Need to Know

Statistics

The rules you must have ready for the exam.

How to use this: say each rule in your head, then tap Show me to check.

Data & averages

1 rule

Rule 01

Types & averages

Numerical (discrete/continuous) or categorical (ordinal/nominal). Mode = most often, median = middle in order, mean = sum÷count. Watch for outliers.

1 rule

Rule 02

Working with the mean

Total = mean × how many — use it for missing values and combined means. Range = highest − lowest; IQR = upper − lower quartile.

1 rule

Rule 03

Totals & mean

Total = add the frequencies; mode = highest frequency; mean = Σ(value×frequency) ÷ total frequency.

2 rules

Rule 04

Which chart

Line plot: stacked ×. Bar chart: gaps (categorical). Histogram: touching bars (continuous). Height = frequency.

Rule 05

Standard deviation

σₓ measures spread about the mean, from the calculator's stats mode.

2 rules

Rule 06

Scatter & r

Bivariate data plotted as points. r between -1 and +1: sign = direction, size = strength.

Rule 07

Normal & margin of error

Normal curve: mean=median=mode; 68/95/99.7 within 1/2/3 SD. Margin of error E = 1÷√n.

Mathslive.ie · Statistics · Need to Know