Reading Graphs

1
Reading Graphs Graphing Equations
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• Igor Lukashin
• Lecture 1A ECN 112 Microeconomic Principles
• Spring 1999

2
Types of Graphs

• Line Graph.
• Bar Graph
• Pie Chart

3
Variables and Their Relationships

• Independent Variable
• Value does not depend on the value of other
  variables. Example Time.
• Dependent Variable
• Value (possibly) depends on the value of
  independent variable(s). Example Distance is a
  function of time and speed.
• Direct (positive) Relationship
• Values of related variables move in the same
  direction. Example Income and Consumption
• Inverse (negative) Relationship
• Values move in opposite directions. Example
  Speed and Time, holding Distance constant.

4
Constructing a Two-Dimensional Line Graph

• Draw Axes
• Vertical(for dependent variable) Y
• Horizontal (for independent variable) X
• Label Axes
• Put the variable name or symbol near the end of
  the axes, denote measurement units for each.
• Use pairs of data to plot points on the graph
• Find the value of the X variable on the
  horizontal axis and draw a vertical line through
  it.

5
Constructing a Two-Dimensional Line Graph 2

• Find the value of the Y variable on the vertical
  axis and draw a horizontal line through it.
• Mark the point where the two lines you just drew
  intersect this will be a point of the graph
  corresponding to the first pair of values
• Repeat the procedure for the remaining points
• Connect the points of the graph with line segments

6
Reading a Two-Dimensional Line Graph

• Select a point on the graph
• Draw horizontal and vertical lines through the
  point
• Note the values at which they intersect with
  vertical and horizontal axes, respectively
• These are the values of the variables
  corresponding to the point on the graph

7
Slopes, Equations

• Slope the steepness of a curve, measured as the
  ratio of the rise to the run, or change in the
  vertical axis variable over the change in the
  horizontal variable between two points
• Slope ?Y/?X
• Slope of a straight line is constant
• Generic Straight Line Equation
• Y a bX
• a, b - parameters

8
Assignment Due Monday 1/25

• Use your credit card bill to create a graph
  showing your daily balance as a function of the
  day of the month (or use some other variable you
  have records of)
• Is it a positive or negative relationship?
• Is it a straight line?
• What are the average, maximum and minimum values
  of the slope?