# Grand Exchange Market Watch/FAQ

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## What is the Grand Exchange Market Watch?

The Grand Exchange Market Watch is a project set up to monitor and record the prices and other information on items on the Grand Exchange.

## Who created this project?

This project was thought up and originally coded by Robert Horning. Many other people over the years since it was created have been involved in "tweaking" the organization of this project and have contributed a huge amount of information to make everything work and to keep the information up to date. Adding in data about the prices can be done by anyone, if you would like to help then you just need to:

1. Click "Update price"

Usually, users do not need to manually update the Grand Exchange Market Watch as automated software keeps it up to date. However, the "bots" require several hours before all prices can be updated. If users require updated prices urgently, the Wiki provides the option of manually updating individual price data.

### List of bots

Below is the list of bots currently involved in the project (as of July 2012):

## What is the common trade index?

The Common Trade Index is a figure which is designed to help show the health of the RuneScape economy in general. The common trade index is worked out using a list of items which are traded very often. There are also other indices such as the Rune, Log, Food, Metal, Herb indices, all of which show the health of that aspect of the RuneScape economy and work in the same way as the Common Trade index.

### List of indices

Index Base date Current Change
Common Trade Index 9 December 2007 95.97  +0.97
Discontinued Rare Index 31 December 2008 1,755.34  -16.67
Rune Index 15 December 2007 65.09  -0.22
Food Index 12 January 2008 134.29  +0.37
Metal Index 10 December 2007 138.28  -0.46
Herb Index 9 June 2009 170.85  +0.50

### How do I understand the Common Trade Index?

The Common Trade Index is displayed as 2 numbers. The first is the index number. This shows the comparative prices of all the items on the list compared to their price on the base date. A value above 100 means that in general the prices have risen and consequently that the RuneScape economy is doing quite well, while a value below 100 means that overall prices have dropped.

Next to that number is index change number accompanied by an arrow, either red or green. This shows whether the index has gone up or down since it was last calculated, a negative number and a red arrow indicate that the index has dropped as prices have generally fallen, while a positive number shows that the prices have generally risen.

### How is the Common Trade Index calculated?

When the index was started, it was easily calculated by summing the current price of items divided by their base prices (i.e. the price on the base date) and then dividing the sum with the number of items at the time.

$X = 100 \times {\sum_{i=1}^{n} \left ( \frac{p_i}{q_i} \right ) \over n} = {\frac{p_1}{q_1} + \frac{p_2}{q_2} + \dots + \frac{p_n}{q_n} \over n}$
\begin{align} \text{where:} \qquad p & = \text{current price} \\ q & = \text{base price} \\ n & = \text{number of items} \end{align}

For example, let us assume the index is composed of four fictional items: A, B, C, and D, with their prices at 30, 40, 70, and 60 coins, respectively. When the index was started, the index would result in:

$X = 100 \times {\sum_{i=1}^{n} \left ( \frac{p_i}{q_i} \right ) \over n} = 100 \times {30/30 + 40/40 + 70/70 + 60/60 \over 4} = 100$

Now, let us change the prices of the four items (Item A at 22, Item B at 31, Item C at 85, and Item D at 64). Notice that the index is down by 5.27 points:

$X = 100 \times {22/30 + 31/40 + 85/70 + 64/60 \over 4} = 94.73$

However, this simple procedure becomes more complicated whenever items are removed and/or added to the Index. While the value of the index remains the same at the moment of the removal and/or addition, the divisor changes. Thus, we need to calculate a new divisor:

\begin{align} X_{\text{new}} & = X_{\text{old}} \\ {\sum \left ( \frac{p}{q} \right )_{\text{new}} \over {div}_{\text{new}}} & = {\sum \left ( \frac{p}{q} \right )_{\text{old}} \over {div}_{\text{old}}} \\ {div}_{\text{new}} & = {div}_{\text{old}} \times \frac{\sum \left ( \frac{p}{q} \right )_{\text{new}}}{\sum \left ( \frac{p}{q} \right )_{\text{old}}} \end{align}

From the four fictional items earlier, let us remove Item B and add two items (Item E at 120 coins and Item F at 354 coins). This gives us five items in the index. However, the new divisor is not five, but:

${div}_{\text{new}} = 4 \times {{22/30 + 85/70 + 64/60 + 120/120 + 354/354} \over {22/30 + 31/40 + 85/70 + 64/60}} = 5.2931$

To check if the new divisor is correct, we calculate the "new" index, which should be the same as the "old" index:

$X_{\text{new}} = 100 \times {{22/30 + 85/70 + 64/60 + 120/120 + 354/354} \over 5.2931} = 94.73$