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1. Exploratory Data Analysis
1.3. EDA Techniques
1.3.3. Graphical Techniques: Alphabetic Scatter Plot

Scatter Plot: Quadratic Relationship

Scatter Plot Showing Quadratic Relationship scatter plot showing quadratic relationship
Discussion Note in the plot above how no imaginable simple straight line could ever adequately describe the relationship between X and Y--a curved (or curvilinear, or non-linear) function is needed. The simplest such curvilinear function is a quadratic model
    \[ Y_{i} = A + BX_{i} + CX_{i}^{2} + E_{i} \]
for some A, B, and C. Many other curvilinear functions are possible, but the data analysis principle of parsimony suggests that we try fitting a quadratic function first.
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