Distinguish chance differences between random samples from systematic selection bias.
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First lesson in this subject: Read data before calculating
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A population has 8 members of A, each with value 0, and 4 members of B, each with value 12. Its mean is (8 × 0 + 4 × 12)/12 = 4.
A sample contains only some members. In a simple random sample of fixed size, every subset of that size has the same chance. That fair method does not guarantee that each sample copies the population’s exact group proportions.
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First lesson in this subject: Read data before calculating
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Practice · Compare weighted populations and sampling methods. Separate a method’s bias from variation in individual samples.
Book · Read distributions, question samples, and explore how groups and outliers change a conclusion.
Previous lesson · See why equal means can hide different variability and what standard deviation summarizes.
Next lesson · Combine totals correctly and see how changing group proportions can reverse a comparison.
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