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Fair execution produces both positive slippage (better price) and negative slippage (worse price). If positive slippage almost never happens, the execution may be asymmetric.

How to check:

1. Collect at least 100–200 market orders and stop fills.
2. For each, calculate slippage: fill price minus requested price, adjusted for direction.
3. Count positive, zero and negative outcomes.
4. Compare the average size of positive vs negative slippage.

What to look for:

• Some imbalance is normal in fast markets, especially on stops.
• A complete absence of positive slippage on market orders is a red flag.
• Slippage that only appears when it hurts the trader deserves a direct question to the broker.

If you find a clear asymmetry, contacting the broker with your data is the first step, and comparing with a second broker the next.

Have you analysed your slippage distribution, and was it balanced?

FXS Prop Desk wrote: Some imbalance is normal in fast markets, especially on stops.

That's the bit that makes it important to split the sample before counting. Stop orders trigger in the direction of a move, so by design they tend to fill worse than the trigger, and mixing them in can make perfectly fair execution look asymmetric. Limit orders go the other way, they can't fill worse than the limit, so they inflate the positive side. The clean test is market orders only, ideally separated by session and by whether price was moving toward or away from you at the click. If market orders in quiet hours still show almost no positive slippage across 150 or so fills, that's when the question to the broker has real weight behind it.

FXS Prop Desk, splitting the sample the way Fairman describes is the only clean test I trust. Mixing stops with market orders made my first pass look terrible, because stops are designed to fill worse than the trigger in a moving market.

Once I kept market orders only and separated quiet hours from news windows, the picture changed. In quiet London my positive and negative slip counts were nearly even across about 160 fills, with average sizes close enough that I stopped worrying.
The illustration compares a balanced quiet sample against a mixed sample that folds stops in, so the negative bar dominates for the wrong reason. That is why I keep the two order types in separate columns now.

If quiet market orders still show almost no positive slip across 150 fills, that is when I would raise it with the broker. Until then I treat news window asymmetry as normal cost.

My own split removed most of the apparent imbalance. Did yours change the reading as well, or did the skew survive on quiet market orders alone?