Measures of Central Tendency: Mean (average — sum of observations ÷ number of observations. For grouped data: Mean = Σfx/Σf where f=frequency, x=class mark), Median (middle value when data arranged in order — for odd n: (n+1)/2 th term, for even n: average of n/2 and (n/2+1) terms), Mode (most frequently occurring value — a dataset can have no mode, one mode, or multiple modes).

Mean for grouped frequency distribution — three methods: Direct Method (Σfx/Σf), Assumed Mean/Short-cut Method (A + Σfd/Σf, where d=x-A), Step Deviation Method (A + (Σfu/Σf)×h, where u=(x-A)/h). Step deviation is most efficient for large class intervals.

Median for grouped data: Median = l + [(n/2 - cf)/f] × h, where l=lower limit of median class, n=total frequency, cf=cumulative frequency before median class, f=frequency of median class, h=class width. Mode for grouped data: Mode = l + [(f₁-f₀)/(2f₁-f₀-f₂)] × h, where f₁=modal class frequency, f₀=preceding class frequency, f₂=succeeding class frequency.

Relationship: 3 Median = Mode + 2 Mean (empirical relationship). Measures of Dispersion: Range (highest - lowest), Mean Deviation, Variance, Standard Deviation (σ = √variance). Ogive: Cumulative frequency graph — 'less than' ogive (ascending) and 'more than' ogive (descending). Their intersection point gives the Median.

REET Exam Tips: 5 questions — focus on Mean-Median-Mode calculation formulas, the empirical relationship (3Median = Mode + 2Mean), and step deviation method. Word problems involve calculating mean/median from frequency tables.