5.5. Predicted probabilities (worked example)#
A logistic regression model describes whether the probability of voting for Candidate X in an election depends on \(x\) = the voter’s total family income (in thousands of dollars) the previous year. The prediction equation is:
\(\log{ \left[ \frac{p(y=1)}{1-p(y=1)} \right]} = -2.00 + 0.03x \)
Before we get to python, this exercise is a chance to practice converting logistic regression output into predicted probabilities by hand (i.e., by calculator or in Excel!) to help you see what is going on.
Identify \(\beta\) and interpret its sign
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0.03. The estimated probability of voting for Candidate X increases as income increases.
Find the estimated probability of voting for the candidate when income = 10,000.
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When we plug in a value of \(x = 10\), we find logit(p) = -1.7.
The alternative equation for logistic regression (derived from the equation above, see lecture slides) that expresses the probability directly is:
Thus
The probability of voting for Candidate X when income = 10,000 (i.e., x = 10) is 0.15, or 15%.
At which income level is the estimated probability for the candidate equal to 0.50?
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When \(p(y=1) = 0.5\), the odds \(\frac{p(y=1)}{1-p(y=1)} = 0.\)
So, we take \(0 = \alpha + \beta x\) and solve for \(x\)
We then find that \(p(y=1)=0.5\) when \(x=\frac{-\alpha}{\beta}\).
Thus, for this example, \(p(y=1) = \frac{2}{0.03}\) = 66.67 thousand dollars