The errors you are seeing are due to chart differences, not to differences in the calculation and/or plotting of the SAR indicator.
I'll cover the SAR first, and then try to untangle the chart differences for you.
The PSAR Indicator
SAR stands for "stop and reverse". That's the name given to it by its creator, J. Welles Wilder, Jr. Because the indicator generates a modified parabolic shape, many traders refer to it as the PSAR (parabolic stop and reverse indicator). For those who don't know what we're talking about, this indicator plots a stop-loss point above or below each bar or candle on a price chart. Wilder's system for trading this indicator (in trending markets only) involves reversing your position from long to short, or vice versa, whenever the market price hits the PSAR.
The indicator, as developed by Wilder, allows traders to make small adjustments to a parameter which Wilder called the "acceleration factor" or AF. Wilder said that 0.02 was the optimum value for the AF, but that it could be set anywhere between 0.018 and 0.021. The algorithm which generates the PSAR increases the AF, candle-by-candle, as the trend continues, up to a maximum value which Wilder specified as 0.20. He said that this maximum value could be adjusted within a range of 0.20 to 0.22.
In the MT4 platform, the PSAR parameters are set to the default values 0.02 and 0.20, but you can change them. In the FXCM platform, the same default values are used, but you can't change them. (FXCM doesn't even identify the default values.)
I recommend that you stick with the default values in any platform which you may be using.
So, the default settings for the PSAR indicator in MT4 and the fixed settings for the PSAR indicator in FXCM are the same, and would plot exactly the same way, if they were applied to identical charts. But, they aren't applied to identical charts; and that is the reason for the differences you are seeing.
Comparing Charts on Different Trading Platforms
If you are seeing noticeable differences between the PSAR on your FXCM charts, and the PSAR on your MT4 charts, I'm guessing that you are looking at daily (D1) charts. If so, here's the source of the problem:
FXCM (USA) begins each daily candle at 5pm EDT (New York time) --- I don't know the situation regarding FXCM accounts and trading platforms in other parts of the world.
The MT4 platform, on the other hand, begins each daily candle at EITHER midnight BST (London time), OR midnight CEST (Zurich time), depending on which broker supplies the platform.
Midnight London time = 7pm New York time. And, midnight Zurich time = 6pm New York time. So, when you compare one platform to another, you are likely comparing daily charts which do not coincide. As a result, the indicators placed on those charts do not coincide, either.
I have accounts (live and demo) with FXCM (USA), plus demo accounts with FXCM (UK) and Alpari (UK). Here's a comparison of the Open and Close bid prices for Monday, May 18, 2009 (a date I picked at random) for the EUR/JPY, for three different platforms:
FXCM (USA) standard account - Open 128.295 --- Close 130.560 --- daily candles open and close at 5pm EDT
Alpari (UK) MT4 demo --------- Open 128.07 ----- Close 130.44 ---- daily candles open and close at 6pm EDT
FXCM (UK) MT4 demo --------- Open 128.217 --- Close 130.675 --- daily candles open and close at 7pm EDT
Three different Open prices and three different Close prices for the same day on three different platforms. But, as we now know, it's not really the "same" day.
Most of the differences seen above are due to time differences. But, there is another factor. Every trading platform relies on a price-stream which has at least one market-maker in it somewhere. There is definitely a market-maker at the interbank level. And there may also be a market-maker at the retail broker level. Different market-makers establish different prices, both Bid and Ask.
It's easy to imagine that traders all over the world are seeing the same prices, and trading off the same charts. But, it just ain't so. Consider all the chatter on this forum, and others, about broker spreads. If different brokers offer different spreads, then --- by definition --- they are offering different prices. These prices generate the charts in their trading platforms --- so, inevitably different brokers will be displaying slightly different charts.
Which broker supplies the "correct" prices and the "correct" charts? All of them. And none of them. And this is the defining difference between an "off-exchange" market, such as our retail spot forex market, and markets such as stocks or commodities which are exchange-traded.
To illustrate how variable prices can be, consider the prices quoted in two different types of account from the same broker --- your broker, FXCM. Again, I will use the Open and Close bid prices from Monday, May 18:
FXCM (USA) standard account - Open 128.295 --- Close 130.560
FXCM (USA) micro account ---- Open 128.304 --- Close 130.569
These differences are small, but they illustrate a point. FXCM advertises tighter spreads in their micro account than in their standard account. And, as you would expect, this shows up as slightly higher Bid prices (and slightly lower Ask prices) in their micro account, as compared to their standard account.
Switching Between Platforms
Like you, I use my MT4 platforms for trade prep, but I do my actual trading on my FXCM platform. This routine works just fine, as long as we don't demand identical prices and identical charts from different platforms.
By the way, I have studied the PSAR in some detail, but I do not use it in my trading.
Thanks Clint. I purchased a FX course on EBAY recently and been trading live about a month now. The concept is basicially using simple indicators, moving Ave and Psar on the daily charts.
Prior to going live i demoed using both fxcm and MT, and couldn't believe the Psar on both charts. Not only were they different but considerably so. Talk about paranoia. I probaby chatted with fxcm couple times trying to figure out what is going on.