Bid/Ask Data and Realtime Trading

[dweems]

I'm developing a strategy that depends on knowing the extent of trading prices. So I plot both the bid and ask data on my charts and use that data in my analyses. I'm performing real time strategy simulations and I regularly see cases where trades trigger outside the range of the bid/ask prices. Am I not understanding correctly the values provided by the bid and ask data? Can trades exist outside of those values? I'm trading Forex data and therefore "last" data is simply the bid data. I had previously thought that last data was the actual trading values and bid/ask were the prices defined by the exchanges of what prices are offered. But that would only be valid if the last data was the actual trade values. Is this a problem with Forex that there is no legitimate last data values to review? There being no difference between bid and last data made me think that the bid/ask data would be the true range of all trades. Hopefully you can clarify how I'm not understanding the bid/ask data range and why trades trigger outside of the bid/ask data range. I'm rather hoping you tell me that it's an idiosyncrasy of the simulation process.