←All concepts
Concept · Probability & Statistics
Combining groups, transformations, regression
Practice this concept→
A Probability & Statistics set, untimed
Named traps
7 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- Between-group variance omittedOmits the between-group (mean-shift) contribution when pooling variances.→
- Histogram frequency/width confusionReads frequency/class-width (density) as the frequency directly, mis-estimating counts/mode/median.→
- Pooled variance simple-averagedAverages stratum variances directly instead of using the weighted between+within formula.→
- Regression slope numerator/denominator swapSwaps the numerator/denominator of the least-squares slope or forgets the n·xbar·ybar term.→
- SD vs variance (missing sqrt)Reports the variance as the standard deviation (forgets the square root).→
- Variance scaling on nonlinear relationApplies linear variance-scaling rules to a nonlinear relationship between variables.→
- Variance scaling without squareUses D(aX+b)=aD(X), forgetting to square a (and that +b does not change variance).→
Key formulas
15- Classical probabilityP(A) = n(A)/n(S), 0 ≤ P(A) ≤ 1P(certain)=1, P(impossible)=0
- Combinations C(n,k)=n!/(k!(n−k)!)permutations A(n,k)=n!/(n−k)!
- Addition ruleP(A∪B) = P(A) + P(B) − P(A∩B)
- Complement: P(Ac) = 1 − P(A)at-least-one: P(≥1) = 1 − P(none)
- Conditional: P(A|B) = P(A∩B)/P(B)Multiplication: P(A∩B)=P(A)P(B|A)Independence: P(A∩B)=P(A)P(B)
- Meanx̄ = (Σ xi)/nweighted/combined mean weights by group sizes
- Variances2 = (1/n) Σ (xi − x̄)2standard deviation s = √s2range = xmax − xmin
- Linear transformE(aX+b)=aE(X)+bD(aX+b)=a2 D(X)
- Pooled variance of two groupsweight within-group variances by size AND add between-group (mean-shift) term
- Normal PDFf(x) = 1/(σ√2π) · e−(x−μ2/(2σ2))curve symmetric about μtotal area = 1
- Empirical rule≈68% within μ±σ, ≈95% within μ±2σ, ≈99.7% within μ±3σ
- Standard normalZ = (x−μ)/σΦ(x)+Φ(−x)=1
- Estimated count in an interval = P(interval) × population size
- Least-squares slopeb̂ = (Σ xi yi − n x̄ ȳ)/(Σ xi2 − n x̄2)intercept â = ȳ − b̂ x̄
- Normal approximation to binomial (De Moivre-Laplace)X~B(n,p) ≈ N(np, np(1−p))
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
FoundationsStatistics and probability