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Line of best fit: smoking in 1945

Sal estimates the percentage of American adults who smoked in 1945 using a scatter plot. Created by Sal Khan.

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  • starky tree style avatar for user Zavier Tsai
    I don't understand this at all... can someone please explain this to me?
    (12 votes)
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    • duskpin ultimate style avatar for user Polina Vitić
      We have a graph with various data points, and it looks like there is a linear relationship between the data points (because if you squint you can kinda see where a line could go, right in the middle of all the points).

      Once you sketch this line, you know (even though you can't see it) that the line goes on forever in both directions. We know that 1965 on the graph is where x=0, and about 41 or 42% of Americans smoked... but we want to know how many Americans smoked in 1945.

      Even though the graph doesn't show 1945, we can draw the line backwards (to the left of the y-axis) and estimate the y-value from the graph. In the video (at ) it looks like the y-value is about 51 or 52%.

      Hope this helps a little!
      (23 votes)
  • leafers ultimate style avatar for user Ivan
    Is it possible to calculate a perfect line through the points?
    (17 votes)
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  • aqualine seed style avatar for user JessieC
    I am confused because I thought the data started at 1945
    (7 votes)
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    • female robot grace style avatar for user Ayani
      The data starts at 1965 can can tell this by looking at the label for the x-axis. The question asks about 1945 and says that it has been consistent since then, which is why you can simply continue drawing the line in the positive direction.
      Hope this helps your confuzzlement!
      (11 votes)
  • duskpin sapling style avatar for user Elizabeth
    Is there a way to make the equations easier to understand and do? I am good at drawing the line of best fit, but not the rate of change...
    (8 votes)
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  • blobby green style avatar for user Bhuvnesh Kumar
    we continue the trend like that backwards, then is it possible to show that at some year ~100% population smokes?
    (6 votes)
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  • winston default style avatar for user Jack Wohl
    Is this a factual chart?
    (6 votes)
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  • blobby green style avatar for user MADISON386
    what if that estimate were to be a faction? And what would that fraction be?
    (6 votes)
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  • blobby green style avatar for user Mario the handsome
    Confusing because it started in 1945
    (6 votes)
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  • aqualine seed style avatar for user tukm1999
    are there any standard to how to get the "best" line ? how do you know that is the right line and this is not ?
    (5 votes)
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  • blobby green style avatar for user jordan.jaden
    HOw do you approximately caculate the points in the first place.
    (5 votes)
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Video transcript

The graph below shows the percentage of American adults who smoke over time. Assuming the trend shown in the data has been consistent since 1945, use the graph to estimate the percentage of American adults who smoked in 1945. So let's see what's going on here. The horizontal axis here, they say years since 1965. So at this point right over here, this is 0 years since 1965. So this really represents 1965. And we see it looks like around-- let's see, if I were to eyeball it, it looks like it's around 42% of Americans, just looking at this graph. I know that's not an exact number. Roughly 41% or 42% of Americans smoked in 1965 based on this graph. And then five years later, this would be 1970. 10 years later, that would be 1975. And they don't sample the data, or we don't have data from every given year. This is just from some of the years that we happen to have. But what is clear, it looks like we have a negative linear relationship right over here, that it would not be difficult to fit a line. So let me try to do that. So I'm just going to eyeball it and try to fit a line to this data. So our line might look something like that. So it looks like a pretty strong negative linear relationship. When I say it's a negative linear relationship, we see that as time increases, the percentage of smokers in the US is decreasing. So that's what makes it a negative relationship. Now, what are they asking? They want to estimate the percentage of American adults who smoked in 1945. Well, 1945 would be to the left of 0. So we could even think of it as if 1945 is 20 years before 1965. So let me see if I can draw that. So 20 years before 1965. Let's see, this would be 5 years before 1965, 10 years, 15 years, 20 years before 1965. So I could even put that as negative 20 right over here. Negative 20 years since 1965 you could view as 20 years before 1965. So that would represent 1945 right over there. And one thing that we could do is very roughly just try to extend this negative linear relationship backwards. And they allow us to do that by saying assuming the trend shown on the data has been consistent. So the trend has been consistent. This line represents the trend. So let's just keep going backwards, keep going backwards at the same rate, so something like that. I want to make sure that it looks like it's the same rate right over here. And you could just try to eyeball it. You could say, well, let's see, 20 years ago, 1945. If I were to extend that line backwards, it looks like there were about 52% of the population was smoking. It seems like we're about 52% right over here. Another way to think about it would be to actually try to calculate the rate of decline. And let's say we do it over every 20 years, because that will be useful because we're going 20 years back. So if we go 20 years from this point, so this is 1965, you go 20 years in the future. So that is 10 years, and then that is 20 years. So my change in the horizontal is 20 years. What's the change in the vertical? Well, it looks like we have a decrease of a little bit more than 10%. It looks like it's 11% or 12% decrease. So I'll just say minus 11% right there. And let's see if that's consistent. If we were to go another 20 years. So if we go another 20 years, it looks like once again we've gone down by about 10%. So that looks like roughly 10%. If we're following the line, it should actually be the same number. So let me write it this way. It's approximately down 10%. So that little squiggly line, I'm just saying approximately negative 10% every 20 years. So if you go back 20 years, you should increase your percentage by 20%. So this should go up by-- or you should increase your percent by 10%, I should say. So if we started at 41% or 42%, once again, this was what we saw when we just eyeballed it, you should get to 51% or 52%. So my estimate of the percentage of American adults who smoked in 1945 would be 51% or 52%.