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As you know, most of the EOD data available have only OHLC price. I used to do back-testing using the Close price as both bid and ask. however, in real world, the bid and ask spread is huge and the bid/ask quantity is also never constant. Above that, the bid/ask price changes with every shares sold/bought. Therefore, my earlier back-test could never be implement in the real world. What approach should i do to get/estimate ask/bid price for backtesting for OHLC data?

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This is too broad. The answer is going to depend on what you're trading (stocks, ETFs, futures, options, etc), the liquidity of what you're trading (bid/ask is not always huge; some markets are deeper than others), the quantity you want to trade, etc. –  Joshua Ulrich Jul 15 '13 at 18:53
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closed as too broad by olaker Aug 9 '13 at 7:20

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

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If you want to backtest with closing prices, the best bet is to add a slippage to the trade price. Note, however, that transaction cost modeling is a large field within quantitative finance and there is no simple solution to estimate this.

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There is a recent paper with a procedure to estimated the bid/ask spread from a series of daily high/low prices. Link: http://www3.nd.edu/~scorwin/papers/high-low_spreads.pdf

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