What is tick data?

What it means

A one-minute bar reduces potentially hundreds of price changes into four numbers. That summary discards the order in which the high and low were reached, which is exactly the information needed to know whether a stop or a target was hit first.

Why it matters

Testing on bar data therefore requires an assumption about intrabar sequence, and the assumption chosen determines the result for every trade where both levels were touched. Optimistic assumptions inflate backtests substantially, and the effect is largest on short-target strategies.

What this changes in practice

Real tick data is not a complete solution either — it is broker-specific, often incomplete, and heavy to store. What matters is knowing which you used. Our replay runs on real broker bars with an explicit and conservative intrabar rule, and two of our gold specialist strategies cannot be evaluated at all in replay because they need a tick stream the engine does not provide.