Diagnosing Experiment Results

Diagnosing experiment results

This skill answers: My PostHog experiment results look wrong, biased, or empty — what's going on?

Match the user's complaint in the dispatch table, then read the matching reference file for the

diagnostic.

Each diagnostic in the reference files is tagged [HIGH], [MEDIUM], or [LOW] based on how

strongly it's verified — [HIGH] is verified directly in PostHog code, [MEDIUM] is partially or

team-source verified, [LOW] describes SDK/external behavior that wasn't verified here. Treat [LOW]

items as hypotheses to test, not facts to assert.

Step 1 — Resolve the experiment

If the user refers to an experiment by name or description, load the finding-experiments skill first to

resolve it to a concrete ID.

Call experiment-get and pull these fields. They are inputs for almost every diagnostic:

• parameters.feature_flag_variants[].rollout_percentage — the variant split
• parameters.rollout_percentage — the overall rollout (% of users entering the experiment)
• exposure_criteria.multiple_variant_handling — defaults to "exclude" if absent
• exposure_criteria.exposure_event — null means default $feature_flag_called
• exposure_criteria.filterTestAccounts — defaults to true
• feature_flag.active, status (draft / running / paused / exposure_frozen / stopped), start_date, end_date
• feature_flag.filters.groups[] — for each group read variant, properties, and

rollout_percentage. Any non-null variant is a forced-variant override on the matched cohort

(release-condition assignment, not randomized) — surfaces A7. Watch for the severe shape (A7b): a

variant-pinned group with broad/empty properties at high rollout, or no group left randomized

(variant: null) / no release path to one arm — that starves the other variant (one arm gets ~0

analyzable exposures). See references/bias-and-skew.md.

• stats_config — Bayesian (default) or Frequentist

Step 1.5 — Pull a diagnostic snapshot (verify before asking)

Before asking the user clarifying questions, pull the diagnostic snapshot in

references/diagnostic-snapshot.md. Most diagnostics in this skill

can be confirmed or ruled out from that data without an interview.

Step 2 — Match symptom to diagnostic

| User says... | Diagnostic group |

| ------------------------------------------------------------------------------------------ | -------------------------------------------- |

| "Smaller variant looks biased" / banner says bias | A — bias & skew |

| "Variant ratio doesn't match my split" / SRM warning | A — bias & skew |

| "Why isn't it 50/50?" / "users in both groups" | A — bias & skew |

| "Users in both control and test" / high $multiple % | A — bias & skew |

| Multi-variant exposure on a server-rendered app | A — bias & skew |

| Banner about feature-flag/experiment state mismatch | A — bias & skew |

| "Migrating distinct_id" / "switching from anonymous to user_id" mid-run | A — bias & skew |

| Metric count is much smaller than exposures (e.g. 10× or 100× gap) | A — bias & skew (route here before D) |

| "Experiment shows 0 / not enough data" / empty | B — empty experiment |

| "Variant always undefined / false" | B — empty experiment |

| "$feature_flag_called fires but no exposures show up" | B — empty experiment |

| "Experiment says running but exposures haven't moved in weeks/months" | B — empty experiment |

| "Significance keeps flipping as we run longer" | C — interpretation traps |

| "Significance was declared, then it wasn't significant anymore" | C — interpretation traps |

| "30/16 split at 46 exposures, is this broken?" | C — interpretation traps |

| "A/A test is showing significant results" | C — interpretation traps |

| "Many metrics — some significant, some not" | C — interpretation traps |

| "Bayesian says 96% chance to win — should we ship?" | C — interpretation traps |

| "Confidence intervals overlap — does that mean not significant?" | C — interpretation traps |

| "An external tool (significance calculator or AI agent) disagrees with PostHog" | C — interpretation traps |

| "Should I ship? Primary is up but a secondary is down" | C — interpretation traps |

| "PostHog numbers ≠ my SQL count" | D — numbers vs SQL |

| "Funnel says X% but my raw event count says Y" | D — numbers vs SQL |

| "Sum of revenue looks wrong" / "breakdown shows 'none'" | D — numbers vs SQL |

| "Recordings panel doesn't match the stats" | D — numbers vs SQL |

| "I applied a filter but the user count didn't change" | D — numbers vs SQL |

| "I want to slice results by current person properties (as of now, not as of exposure)" | D — numbers vs SQL |

| "Changed split / rollout / metric / criteria mid-run, now odd" | E — mid-run changes |

| "Ended/shipped — flag now flipped to 0/100 unexpectedly" | E — mid-run changes |

| "Long-term metric moves opposite from primary" | E — mid-run changes |

| "Retention metric counts users I didn't expect" | E — mid-run changes |

| "Can't convert the feature flag back to a simple (boolean) flag after the experiment ends" | E — mid-run changes |

| "How do I restart an experiment with new variants?" | E — mid-run changes |

| Metric line is rendered but the result block is empty / no chance-to-win or significance | E — mid-run changes (E13 legacy methodology) |

| "Results won't load" / many metric rows show data: null (not a legacy experiment) | Step 1.5 — diagnostic snapshot (null rows) |

If the symptom is unclear, ask one clarifying question before picking. Most diagnostics have different fixes

— do not guess.

Step 3 — Surface every diagnostic the evidence supports

After matching the symptom in Step 2 and reading the relevant reference file(s), list each diagnostic

that applies before recommending an action.

Surface co-occurring mechanisms independently — even when one is more salient, don't collapse them

into a single "wait" or "fix" recommendation. Different mechanisms have different fixes: a

_systematic_ bias (e.g. uneven-split + Exclude) doesn't resolve by waiting; a _statistical_ pattern

(e.g. small-sample variance) does. Bundling them leaves the bias in place after the user follows the

bundled advice.

Only list mechanisms that have a path to verification in the project state — config (from

experiment-get), snapshot data, activity log, or repo source. Config-derived mechanisms count: an

80/20 split with default multiple_variant_handling="exclude" is visible in experiment-get and is

therefore enumerable. Naming a mechanism with no source (e.g. SRM when the snapshot shows a clean

variant ratio) is not.

Diagnostic groups

A — Bias & skew

Variants don't look balanced, one variant looks biased, the in-app warning banner appeared, or users are

showing up under multiple variants. Covers the uneven-split + Exclude interaction, SRM, identity

fragmentation, bootstrap × /decide mismatch, and flag/experiment state inconsistency.

→ See references/bias-and-skew.md

B — Empty experiment / 0 exposures / "not enough data"

A frequent pain point. Covers SDK call (wrong evaluation method, identify() timing, dedup),

exposure capture (custom event missing variant property, required properties, ad-blockers), and

exposure-criteria match (test-account filter, eligibility ordering, events firing before exposure).

→ See references/empty-experiment.md

C — Significance / interpretation traps

Significance flipping, A/A test showing significance, Bayesian vs Frequentist confusion, multiple

comparisons, low-volume variance, peeking / early stopping. Includes the legacy stats issue (A/A tests

historically over-fired before the new Bayesian module) and how the win-probability methodology changed in

Jan 2025 (single test vs control, not control vs all variants).

→ See references/interpretation.md

D — Numbers don't match (PostHog vs the user's SQL / raw count)

The experiment page applies an exposure scope, $multiple exclusion, test-account filter, and date range

that ad-hoc SQL almost never replicates. Covers funnel attribution (only first→last step counts for stats),

breakdowns (read from the exposure event, not the metric event), the "sum of revenue" mean-of-per-user

confusion, and the recordings-panel-vs-stats divergence.

→ See references/numbers-vs-sql.md

E — Surprises after mid-run changes (incl. lifecycle and retention quirks)

Increasing rollout is safe; decreasing is caution; changing the variant split is an anti-pattern; adding

metrics mid-run is p-hacking; ship-variant can rewrite the flag in surprising ways; reset clears

results not the flag. Also covers retention-metric quirks (first-event-must-be-after-exposure design),

"matured users" filtering, and long-term vs short-term metric divergence.

→ See references/mid-run-changes.md

Step 4 — Calibrate recommendations to experiment state

Surface diagnostics first (Step 3). Then recommend — but scope what you recommend to what the

experiment's current state permits.

• Draft — config changes are free; recommend and apply.
• Running — every change has a tradeoff. Explain the mid-run impact (anti-pattern? safe?

user-visible?) before recommending. See configuring-experiment-rollout and its reference file

references/changing-distribution-after-launch.md for the mid-run rules.

• Stopped / archived — the experiment AND its feature flag represent the documented outcome of

the run. Recommendations are scoped to (a) interpretation of the existing data, (b) what to do for

the _next_ experiment, or (c) explaining what happened.

On a stopped or archived experiment, don't preemptively offer reversal of a state mutation

(ship-variant flag rewrite, manual flag edit, reset, archive). If the user asks "why did X happen?",

explain X — don't append a "here's how to undo it" coda. That pattern assumes intent the user didn't

signal. Conditional offers like _"if this wasn't intended, you could…"_ or _"want me to revert it?"_

count as preemptive too — only the user explicitly naming the reversal action ("how do I undo this?",

"can I roll back ship-variant?", "how do I get the 50/50 split back?") is a request to surface

reversal mechanics.

Use consistent terminology: variant _split_ (between variants) is distinct from _rollout_ (overall %

entering); the $feature_flag_called exposure event is distinct from a _custom exposure event_; the

_Exclude_ / _First seen_ options control multivariate handling, not exposure.