Scatterplot

Scatterplots are statistical graphs using coordinates to display two variables for a set of data pairs. The relationship between variables is called correlation. Scatterplots are used to understand the type and the strength of correlation between two variables.

**Example:** The following table shows the test average
and course overall grades of 20 students.

Test-Average | Course-Overall |
---|---|

80 | 74 |

66 | 65 |

72 | 75 |

76 | 71 |

78 | 61 |

73 | 65 |

74 | 69 |

80 | 79 |

79 | 72 |

80 | 79 |

94 | 95 |

77 | 70 |

65 | 64 |

82 | 73 |

89 | 88 |

92 | 88 |

96 | 96 |

97 | 98 |

88 | 85 |

77 | 75 |

Draw a scatterplot for the data and use it to answer the following questions.

Does there seems to be a positive linear relationship? Why or why not?

Does the relationship seems strong or weak? Why?

**Solution:** To draw a scatterplot, you may follow the
instructions below.

**Step 1:** Enter the set of data pairs, say, in the
region `A1:B21`

in a new worksheet.

**Step 2:** Select data arrays in the range
`A1:B21`

.

**Step 3:** On the Insert tab, click the XY (Scatter)
chart command button.

**Step 4 (optional):** Add a trendline by clicking the
Add Chart Element menuâ€™s Trendline command button.

**Step 5 (optional):** Change bounds for the *x*-axis by clicking the *x*-axis and then right clicking to
format axis.

**Step 6 (optional):** Change the title of the picture
by clicking the title.

From the scatterplot, we can see that there is a strong positive linear correlation between the test scores and the course overall scores.

The following data set consists of the midterm and the final exam scores of 20 students.

Midterm | Final |
---|---|

72 | 72 |

93 | 88 |

81 | 82 |

82 | 82 |

94 | 88 |

80 | 77 |

73 | 78 |

71 | 77 |

81 | 76 |

81 | 76 |

63 | 68 |

73 | 82 |

88 | 89 |

88 | 92 |

96 | 96 |

98 | 97 |

85 | 88 |

75 | 77 |

94 | 95 |

92 | 88 |

Draw a scatterplot for the data and use it to answer the following questions.

Does there seems to be a positive linear relationship? Why or why not?

Does the relationship seems strong or weak? Why?