- Summary statistics for groups identical
- Mean x = 9.0
- Mean y = 7.5
- Variance of x = 10.0
- Variance of y = 3.75
- Linear regression model: y = 0.5x + 3
- Are four data sets really similar?
- Sometimes, Statistics about the data is not the same as the data
- Use visualization tools to look at the data itself
Lying with Pictures
- Telling the Truth with Pictures
- Moral: Look carefully at the axes labels and scales
GIGO (Garbage In, Garbage Out)
- Moral: Analysis of bad data can lead to dangerous conclusions.
- “Convenience sampling” not usually random, e.g.,
- Survivor bias, e.g., course evaluations at end of course or grading final exam in 6.00.2x on a curve
- Non-response bias, e.g., opinion polls conducted by mail or online
- Moral: Understand how data was collected, and whether assumptions used in the analysis are satisfied. If not, be wary.
A Comforting Statistic?
- 99.8% of the firearms in the U.S. will not be used to commit a violent crime in any given year
- How many privately owned firearms in U.S.?
- 300,000,000*0.002 = 600,000
- Moral: Context matters. A number means little without context.
Relative to What?
- Consider drugs X and Y for treating acne
- X cures acne twice as well as Y
- X kills twice as many acne patients as Y
- Do you want to take X or Y?
- Suppose Y kills 0.00001% of cases, and cures 50% of them
- Moral: Beware of percentages when you don’t know the baseline
- Does going to school contribute to the spread of flu?
- Establishing Causation
- Attempt to control for all variables other than the variables of interest
- Randomized control studies the gold standard
- Start with a population
- Randomly assign members to either
- Control group
- Treatment group
- Deal with two groups identically except with respect to the one thing being evaluated
- Very hard to do