Mode = most often
Median = middle in order
Mean = sum ÷ how many
Range = highest − lowest
IQR = upper − lower quartile
Total = mean × how many
Total = add frequencies
Mean = Σ(v×f) ÷ Σf
Bar chart: gaps (categorical)
Histogram: touching (continuous)
r between -1 and +1
sign = direction, size = strength
68 / 95 / 99.7 within 1/2/3 SD
Margin of error E = 1÷√n