Chapter 4 Scatterplots – Descriptions

Scatterplots Graphical display of two quantitative variables We plot the explanatory (independent) variable on the x-axis and the response (dependent) variable on the y-axis Each dot represents a single observation and it’s two values Categorical variables may be included by adding in different colors or symbols

Determine Explanatory and Response Variables Hours of studying & grade on a test # of tumors the mice develop & # of grams of a toxin given to lab mice Yearly income & life expectancy

Describing Scatterplots 4 things that must be talked about: ◦ Direction ◦ Form ◦ Strength ◦ Unusual elements

Direction Positive: as values of the explanatory variable increase, values in the response variable tend to increase

Direction Negative: as values of the explanatory variable increase, value in the response variable tend to decrease

Direction None: no discernable change in the response variable

Form (Shape) Linear: The shape has the appearance of a linear relationship. Does not have to fit it perfectly.

Form Curved

Form None No discernable form

Strength (Scatter) Strong association: very little scatter

Strength Moderate strength:

Strength Weak strength: lots of scatter

Unusual Features Outliers

Practice Negative Curved Moderately Strong No outliers

Positive Linear Weak No outliers

Positive Linear Moderately strong Possible outlier around (9, 35).

No direction No form No apparent association Possible outlier around (60,8).

Positive Curved Strength weakens as the explanatory variable increases No outliers