428 McGRAW-HILL’S SAT
Lesson 5: Data Analysis
Scatterplots and Lines of Best Fit
A scatterplotis a collection of points plotted on a
graph that is used to visualize the relationship be-
tween two variables. A line of best fit is a straight line
that best “hugs” the data of a scatterplot. This line
usually divides the points roughly in half. This line
can be used to make predictions about how one of
the variables will change when the other is changed.The SAT may ask you to describe the basic
features of a line of best fit for a set of data, but
it won’t ask you to find this equation exactly.
You can usually just eyeball it: draw a line that
fits the data and cuts the points roughly in
half, and then notice whether the slope of the
line is positive, negative, or 0, and then notice
roughly where its y-intercept is.
Example:
If the Smiths own as many pieces of electronic
equipment as the Carsons do, how many cell
phones do the Smiths own?
Start with the first table. The Carsons own 3 + 4 +
2 = 9 pieces of equipment. If the Smiths own the same
number of pieces, they must own 9 4 = 5 tele-
phones. Now go to the “Telephones” table. Since the
Smiths have five telephones and two are not cell
phones, they must own 5 2 = 3 cell phones.As with tables, always carefully read the labels
of pie chartsto understand what the data rep-
resent before tackling the question. In a pie
chart, a sector containing x% of the data has a
central angle of (x/100)(360°).Example:
In the pie chart above, what is the angle measure
of the sector represented by the color purple?
Purple accounts for 20% of the circle, and 20%
of 360° = (0.2)(360°) = 72°.Example:
What is the approximate slope of the line of best
fit in the scatterplot above?
To estimate the slope of a line of best fit, pick two
points on your line of best fit and use the slope equation
(from Chapter 10, Lesson 4) to solve for m. It appears
that the line of best fit connects the points (30, 40)
and (90, 90), so.
Tables, Charts, and Graphs
A tableis a set of data arranged in rows and
columns. If an SAT question includes a table, al-
ways read the table carefully first, paying special
attention to the axis labels, and try to understand
what the table represents before tackling the
question. If you are given twotables, make sure
you understand how the two tables are related.m=