Numerical: discrete (counts) / continuous (range)
Categorical: ordinal (order) / nominal (no order)
Primary = you · Secondary = others
Mean = sum ÷ how many
Median = middle (order first)
Mode = most common
Use the mid-interval value of each class
Mean = Σ(mid × f) ÷ Σf
Median at (n+1)/2
Range = highest − lowest
IQR = Q₃ − Q₁
Q₁ at (n+1)/4, Q₃ at 3(n+1)/4
Survey, online, interview, experiment, observation
Bias: a question that favours one side
Sample = part; Population = all (census)