RuneScape:Grand Exchange Market Watch/Common Trade Index

• Adjusted index: Common Trade Index
• Adjustment date: 10 January 2008
• Affected templates: Template:GE Common Trade Index and Template:GE Common Trade Index/Diff
• Added item(s): 8 — Mithril ore, Raw swordfish, Raw monkfish, Snape grass, Clean ranarr, Clean kwuarm, Clean snapdragon, Vial of water
• Removed item(s): None
• Items before adjustment: 15
• Items after adjustment: 23
• Divisor before adjustment: 14.464460143296
• Divisor after adjustment: 21.791759207424

• Adjusted index: Common Trade Index
• Adjustment date: 12 February 2012
• Affected templates: Template:GE Common Trade Index and Template:GE Common Trade Index/Diff
• Added item(s): 10 — Abyssal whip, Dragon boots, Rune armour set (lg), Red chinchompa, Oak plank, Shark, Green dragonhide, Dragon bones, Cannonball, Dragonfire shield
• Removed item(s): 6 — Mithril ore, Law rune, Big bones, Raw swordfish, Vial of water, Clean ranarr
• Items before adjustment: 23
• Items after adjustment: 27
• Divisor before adjustment: 21.791759207424
• Divisor after adjustment: 23.9535

Calculations

From the old divisor obtained from the templates:

$ {div}_{\text{old}} = 21.791759207424 $

We need to calculate a new divisor:

$ {div}_{\text{new}} = {div}_{\text{old}} \times \frac{\sum \left ( \frac{p}{q} \right )_{\text{new}}}{\sum \left ( \frac{p}{q} \right )_{\text{old}}} $

To calculate the new divisor, we need to find:

$ \begin{align} \sum \left ( \frac{p}{q} \right )_{\text{old}} & = \text{sum of ratios prior to change} \\ & = \left (\frac{320}{159} + \frac{1,335}{540} + \dots + \frac{1,828}{3,030} \right ) + \left (\frac{363}{303} + \frac{621}{419} + \dots + \frac{4,958}{5,959} \right ) \\ & = 35.26980062 \text{ (up to 8 d.p.)} \end{align} $

And also:

$ \begin{align} \sum \left ( \frac{p}{q} \right )_{\text{new}} & = \text{sum of ratios prior to change} - \text{sum of removed ratios} + \text{sum of added ratios} \\ & = \sum \left ( \frac{p}{q} \right )_{\text{old}} - \text{sum of removed ratios} + \text{number of added items} \\ & = 35.26980062 - \left ( \frac{363}{303} + \frac{621}{419} + \dots + \frac{4,958}{5,959} \right ) + 10 \\ & = 38.76853218 \text{ (up to 8 d.p.)} \end{align} $

Thus, the new divisor is:

$ \begin{align} {div}_{\text{new}} & = {div}_{\text{old}} \times \frac{\sum \left ( \frac{p}{q} \right )_{\text{new}}}{\sum \left ( \frac{p}{q} \right )_{\text{old}}} \\ & = 21.791759207424 \times \frac{38.76853218}{35.26980062} \\ & = 23.9535 \text{ (4 d.p.)} \end{align} $