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To use the scatter graph to estimate a yvalue for a given xvalue (e.g. estimate a students mark on exam 2 given his result on exam 1) then find your xvalue along the xaxis and then go up until you get to the line of best fit. Then go across and you'll get a y value.
The same works the opposite way around if you are given a yvalue just start on the yaxis.
Suggested reading…
Use a line of best fit to estimate values on a scatter graph and describe the correlation
Scatter graphs, also called scatter plots, are used to compare two quantities.
Let’s say someone’s interested in whether taller people have bigger feet, they can measure the height and shoe size for a whole bunch of people, then plot them on a scattergraph. When they do, the dots will either seem to go upwards from left to right, go downwards, or just be scattered all over with no pattern.
Correlation explained:
 Positive correlation
If looking at a scatter diagram and the points increase with each other, i.e. the points go upwards from left to right. This means that as one data set increases, the other is also increasing.
 Negative correlation
If looking at a scatter diagram and the points decrease with each other, i.e. the points go downwards from left to right. This means that as one data set increases, the other decreases
 No correlation
If looking at a scatter diagram and there is no visible pattern to the points, i.e. the points look as if they are randomly placed. This means that there is no connection between the two data sets.
 Strong correlation 
If the points on the graph all seem really close to our line of best fit, then we say there is strong correlation between the two things we measured e.g. strong positive correlation between foot size and height would mean that it is *very* likely that you will have bigger feet, if you are a taller person.
 Weak correlation
If many of the points are relatively far from the line of best fit, we call it weak correlation.
Using a line of best fit
To make the pattern more clear, we can draw a line of best fit over the points. We then use this line to estimate values that we don’t have measurements for.
For example
If we want to find out from our graph of height vs shoe size, what is the height of someone with shoesize of size 10. We only have to find 10 on the axis with shoesize, then follow that to the line of best fit, then read off what the corresponding height is (roughly 174.8).
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