1 Working With Graphs

2 Graphs In General: A graph is a visual representation of the relationship between two ormore variables. We will deal with just two variables at a time.

3 Graphs In General: 1. Independent variable: This is the variable that influences the dependent variable. (X variable) 2. Dependent variable: Its value is determined by the independent variable. (Y variable)

4 Graphs In General: 3. We say that the dependent variable is a function of the independent variable: function of the independent variable: Y = f(X)

5 The Axis of a Graph: Dependent Variable Dependent Variable (Y-axis) (Y-axis) Independent Variable ( X-axis)

6 Direct Relationships: v A person's weight and height are often related. v If we sample 1000 people and measure their weight and height we would probably find that as weight increases so does height.

7 Direct Relationships: HeightWeight

8 v There is a direct relationship between height and weight. v Have a direct relationship when:  indep. variable  dep. variable   indep. variable  dep. variable   indep. variable  dep. variable   indep. variable  dep. variable 

9 Inverse Relationships: There is strong evidence indicating that as price rises for a specific commodity, the amount purchased decreases.

10 Inverse Relationships: Price per Unit DemandCurve Quantity Purchase per Unit Time

11 Inverse Relationships: v There is an inverse relationship between price per unit and the quantity purchased per unit of time. v Have an inverse relationship when: (1) indep. variable    dep. variable (2) indep. variable  dep. variable

12 Complex Relationships: v Evidence suggests that income from wages increases up to a certain age, and then decreases until death.

13 Complex Relationships: Income from Wages ($) Age

14 Complex Relationships: v There is a direct relationship between wage income and age up to a certain point known as retirement, v then an inverse relationship exists from retirement to the individuals expiration date.

15 Complex Relationships: Income from All Sources ($) Age

16 Complex Relationships: Income from All Sources ($) Age

17 Complex Relationships: Income from All Sources ($) Age What should the slope of this line be equal to at the minimum?

18 Constructing A Graph We start with a horizontal number line:

19 Constructing A Graph 1.The points on the line divide the line into segments. 2.All the line segments are equally spaced 3.Numbers associated with the points increase in value from left to right. 4.Use a distance, so many points, to represent a quantity.

20 Constructing A Graph       

21 Add a Vertical Number Line: 1. Construct a vertical number line. 1. Construct a vertical number line. 2. Points divide the line into equal 2. Points divide the line into equal segments. segments. 3. Numbers associated with points 3. Numbers associated with points increase in value from the bottom increase in value from the bottom to top. to top. 4. The scale can be different from the 4. The scale can be different from the horizontal number line. horizontal number line.

22 Add a Vertical Number Line:         

23 To Make A Graph: 1. The vertical and horizontal number lines must intersect at each others zero point. 2. They must be perpendicular.  

24 To Make A Graph: The vertical and horizontal number lines should look like the illustration below:

25 To Make A Graph: 3. Result: We get a set of coordinate axis, or a coordinate number system. e.g. Sighting in a rifle scope on the range.

How would you call out the location of this three shot group? X-Axis Y-Axis

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30 To Make A Graph: 4. With a graph, you need two numbers to specify a single point OR OR When you see a point on a graph, you know that point represents two numbers ! When you see a point on a graph, you know that point represents two numbers !

31 BASICS YOU NEED TO KNOW ABOUT GRAPHING AND THE COORDINATE NUMBER SYSTEM Axis defined: v The vertical number line is reserved for the Dependent variable and is referred to as the Y AXIS. v The horizontal number line is referred to as the X AXIS and is reserved for the Independent variable.

32 The origin and points on the graph v The point of intersection of the two number lines is referred to as the ORIGIN.

33 Point A represents two numbers: A value for x and a value for y.   Point A

34 The origin and points on the graph v Every point on a graph represents a pair of observations of x and y. (x,y) v In this class, y will often represent price and x will often represent quantity.

35 The Slope 1. Slope = change in Y values / change in X values = (y 1 - y 0 ) / (x 1 - x 0 ) = (y 1 - y 0 ) / (x 1 - x 0 ) = RISE / RUN = RISE / RUN

36 The Slope Price Quantity demanded per unit time 2 3 Quantity demanded per unit time  x 1, y 1 )  x 0, y 0 )

37 The Slope 2. As X goes from 2 to 3, Y goes from 8 to 6. Y goes from 8 to  Y = RISE = (TO - FROM) = = -2  X = RUN = (TO - FROM) = = 1

38 The Slope 4. SLOPE =  Y /  X = -2 / 1 = The slope of a straight line is CONSTANT. Class Exercise

39 References: v N.c.State university-College of Agriculture and Life science –Dr. herman_sampson