Time Series
Gujarat Board · Class 12 · Statistics
Quick revision notes for Time Series — Gujarat Board Class 12 Statistics. Key concepts, formulas, and definitions for last-minute revision.
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4.1 Meaning, Definition and Importance of Time Series
- A Time Series is a set of observations of a variable recorded at successive, equally spaced intervals of time.
- Time (t) is always the INDEPENDENT variable; the observed quantity (y) is the DEPENDENT variable.
- Data must be arranged in CHRONOLOGICAL ORDER — from earliest to latest.
4.2 Components of Time Series
- There are FOUR components: Trend (T), Seasonal (S), Cyclical (C), and Random/Irregular (I).
- ADDITIVE MODEL: y = T + S + C + I — the observed value is the SUM of all four components.
- Trend (T): Long-term, sustained, overall direction of change (rise or fall) over many years.
4.3.1 Graphical Method of Measuring Trend
- Easiest and quickest method — requires no mathematical formula.
- Steps: (1) Plot points with time (t) on X-axis and variable (y) on Y-axis. (2) Join consecutive points with straight line segments. (3) Draw a smooth freehand curve through the MIDDLE of all points us
- The smooth curve drawn represents the trend — short-term ups and downs are ignored.
4.3.2 Method of Least Squares – Linear Trend
- This is a MATHEMATICAL method — completely objective, not subjective like graphical method.
- A linear equation ŷ = a + bt is fitted to the time series data to represent the LINEAR TREND.
- Constants a and b are calculated using two normal equations derived from the principle of least squares.
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