LSP 120: Quantitative Reasoning and Technological Literacy

Section 202

Özlem Elgün

Why are we here?

• Data: numbers with a context• Cell: each data point is recorded in a cell• Observation: each row of cells form an

observation for a subject/individual• Variable: any characteristic of an

individual

Why Data?

1) Data beat anecdotes“Belief is no substitute for arithmetic.”

Henry Spencer

Data are more reliable than anecdotes, because they systematically describe an overall picture rather than focus on a few incidents .

Why Data?

2. Where the data come from is important.“Figures won’t lie, but liars will figure.”

Gen. Charles H. Grosvenor (1833-1917), Ohio Rep.

Familiarizing with Data

• Open Excel

• Collect data:– Ask 5 classmates the approximate # of text messages they

send per day– Record the data on Excel spreadsheet

• Calculate average using the Average function on Excel. (There are many functions such as sum, count, slope, intercept etc. that we will use in this class)

What is a linear function?

• Most people would say it is a straight line or that it fits the equation y = mx + b.

• What is the relationship between the variables in a linear function?

• A linear function indicates a relationship between x and y that has a fixed or constant rate of change.

Is the relationship between x and y is linear?

The first thing we want to do is be able to determine whether a table of values for 2 variables represents a linear function:

x y 3 11 5 16 7 21 9 26

11 31

• To determine if a relationship is linear in Excel, add a column in which you calculate the rate of change.

• You must translate the definition of “change in y over change is x” to a formula using cell references.

  A B C

1 x y Rate of Change

2 3 11  

3 5 16 =(B3-B2)/(A3-A2)

4 7 21  

5 9 26  

6 11 31  

This is a cell reference

• If the rate of change values are constant then the relationship is a linear function.

• So this example does represent a linear function. Rate of change is 2.5 and it is constant.

• This means that that when the x value increases by 1, the y value increases by 2.5.

  A B C1 x y Rate of Change2 3 11  3 5 16 2.54 7 21 2.55 9 26 2.56 11 31 2.5

How to Write a Linear EquationNext step is to write the equation for this function.

y = mx + b. y and x are the variables m is the slope (rate of change) b is the y-intercept (the initial value when x=0)

We know x, y, and m, we need to calculate b:Using the first set of values (x=3 and y=11) and 2.5 for "m“ (slope):

11=2.5*3 + b. Solving: 11=7.5 + b

3.5 = b.

The equation for this function is : y = 2.5 x + 3.5

Another way to find the equation is to use Excel’s intercept function.

  A B C

1 x yRate of Change

2 3 11  3 5 16 2.54 7 21 2.55 9 26 2.56 11 31 2.5

PracticeOn the Class Website Open Rate of Change Calculationsdetermine whether the function is linear and if so, write

the equation for the function.

x y

5 -4

10 -1

15 2

20 5

x y

1 1

2 3

5 9

7 18

x y

2 20

4 13

6 6

8 -1

"Real world" example of a linear function:• Studies of the metabolism of alcohol consistently show that blood

alcohol content (BAC), after rising rapidly after ingesting alcohol, declines linearly. For example, in one study, BAC in a fasting person rose to about 0.018 % after a single drink. After an hour the level had dropped to 0.010 %. Assuming that BAC continues to decline linearly (meaning at a constant rate of change), approximately when will BAC drop to 0.002%?

• In order to answer the question, you must express the relationship as an equation and then use to equation. First, define the variables in the function and create a table in excel.

• The two variables are time and BAC. • Calculate the rate of change.

Time BAC0 0.018%1 0.010%   

Time BACRate of change

0 0.018%

1 0.010% -0.008%

 

This rate of change means when the time increases by 1, the BAC decreases (since rate of change is negative) by .008. In other words, the BAC % is decreasing .008 every hour. Since we are told that BAC declines linearly, we can assume that figure stays constant. Now write the equation with Y representing BAC and X the time in hours. Y = -.008x + .018.This equation can be used to make predictions. The question is "when will the BAC reach .002%?" Plug in .002 for Y and solve for X..002 = -.008x + .018-.016 = -.008xx = 2Therefore the BAC will reach .002% after 2 hours.

Warning: Not all graphs that look like lines represent linear functions

The graph of a linear function is a line. However, a graph of a function can look like a line even thought the function is not linear. Graph the following data where t is years and P is the population of Mexico (in millions):

• What does the graph look like?• Now, calculate the rate of change

for each set of data points (as we learned under Does the data represent a linear function?) Is it constant?

t P1980 67.38

1981 69.131982 70.931983 72.771984 74.671985 76.611986 78.60

• What if you were given the population for every ten years? Would the graph no longer appear to be linear? Graph the following data.

• Does this data (derived from the same equation as the table above) appear to be linear? Both of these tables represent an exponential model (which we will be discussing shortly). The important thing to note is that exponential data can appear to be linear depending on how many data points are graphed. The only way to determine if a data set is linear is to calculate the rate of change (slope) and verify that it is constant.

t P1980 67.38

1990 87.102000 112.582010 145.532020 188.122030 243.162040 314.32