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# Scatterplots — Basic example

Watch Sal work through a basic Scatterplots problem.

## Want to join the conversation?

• How do we create the best fit line, as sometimes it is not given in the questions.
• Basically, you really don't want to find the exact best fit line, because it is going to take you (and probably everyone else in the world, except computers) 1,000,000 years to find it because you have to find the absolute minimum linear regression for the line. You also have to use standard deviation, so just don't do it unless the problem tells you to eyeball it, which you can do.
• Why do you always have to draw the best fit line to determine the answer ?
• Lucky for us, in this case, WE didn't have to draw the line of best fit--it was given to us.
If we don't have a line of best fit, how do we find the rate of change? Well, that would be how much the data increases between points, divided by the measurement increase along the x-axis, which in this case is years.

But then we have to choose points to use to find the rate of change.
Here are some of the coordinates from that graph that we can use:

`0 42`
`1 46`
`2 48`
`3 54`
`4 47`
`5 48`
`6 56`
`7 58`
`8 54`
`9 50`

If we choose 0 years and 1 year, the percentages are 42 and 46
The calculation is (46 - 42)/(1 - 0) which is 4/1 or `4% per year`

If we choose 1 year and 4 years, the percentages are 46 and 47
The calculation is (47 - 46)/(4 - 1) which is 1/3 or `0.33% per year`

If we choose 3 years and 4 years, the percentages are 54 and 47
The calculation is (47 - 54)/(4 - 31) which is -7/1 or `-7.0% per year`

If we choose 2 years and 5 years, the percentages are 48 and 48
The calculation is (48 - 48)/(5 - 2) which is 0/3 or `0% per year` (no change)

If we choose 0 years and 9 years, the percentages are 42 and 50
The calculation is (50 - 42)/(9 - 0) which is 8/8 or `1% per year`
4% .33% -7% 0% 1% are all correctly calculated from the data points, but they are wildly different and none of them is a good answer. THAT is why we draw a line of best fit.

In other words, we use the slope of an imaginary line that passes through the points to help us choose good points for calculating the answer.
• When given a scatterplot without the line of best fit already provided for you what is the best approach to take to find that line and the slope for that line.
• use your test booklet, use your answer key as a ruler and do your best to draw a line of best fit. Once you find the slope, just go with whats closest.
• In this case, the vertical axis is already in percents (Percentage of U.S. adults that have this opinion), while the horizontal axis is in years, so percents are already there in the problem. So this is not about calculating percentage based on numbers of people.

Instead, we need to find the average yearly change in percentage. That is a rate of change problem, so we need to find the amount the vertical height of the data points is changing for every increase in the horizontal axis. To find our the rate of change, we calculate the slope of the line that is closest to all the points, running at about the same slope as the group of points. That is the line of best fit.

It is best not to choose two points that are really close together, and your result will be most accurate if you find a point that is exactly at the crossing of two grid marks AND on the line of best fit.
Sal chose to look at the percentage at 1 year and at 9 years. The y- coordinates were 45 and 55 and those are in percent, remember.
To calculate the slope, (55-45)% is the amount the vertical height changed and the length of time in years was 9 - 1 = 8 years.

So the slope of the line is 10/8 percent per year which is 5/4 % per year or about 1.25% per year. Because we are using a line of best fit and the scale of the vertical axis is large, this answer is a little off of the alternatives we are given. Two alternatives are silly because they show a negative slope, but our data is definitely increasing. Therefore it is clear which result is correct.

Hope that helps.
• I am so confused but I learned this already.
Approximately.
• how come the line of best fit ended on 9th year? To be more precise i could have used 8ht year as an end to line of best fit because i can see a pint there , and also slope of line = y1 - y2/ x1 - x2. these points can easily define the slope
• What you do to find a line of best fit is you normally go by the average angle and direction the points are trending... it won't necessarily go through any specific point if that's what you mean...
(1 vote)
• how to get the slope on a scatter plot
(1 vote)
• For a scatterplot on the SAT, the fastest and easiest way to get a line of best fit is to just eyeball it. The answers will be different enough from each other that errors will be slight and you can pick the closest answer. Just draw a line through the "center" of the scatterplot, or if you prefer, through the furthest-apart two points that you can find that aren't obvious outliers. Then you can calculate the slope through rise-over-run.