How to Detect Such Movements
Only a fraction of the orders that reach the market are executed. Executed
orders create transactions between a buyer and a seller. The buyer’s and
the seller’s respective buy and sell orders are mutually filled. If you only
study the transactions, it is very difficult to see the direction of these transactions.
Because for each transaction there is a buyer and a seller, it is
impossible to tell if sellers are stronger than buyers.
The direction of the trade is indicated by the small price change that occurs
on the transaction: If the price increased on the transaction, the buyer
was stronger (pushed the price up). Otherwise, the seller was stronger. Because
institutions split up large orders into numerous small orders, studying
the transaction size does not help in figuring out whether the transaction
was generated by an institution. What we therefore need to do is study
all the aggregated transactions within regular time intervals of one minute.
The idea is to reconstruct the size of the original order by adding up all the
transactions that occurred within one minute and to compare that number
to the price variation.
Let’s study this idea in one example: a large buyer. Let us suppose that
an institution wants to buy 100,000 shares on the market of a stock that
is trading 500,000 shares per day. The institution will probably have to use
one of the following four tactics:
1. Place a large buy order at the bid.
2. Place regular small buy orders at the bid.
3. Place regular small buy orders at the ask.
4. Place a large buy order at the ask.