Direct answer: common mistakes
Execution comparison is the practice of evaluating how different brokers or trading setups handle orders (for example, how orders fill versus expected prices). Common mistakes usually come from comparing the wrong things, mixing definitions, or treating sample results as if they were stable.
What “execution comparison” means (and what can be misunderstood)
Execution can refer to several measurable outcomes. Typical examples include fill price relative to an expected reference, the difference between requested and filled quantity, and timing (how quickly an order gets filled). A frequent misunderstanding is assuming that “better execution” is one single metric. In reality, metrics trade off: improving one aspect (like reducing average slippage) can hide other issues (like higher spread costs or more partial fills).
Another mistake is using an undefined reference price. For instance, one comparison might measure slippage versus the moment the order was sent, while another measures versus the price at a later time. If those reference points differ, the “numbers” are not describing the same phenomenon.
Evidence and example: how comparisons go wrong in practice
A simple comparison can become misleading when key inputs are not controlled. Consider two setups, A and B, evaluated on “average slippage” during a chosen period. If A was tested with smaller order sizes and B with larger sizes, results can differ because market depth and liquidity affect how far prices move when orders execute. Without stating order size assumptions and whether orders are placed during similar liquidity conditions, the average can reflect market impact rather than execution quality.
A second failure mode is mixing fee and cost components. Some people compare only price movement (slippage) but ignore spreads, commissions, or other trading costs. The result is a cost picture that is incomplete. Even if execution in terms of fills looks similar, total realized cost can diverge.
A third issue is survivorship and selection effects. If you look only at instances where fills were favorable, or you cherry-pick sessions with calmer volatility, the comparison can overstate consistency. Markets are not stationary: volatility regimes change, liquidity can thin, and spreads widen, all of which can alter observed execution.
Limitations and risks: what can’t be safely concluded
Execution comparison does not produce a certainty about future results. Even when two setups look similar in one sample, outcomes can vary with market conditions, order characteristics, and execution environment. Historical relationships do not establish future results, especially when volatility, liquidity, and spread behavior change.
Material limitations also include definition drift. If one provider’s execution behavior changes (for example, due to internal routing rules or risk controls), older comparisons may no longer represent current operation. You should also treat any single metric as incomplete because execution quality can involve partial fills, timing effects, and total cost.
Neutral verification: checks you can apply independently
To verify a comparison claim neutrally, require clear assumptions: (1) which execution metrics are used, (2) the reference price definition for “slippage,” (3) the order size and order type assumptions, and (4) whether spreads and commissions are included in total cost. Next, check whether the comparison period includes different market conditions (not only calm days), and whether results are reported with enough context to avoid cherry-picking.
A helpful next question to ask is: “If we change the reference point, order size, or cost components, do the conclusions still hold?” If not, the comparison likely suffers from one of the common mistakes above.