How does one change the spread used in the Strategy Tester?

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It appears that the strategy tester uses the last official spread for each particular currency, and keeps that value constant for the entire duration of the backtesting period. How does one change this value? Does anyone log these spreads?
 

You are correct; Spread is fixed for duration of backtest. With some 'smart' coding it's possible to effectively vary this during a backtest & tweak the Entry, SL & TP points.

It is possible to log spreads and so make a Spread history, and so then it's possible to take it into account for back testing. I'm sure a search will help you find examples of how others have done it.

Note that Broker A's price history & Broker B's spread history could be worse than not taking Spread into account. Indeed, 2 people using Broker A and recording Spread history could well get 2 different values. e.g. One broker gives different spreads for different geographical areas, and gives a wider spread than native clients if a client is introduced from their US sister company. https://alpari.com/en/trading_accounts/account_comparison_table.html#alpari_us

Personally, I ignore spreads and calc my EAs just on Bid price, & regard spread as the cost of entering or exiting the trade. This means that my Entry & Exit points exactly match the historical Bid price (rather than being above when Ask price is used) . Some laugh at me for doing that, but then again no-one has shown where I can get Bid+Ask or Spread history.

 
It is possible to obtain spread data then model it in your backtest ..I have seen discussion on this but did not look any further into implementing it..personally I dont see it as particularly significant unless youre using a scalping strategy (which Im not) alternatively you can put code into your EA to use a variable that you define in place of the spread and add code to model the value whichever way.. Ive written a program to log spread data https://www.mql5.com/en/code/10102
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