## Section4.3Exploring Two-Variable Data and Rate of Change

###### ObjectivesPCC Course Content and Outcome Guide

This section is about examining data that has been plotted on a Cartesian coordinate system, and then making observations. In some cases, we'll be able to turn those observations into useful mathematical calculations.

### Subsection4.3.1Modeling data with two variables

Using mathematics, we can analyze real data from the world around us. We can use what we discover to better understand the world, and sometimes to make predictions. Here's an example of data about the economic situation in the US:

If this trend continues, what percentage of all income will the top 1 % have in the year 2030? If we model data in the chart with the trend line, we can estimate the value to be 28.6 %. This is one way math is used in real life.

Does that trend line have an equation like those we looked at in Section 4.2? Is it even correct to look at this data set and decide that a straight line is a good model? These are some of the questions we want to consider as we begin this section. The answers will evolve through the next several sections.

### Subsection4.3.2Patterns in Tables

###### Example4.3.3

Find a pattern in each table. What is the missing entry in each table? Can you describe each pattern in words and/or mathematics?

Explanation
First table

Each word on the right has the opposite meaning of the word to its left.

Second table

Each city on the right is the capital of the country to its left.

Third table

Each number on the right is double the number to its left.

We can view each table as assigning each input in the left column a corresponding output in the right column. In the first table, for example, when the input “big” is on the left, the output “small” is on the right. The first table's function is to output a word with the opposite meaning of each input word. (This is not a numerical example.)

The third table is numerical. And its function is to take a number as input, and give twice that number as its output. Mathematically, we can describe the pattern as “$y=2x\text{,}$” where $x$ represents the input, and $y$ represents the output. Labeling the table mathematically, we have Table 4.3.6.

 $x$ (input) $y$ (output) $1$ $2$ $2$ $4$ $3$ $6$ $5$ $10$ $10$ $20$ Pattern: $y=2x$

The equation $y=2x$ summarizes the pattern in the table. For each of the following tables, find an equation that describes the pattern you see. Numerical pattern recognition may or may not come naturally for you. Either way, pattern recognition is an important mathematical skill that anyone can develop. Solutions for these exercises provide some ideas for recognizing patterns.

### Subsection4.3.3Rate of Change

For an hourly wage-earner, the amount of money they earn depends on how many hours they work. If a worker earns $\15$ per hour, then $10$ hours of work corresponds to $\150$ of pay. Working one additional hour will change $10$ hours to $11$ hours; and this will cause the $\150$ in pay to rise by fifteen dollars to $\165$ in pay. Any time we compare how one amount changes (dollars earned) as a consequence of another amount changing (hours worked), we are talking about a rate of change.

Given a table of two-variable data, between any two rows we can compute a rate of change.

###### Example4.3.10

The following data, given in both table and graphed form, gives the counts of invasive cancer diagnoses in Oregon over a period of time. (wonder.cdc.gov)

 Year Invasive Cancer Incidents 1999 17,599 2000 17,446 2001 17,847 2002 17,887 2003 17,559 2004 18,499 2005 18,682 2006 19,112 2007 19,376 2008 20,370 2009 19,909 2010 19,727 2011 20,636 2012 20,035 2013 20,458

What is the rate of change in Oregon invasive cancer diagnoses between 2000 and 2010? The total (net) change in diagnoses over that timespan is

\begin{equation*} 19727-17446=2281\text{.} \end{equation*}

Since $10$ years passed (which you can calculate as $2010-2000$), the rate of change is $2281$ diagnoses per $10$ years, or

\begin{equation*} \frac{2281\,\text{diagnoses}}{10\,\text{year}}=228.1\,\frac{\text{diagnoses}}{\text{year}}\text{.} \end{equation*}

We read that last quantity as “$228.1$ diagnoses per year.” This rate of change means that between the years $2000$ and $2010\text{,}$ there were $228.1$ more diagnoses each year, on average. (Notice that there was no single year in that span when diagnoses increased by $228.1\text{.}$)

Let's practice calculating rates of change over different timespans:

###### Checkpoint4.3.11

We are ready to give a formal definition for rate of change. Considering our work from Example 4.3.10 and Checkpoint 4.3.11, we settle on:

###### Definition4.3.12Rate of Change

If $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$ are two data points from a set of two-variable data, then the rate of change between them is

\begin{equation*} \frac{\text{change in $y$}}{\text{change in $x$}}=\frac{\Delta y}{\Delta x}=\frac{y_2-y_1}{x_2-x_1}\text{.} \end{equation*}

(The Greek letter delta, $\Delta\text{,}$ is used to represent “change in” since it is the first letter of the Greek word for “difference.”)

In Example 4.3.10 and Checkpoint 4.3.11 we found three rates of change. Figure 4.3.13 highlights the three pairs of points that were used to make these calculations.

Note how the larger the numerical rate of change between two points, the steeper the line is that connects them. This is such an important observation, we'll put it in an official remark.

###### Remark4.3.14

The rate of change between two data points is intimately related to the steepness of the line segment that connects those points.

1. The steeper the line, the larger the rate of change, and vice versa.

2. If one rate of change between two data points equals another rate of change between two different data points, then the corresponding line segments will have the same steepness.

3. When a line segment between two data points slants down from left to right, the rate of change between those points will be negative.

In the solution to Checkpoint 4.3.8, the key observation was that the rate of change from one row to the next was constant: $3$ units of increase in $y$ for every $1$ unit of increase in $x\text{.}$ Graphing this pattern in Figure 4.3.15, we see that every line segment here has the same steepness, so the entire graph is a line.

Whenever the rate of change is constant no matter which two $(x,y)$-pairs (or data pairs) are chosen from a data set, then you can conclude the graph will be a straight line even without making the graph. We call this kind of relationship a linear relationship. We'll study linear relationships in more detail throughout this chapter. Right now in this section, we feel it is important to simply identify if data has a linear relationship or not.

###### Checkpoint4.3.18

Let's return to the data that we opened the section with, in Figure 4.3.2. Is that data linear? Well, yes and no. To be completely honest, it's not linear. It's easy to pick out pairs of points where the steepness changes from one pair to the next. In other words, the points do not all fall into a single line.

However if we stand back, there does seem to be an overall upward trend that is captured by the line someone has drawn over the data. Points on this line do have a linear pattern. Let's estimate the rate of change between some points on this line. We are free to use any points to do this, so let's make this calculation easier by choosing points we can clearly identify on the graph: $(1991,15)$ and $(2020,25)\text{.}$

The rate of change between those two points is

\begin{equation*} \frac{25-15}{2020-1991}=\frac{10}{29}\approx0.3448\text{.} \end{equation*}

So we might say that on average the rate of change expressed by this data is 0.3448 %yr.

### Subsection4.3.4Exercises

###### 1

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $-2$ ${-6}$ $-1$ ${-3}$ $0$ ${0}$ $1$ ${3}$ $2$ ${6}$
###### 2

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $3$ ${12}$ $4$ ${16}$ $5$ ${20}$ $6$ ${24}$ $7$ ${28}$
###### 3

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $5$ ${11}$ $6$ ${12}$ $7$ ${13}$ $8$ ${14}$ $9$ ${15}$
###### 4

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $6$ ${10}$ $7$ ${11}$ $8$ ${12}$ $9$ ${13}$ $10$ ${14}$
###### 5

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $15$ ${23}$ $13$ ${21}$ $6$ ${14}$ $4$ ${12}$ $1$ ${9}$
###### 6

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $17$ ${16}$ $6$ ${5}$ $14$ ${13}$ $19$ ${18}$ $1$ ${0}$
###### 7

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $25$ ${5}$ $1$ ${1}$ $4$ ${2}$ $16$ ${4}$ $9$ ${3}$
###### 8

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $-5$ ${5}$ $-2$ ${2}$ $-3$ ${3}$ $-2$ ${2}$ $-4$ ${4}$
###### 9

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $2$ ${4}$ $3$ ${9}$ $4$ ${16}$ $5$ ${25}$ $6$ ${36}$
###### 10

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $7$ ${49}$ $9$ ${81}$ $11$ ${121}$ $13$ ${169}$ $15$ ${225}$
###### 11

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $43$ ${{\frac{1}{43}}}$ $62$ ${{\frac{1}{62}}}$ $84$ ${{\frac{1}{84}}}$ $58$ ${{\frac{1}{58}}}$ $1$ ${1}$
###### 12

Write an equation in the form $y=\ldots$ suggested by the pattern in the table.

 $x$ $y$ $54$ ${{\frac{1}{54}}}$ $27$ ${{\frac{1}{27}}}$ $25$ ${{\frac{1}{25}}}$ $33$ ${{\frac{1}{33}}}$ $99$ ${{\frac{1}{99}}}$
###### 13

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $0$ ${93}$ $1$ ${100}$ $2$ ${107}$ $3$ ${114}$ $4$ ${121}$ $5$ ${128}$
###### 14

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $0$ ${62}$ $1$ ${70}$ $2$ ${78}$ $3$ ${86}$ $4$ ${94}$ $5$ ${102}$
###### 15

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $5$ ${51}$ $6$ ${49}$ $7$ ${47}$ $8$ ${45}$ $9$ ${43}$ $10$ ${41}$
###### 16

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $10$ ${74}$ $11$ ${72}$ $12$ ${70}$ $13$ ${68}$ $14$ ${66}$ $15$ ${64}$
###### 17

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $3$ ${19}$ $4$ ${27}$ $5$ ${43}$ $6$ ${75}$ $7$ ${139}$ $8$ ${267}$
###### 18

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $8$ ${260}$ $9$ ${516}$ $10$ ${1028}$ $11$ ${2052}$ $12$ ${4100}$ $13$ ${8196}$
###### 19

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $0$ ${17}$ $1$ ${18}$ $2$ ${25}$ $3$ ${44}$ $4$ ${81}$ $5$ ${142}$
###### 20

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $1$ ${11}$ $2$ ${18}$ $3$ ${37}$ $4$ ${74}$ $5$ ${135}$ $6$ ${226}$
###### 21

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $-10$ ${35.92}$ $-9$ ${36.32}$ $-8$ ${36.72}$ $-7$ ${37.12}$ $-6$ ${37.52}$ $-5$ ${37.92}$
###### 22

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $-2$ ${82.57}$ $-1$ ${84.08}$ $0$ ${85.59}$ $1$ ${87.1}$ $2$ ${88.61}$ $3$ ${90.12}$
###### 23

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $5$ ${85}$ $10$ ${125}$ $12$ ${141}$ $16$ ${173}$ $17$ ${181}$ $18$ ${189}$
###### 24

Does the following table show that $x$ and $y$ have a linear relationship?

• yes

• no

 $x$ $y$ $1$ ${18}$ $5$ ${50}$ $7$ ${66}$ $13$ ${114}$ $16$ ${138}$ $19$ ${162}$
###### 25

This table gives population estimates for Portland, Oregon from 1990 through 2014.

 Year Population Year Population 1990 487849 2003 539546 1991 491064 2004 533120 1992 493754 2005 534112 1993 497432 2006 538091 1994 497659 2007 546747 1995 498396 2008 556442 1996 501646 2009 566143 1997 503205 2010 585261 1998 502945 2011 593859 1999 503637 2012 602954 2000 529922 2013 609520 2001 535185 2014 619360 2002 538803

Find the rate of change in Portland population between 2005 and 2006. Just give the numerical value; the units are provided.

$\,\frac{\text{people}}{\text{year}}$

And what was the rate of change between 2008 and 2014?

$\,\frac{\text{people}}{\text{year}}$

List all the years where there is a negative rate of change between that year and the next year.

###### 26

This table and graph gives population estimates for Portland, Oregon from 1990 through 2014.

 Year Population Year Population 1990 487849 2003 539546 1991 491064 2004 533120 1992 493754 2005 534112 1993 497432 2006 538091 1994 497659 2007 546747 1995 498396 2008 556442 1996 501646 2009 566143 1997 503205 2010 585261 1998 502945 2011 593859 1999 503637 2012 602954 2000 529922 2013 609520 2001 535185 2014 619360 2002 538803

Between what two years that are two years apart was the rate of change highest?

What was that rate of change? Just give the numerical value; the units are provided.

$\,\frac{\text{people}}{\text{year}}$