Unit Tests for Position/Portfolio - While I've not mentioned it directly in diary entries #1 and #2, I've actually been writing some unit tests for the Portfolio and Position objects. Since these are so crucial to the calculations of the strategy, one must be extremely confident that they perform as expected. An additional benefit of such tests is that they allow the underlying calculation to be modified, such that if all tests still pass, we can be confident that the overall system will continue to behave as expected.
As we've already stated, trading on margin is trading on money borrowed from your broker. Each time you open a trade on margin, your broker automatically allocates the required margin from your existing funds in the trading account in order to back the margin trade. The precise amount of allocated funds depends on the leverage ratio used on your account.

Options involve risk and are not suitable for all investors. For more information read the "Characteristics and Risks of Standardized Options". For a copy call Interactive Brokers' Client Services on 312-542-6901. Before trading, clients must read the relevant risk disclosure statements on our Warnings and Disclosures page - http://www.interactivebrokers.com/disclosures. Trading on margin is only for sophisticated investors with high risk tolerance. You may lose more than your initial investment. For additional information regarding margin loan rates, see http://www.interactivebrokers.com/interest. Security futures involve a high degree of risk and are not suitable for all investors. The amount you may lose may be greater than your initial investment. Before trading security futures, read the Security Futures Risk Disclosure Statement. For a copy visit http://www.interactivebrokers.com/disclosures. Structured products and fixed income products such as bonds are complex products that are more risky and are not suitable for all investors. Before trading, please read the Risk warning and Disclosure Statement at http://www.interactivebrokers.com/disclosures. There is a substantial risk of loss in foreign exchange trading. The settlement date of foreign exchange trades can vary due to time zone differences and bank holidays. When trading across foreign exchange markets, this may necessitate borrowing funds to settle foreign exchange trades. The interest rate on borrowed funds must be considered when computing the cost of trades across multiple markets.
Local Portfolio Handling - In my opinion carrying out a backtest that inflates strategy performance due to unrealistic assumptions is annoying at best and extremely unprofitable at worst! Introducing a local portfolio object that replicates the OANDA calculations means that we can check our internal calculations while carrying out practice trading, which gives us greater confidence when we later use this same portfolio object for backtesting on historical data.
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