So it seems like you're estimating trading costs using this approach. There are a few things I can't follow here:
- The trading costs are in terms of normalized turnover. I thought this turnover was volatility normalized, but from what you've written above the turnover seems to be normalized by the average absolute number of blocks held. Is the average absolute number of blocks held equal to the volatility?
- Wouldn't it just be simpler to price in the cost of trading directly in the account currency, e.g. if 300 blocks of an instrument are bought on a particular day, and the cost per block is $8, then just subtract 300*$8 from the account value at the end of the day in the back test. What advantage does using the normalized turnover and standardized cost have over this simple approach?
The normalisation is by a position with a forecast of +10 (the most accurate way) will depend (inversely) on the volatility. So on a particular day the position with a forecast of +10 might be 7 contracts. But if volatility halves it would be 14 contracts.
To put it another way, if I am trading 1 contract a day with a +10 forecast position of 7 contracts, then all other things being equal I should be trading 2 contracts a day if volatility halves.
Using the average absolute position (or a moving average of it) is an approximate for using the position with a +10 forecast. Over long periods of time, if volatility is stable, it should be correct (since everything should be scaled so that you average absolute position is the same as it would be with a fixed forecast of +10). Again your average absolute position will depend inversely on volatility.
The advantages of using a standardised approach are:
- you can subtract the cost estimate directly from the sharpe ratio. This means you can say instantly what proportion of your raw sharpe is being eaten by costs.
- you can compare costs across instruments and across trading rule variations
- you can pool turnover estimates from different estimates in deciding how fast a particular variation is trading
- you can compare costs across time.
GAT
