Step 1: Calculate 4-year moving totals:
Add each set of 4 consecutive years:
2001–2004: $2450 + 1470 + 2150 + 1800 = 7870$
2002–2005: $1470 + 2150 + 1800 + 1210 = 7630$
2003–2006: $2150 + 1800 + 1210 + 1950 = 7110$
2004–2007: $1800 + 1210 + 1950 + 2300 = 7260$
2005–2008: $1210 + 1950 + 2300 + 2500 = 7960$
2006–2009: $1950 + 2300 + 2500 + 2480 = 9230$
2007–2010: $2300 + 2500 + 2480 + 2680 = 9960$
Step 2: Convert to 4-year centered moving averages:
Centered average is the average of two consecutive 4-year totals divided by 4.
E.g., (7870 + 7630)/8 = 1937.5
Continue for all valid periods and get centered values.
Step 3: Calculate seasonal index = (Actual / Moving Average) × 100 for mid-year
E.g., for 2003: Index = 2150, Avg ≈ 1937.5 $\Rightarrow$ SI = $(2150 / 1937.5) \times 100 \approx 111$
Continue this for each year.
Step 4: Group years by quarter or season, average seasonal indices per group.
Then scale the average seasonal indices so that their total is 400 (for 4 seasons).