in none of my points did I challenge the statement that those are the prison populations I'm completely accepting that there are eighty four thousand people in prison in Britain and seventy four thousand people in prison in Spain I'm completely accepting those facts but I'm questioning whether the conclusion follows from them outside critical thinking of course we might well want to check those facts and see if they're true but that isn't a critical thinking task now before we move on there's a couple of questions which it can be helpful to ask yourself to try and see why evidence isn't very good and doesn't help to support a conclusion questions that you can ask yourself are is the evidence relevant and is the evidence sufficient let's see how those questions can help us to see problems in the use of evidence let's look at our argument again hopefully you can see that a statistic about the prison population isn't precisely relevant to the number of criminals that are here in the UK prison population a number of criminals are different things and it's even less relevant to the probability of being a victim of crime the number of people who are imprisoned isn't relevant information or not precisely relevant information to a statement about how you are to be a victim of crime you can also ask the question is it sufficient is this evidence about the prison population of Great Britain compared to Spain sufficient to show that you're more likely to be a victim of crime and as I've tried to explain earlier it's certainly not it's easy to see that the prison population of Great Britain could be higher whereas the probability of being a victim of crime could be lower as I said in Britain it could be that all the criminals are locked up and so you're very safe okay let's look at another example here we're told school leavers are having second thoughts about University aware that an average graduate starting salary of seventeen thousand pounds is hardly going to cover debts of up to fifteen thousand pounds after three years at university it's obvious that graduate salaries no longer match the crippling costs of getting a degree okay again at this point you should pause the video and have a think for yourself what's wrong with the evidence in this argument okay so it's really important to be clear we're not going to question these figures of 17,000 pounds and 15,000 pounds we're not going to ask ourselves where they've come from or if they're true we're going to assume that they're true but think about whether even if they are true does that mean that graduate salaries no longer match the crippling costs of getting a degree so we're going to assume that the evidence is true and question whether the conclusion actually follows from that so first of all you might have noticed that the 17,000 pounds figure is an average whereas the 15,000 pounds figure is an up – so it's clear that actually there could be a lot of graduates who are earning far more than 17,000 pounds whereas the average debt could be much smaller than 15,000 pounds so it's possible that if we compare like with like instead of an average with a maximum if we compared to averages it could be 17,000 pounds and 5,000 pounds or if we compare two maximums it could be 35,000 pounds and 15,000 pounds something else you might have thought of is that this is telling us the graduate starting salary now it may be that graduates are only paid 17,000 pounds when they start work but if their salary rapidly increases when they're in employment it could be that they're very soon able to start paying off their debts so that would be another reason why the graduate salaries may actually be enough to be able to pay off the costs of getting a degree okay there are other problems with this argument as well we can cheerfully accept that graduate starting salaries are 17,000 pounds and in average initial debt is 15,000 pounds but what if you don't have to start paying back the debt straight away or you're able to repay the debt over a considerable number of years so the amount that you have to pay back each year is say only a thousand pounds there's lots of ways that could be true that the 15,000 pound debt is easily repayable even though the average starting salary is only seventeen thousand pounds okay so to summarize the main question that you want to ask when you're asked to look at some evidence and evaluate the use of it is even if this evidence is true even if we have to accept the evidence does that mean that the conclusion follows and you can do that by thinking is the evidence precisely relevant is it telling me the exact thing I need to know in order to draw that conclusion and secondly is it sufficient is that piece of evidence enough to show me that that conclusion is right or is it possible that the conclusion could be wrong even if the piece of evidence is absolutely true okay I hope you found this very short introduction to evaluating use of evidence helpful it's important now to look at lots of examples of the use of evidence and to start to get clear about some of the ways in which evidence and statistical data can be misused in real arguments so thank you very much for watching and I look forward to making the next video for you

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