The fastest way to trust the cost rule is to watch it turn one signal down, line by line. A rejection is not a mysterious verdict — it is arithmetic you can follow. Here is the anatomy of a single rejected signal, from the move it hoped to capture to the moment its edge ran out.
Start with the expected edge
Every signal begins with a gross figure: the move it expects to capture if it is right. This is the number everything else is subtracted from. On its own it looks attractive — which is exactly why it is never the number that decides anything.
Subtract the fee and the spread
First come the costs you pay just to participate. The exchange fee applies on entry and exit. The spread is the gap between bid and ask that you cross the instant you trade at market. Together they take the first bite out of the expected edge, and in calm conditions they are usually the predictable part.
Subtract slippage and the safety buffer
Next come the costs of uncertainty. Slippage is the gap between the price you expected and the price your order actually filled at, and it deepens when liquidity thins. The safety buffer is a deliberate margin on top, because real fills rarely match the textbook. These two are what most often tip a marginal signal under water.
Read the verdict
With every cost subtracted, the net is what is left. If it is positive, the signal clears the bar. In a rejection, the net has fallen below zero — the edge could not cover its own friction — and the signal is logged as rejected. Nothing is hidden: the failing line is right there in the breakdown.
To see the breakdown format itself, read how to read a signal cost breakdown, and to interpret the logged result, how to interpret a transparent rejection. For why the bar rises in volatile markets, see how volatility raises the cost bar. A rejected signal is discipline working, not advice withheld.