Quotes Confusion

I have downloaded Forex quotes from 2 different sources and they are significantly different. Using the different quotes gives me significantly different results from the backtests I am running. I downloaded hourly quotes for EUR/USD and GBP/USD for the last 6 months. I downloaded them from my broker's system (InteractiveBrokers) and from this website that I think I found in a thread here in this forum:

http://freeserv.dukascopy.com/exp/

The quotes for EUR/USD average a difference of 4 pips based on closing price and the quotes for GBP/USD average a difference of a whopping 27 pips. WTF? (Pardon my abbreviation) Why is this happening and what does it mean for my backtesting results? With one set of quotes my system tests great and with the other it barely covers the spread. A little more information: On the good test results the EUR/USD trades have great results but the GBP/USD trades are about breakeven.

Can anyone help me understand why the quotes from these two systems are so different and what it means to my backtesting. I'm beginning to think that backtesting is a complete waste of time.

Steve

I looked at the quotes a little more and it looks like the GBP/USD quotes from the two systems are out of sync by 6 hours (5 hours during Daylight Savings Time). The EUR/USD quotes appear to be in sync between the two systems. If I sync up the GBP/USD quotes then the average difference for those quotes falls to about 4 pips like the EUR/USD quotes. This timing difference shouldn't affect my backtest results because in any case I am not looking at future quotes and the "time slide" is relative in that respect.

Steve

You need to be careful on how time is being measured. Most historical data assumes GMT for the timezone. If you care enough to want the time in your own zone, you will have to do the conversion yourself.

I have seen cases where some historical data (MT4 brokers have this problem) is not in GMT, and I think this is seriously annoying.

Something else to consider is the fact that each individual Borker/Market Maker is allowed to make the following descision...

"Do you want to move in conjunction with other Borkers"
Or
"Do you want to hold your ground."

The only prerequisite being that... around Noon NY time, they are all within a couple of pips of each other.

With this in mind, it is feasible that 2 (two) different Borkers could be 30 pips apart at 10 AM NY Time and Not be in Failure of Compliance.

Aha, I see it now, Arbitrage raises it's virtual head yet again.