> ## Documentation Index
> Fetch the complete documentation index at: https://docs.traffical.io/llms.txt
> Use this file to discover all available pages before exploring further.
# Variance reduction (CUPED)
> Use pre-experiment data to shrink confidence intervals and reach significance sooner — without biasing the result.
Two users rarely behave the same. A lot of the variation you see during an experiment was already visible **before** it started — a high spender keeps spending, a heavy user keeps engaging. CUPED (Controlled-experiment Using Pre-Experiment Data) uses that history to cancel out the noise it can explain, so what's left is a cleaner view of the effect your change actually had.
The result: **narrower confidence intervals and faster significance**, with no change to the lift in expectation.
## The intuition
For each unit, Traffical looks at a **pre-experiment covariate** — typically the same metric measured over a window before the unit was exposed. It then removes the part of the unit's in-experiment outcome that was predictable from that covariate, comparing variants on what remains.
Because the covariate comes from *before* the experiment, it can't have been affected by the variant. Adjusting for it is therefore **unbiased**: CUPED only removes noise, it never invents or hides an effect.
How much you gain depends on how well the past predicts the present. The stronger the correlation between the pre-period covariate and the outcome, the more variance comes out and the tighter the interval — a strongly predictive covariate can cut interval width substantially.
## Turning it on
CUPED is **off by default** and controlled by a setting at two levels:
* **Project default** — set a default for every experiment in the project (for example, "use CUPED everywhere we can").
* **Per policy** — each policy can turn CUPED on or off explicitly, overriding the project default. Left unset, a policy inherits the project default.
The effective rule is simply: **policy setting if set, otherwise the project default, otherwise off.** Turning the project default on is the easy way to get variance reduction across the board; the per-policy switch is there for the experiments where you want to opt in or out individually.
## What it measures
When CUPED is on, it adjusts by a **pre-experiment covariate** — the metric's own value over a window before each unit was exposed (for example, the two weeks before they first saw the experiment). Where a unit has no pre-period history, it's analysed without adjustment. CUPED only ever removes noise — enabling it never makes an interval wider or changes the lift in expectation.
## Configuring it on a metric
CUPED needs **two things** to take effect, by design:
1. **A covariate on the metric.** On the metric itself, enable variance reduction and set the **lookback window** (how many days before exposure to measure the covariate). The covariate defaults to the metric's own value over that window. Available on **average (mean)** and **conversion-rate** metrics.
2. **CUPED enabled on the policy** (or project default), as described above.
If either is missing, the metric is analysed normally — so turning the project default on does nothing until your metrics also carry a covariate, and a metric's covariate is dormant until a policy enables CUPED.
## Where it applies
CUPED works on **average (mean)** and **conversion-rate** metrics. It composes cleanly with the rest of the engine:
Variance reduction narrows the confidence sequence too — you keep the anytime-valid guarantee and reach a conclusion sooner.
Outlier capping and CUPED stack: cap the extreme tail, then remove the predictable variation from the rest.
## When it helps most
* Metrics with **high natural variance** between units — revenue, engagement time, order value.
* Experiments where users have **history** in your product, so the pre-period covariate is meaningful.
* Any time you want a **decision sooner** without lowering your confidence level.
For brand-new users with no pre-period history, the covariate is simply absent and they're analysed without adjustment — CUPED helps where it can and stays out of the way where it can't.