Histogram Guidelines

This document gives the best practices on how to use histograms in code and how to document the histograms for the dashboards. There are three general types of histograms: enumerated histograms, count histograms (for arbitrary numbers), and sparse histograms (for anything when the precision is important over a wide range and/or the range is not possible to specify a priori).

[TOC]

Defining Useful Metrics

Directly Measure What You Want

Measure exactly what you want, whether that's the time used for a function call, the number of bytes transmitted to fetch a page, the number of items in a list, etc. Do not assume you can calculate what you want from other histograms, as most ways of doing this are incorrect.

For example, suppose you want to measure the runtime of a function that just calls two subfunctions, each of which is instrumented with histogram logging. You might assume that you can simply sum the histograms for those two functions to get the total time, but that results in misleading data. If we knew which emissions came from which calls, we could pair them up and derive the total time for the function. However, histograms are pre-aggregated client-side, which means that there's no way to recover which emissions should be paired up. If you simply add up the two histograms to get a total duration histogram, you're implicitly assuming the two histograms' values are independent, which may not be the case.

Directly measure what you care about; don't try to derive it from other data.

Provide Context

When defining a new metric, think ahead about how you will analyze the data. Often, this will require providing context in order for the data to be interpretable.

For enumerated histograms in particular, that often means including a bucket that can be used as a baseline for understanding the data recorded to other buckets: see the enumerated histogram section.

Naming Your Histogram

Histograms are taxonomized into categories, using dot (.) characters as separators. Thus, histogram names should be in the form Category.Name or Category.Subcategory.Name, etc., where each category organizes related histograms.

It should be quite rare to introduce new top-level categories into the existing taxonomy. If you're tempted to do so, please look through the existing categories to see whether any matches the metric(s) that you are adding.

Coding (Emitting to Histograms)

Prefer the helper functions defined in histogram_functions.h. These functions take a lock and perform a map lookup, but the overhead is generally insignificant. However, when recording metrics on the critical path (e.g. called in a loop or logged multiple times per second), use the macros in histogram_macros.h instead. These macros cache a pointer to the histogram object for efficiency, though this comes at the cost of increased binary size: 130 bytes/macro usage sounds small but quickly adds up.

Don't Use the Same Histogram Logging Call in Multiple Places

These logging macros and functions have long names and sometimes include extra parameters (defining the number of buckets for example). Use a helper function if possible. This leads to shorter, more readable code that's also more resilient to problems that could be introduced when making changes. (One could, for example, erroneously change the bucketing of the histogram in one call but not the other.)

Use Fixed Strings When Using Histogram Macros

When using histogram macros (calls such as UMA_HISTOGRAM_ENUMERATION), you're not allowed to construct your string dynamically so that it can vary at a callsite. At a given callsite (preferably you have only one), the string should be the same every time the macro is called. If you need to use dynamic names, use the functions in histogram_functions.h instead of the macros.

Don't Use Same String in Multiple Places

If you must use the histogram name in multiple places, use a compile-time constant of appropriate scope that can be referenced everywhere. Using inline strings in multiple places can lead to errors if you ever need to revise the name and you update one one location and forget another.

Efficiency

Generally, don't be concerned about the processing cost of emitting to a histogram (unless you're using sparse histograms). The normal histogram code is highly optimized. If you are recording to a histogram in particularly performance-sensitive or "hot" code, make sure you're using the histogram macros; see reasons above.

Picking Your Histogram Type

Enum Histograms

Enumerated histogram are most appropriate when you have a list of connected / related states that should be analyzed jointly. For example, the set of actions that can be done on the New Tab Page (use the omnibox, click a most visited tile, click a bookmark, etc.) would make a good enumerated histogram. If the total count of your histogram (i.e. the sum across all buckets) is something meaningful—as it is in this example—that is generally a good sign. However, the total count does not have to be meaningful for an enum histogram to still be the right choice.

Enumerated histograms are also appropriate for counting events. Use a simple boolean histogram. It's usually best if you have a comparison point in the same histogram. For example, if you want to count pages opened from the history page, it might be a useful comparison to have the same histogram record the number of times the history page was opened.

In rarer cases, it's okay if you only log to one bucket (say, true). However, think about whether this will provide enough context. For example, suppose we want to understand how often users interact with a button. Just knowning that users clicked this particular button 1 million times in a day is not very informative on its own: The size of Chrome's user base is constantly changing, only a subset of users have consented to metrics reporting, different platforms have different sampling rates for metrics reporting, and so on. The data would be much easier to make sense of if it included a baseline: how often is the button shown?

If only a few buckets are emitted to, consider using a sparse histogram.

Requirements

Enums logged in histograms must:

be prefixed with the comment:

// These values are persisted to logs. Entries should not be renumbered and
// numeric values should never be reused.

be numbered starting from 0. Note this bullet point does not apply for enums logged with sparse histograms.
have enumerators with explicit values (= 0, = 1, = 2) to make it clear that the actual values are important. This also makes it easy to match the values between the C++/Java definition and histograms.xml.
not renumber or reuse enumerator values. When adding a new enumerator, append the new enumerator to the end. When removing an unused enumerator, comment it out, making it clear the value was previously used.

If your enum histogram has a catch-all / miscellaneous bucket, put that bucket first (= 0). This makes the bucket easy to find on the dashboard if additional buckets are added later.

Usage

In C++, define an enum class with a kMaxValue enumerator:

enum class NewTabPageAction {
  kUseOmnibox = 0,
  kClickTitle = 1,
  // kUseSearchbox = 2,  // no longer used, combined into omnibox
  kOpenBookmark = 3,
  kMaxValue = kOpenBookmark,
};

kMaxValue is a special enumerator that must share the highest enumerator value, typically done by aliasing it with the enumerator with the highest value: clang automatically checks that kMaxValue is correctly set for enum class.

The histogram helpers use the kMaxValue convention, and the enum may be logged with:

UMA_HISTOGRAM_ENUMERATION("NewTabPageAction", action);

or:

UmaHistogramEnumeration("NewTabPageAction", action);

Logging histograms from Java should look similar:

// These values are persisted to logs. Entries should not be renumbered and
// numeric values should never be reused.
@IntDef({NewTabPageAction.USE_OMNIBOX, NewTabPageAction.CLICK_TITLE,
        NewTabPageAction.OPEN_BOOKMARK})
private @interface NewTabPageAction {
    int USE_OMNIBOX = 0;
    int CLICK_TITLE = 1;
    // int USE_SEARCHBOX = 2;  // no longer used, combined into omnibox
    int OPEN_BOOKMARK = 3;
    int COUNT = 4;
}

// Using a helper function is optional, but avoids some boilerplate.
private static void logNewTabPageAction(@NewTabPageAction int action) {
    RecordHistogram.recordEnumeratedHistogram(
            "NewTabPageAction", action, NewTabPageAction.COUNT);
}

Legacy Enums

Note: this method of defining histogram enums is deprecated. Do not use this for new enums in C++.

Many legacy enums define a kCount sentinel, relying on the compiler to automatically update it when new entries are added:

enum class NewTabPageAction {
  kUseOmnibox = 0,
  kClickTitle = 1,
  // kUseSearchbox = 2,  // no longer used, combined into omnibox
  kOpenBookmark = 3,
  kCount,
};

These enums must be recorded using the legacy helpers:

UMA_HISTOGRAM_ENUMERATION("NewTabPageAction", action, NewTabPageAction::kCount);

or:

UmaHistogramEnumeration("NewTabPageAction", action, NewTabPageAction::kCount);

Flag Histograms

When adding a new flag in about_flags.cc, you need to add a corresponding entry to enums.xml. This is automatically verified by the AboutFlagsHistogramTest unit test.

To add a new entry:

Edit enums.xml, adding the feature to the LoginCustomFlags enum section, with any unique value (just make one up, although whatever it is needs to appear in sorted order; pretty_print.py can do this for you).
Build unit_tests, then run unit_tests --gtest_filter='AboutFlagsHistogramTest.*' to compute the correct value.
Update the entry in enums.xml with the correct value, and move it so the list is sorted by value (pretty_print.py can do this for you).
Re-run the test to ensure the value and ordering are correct.

You can also use tools/metrics/histograms/validate_format.py to check the ordering (but not that the value is correct).

Don't remove entries when removing a flag; they are still used to decode data from previous Chrome versions.

Count Histograms

histogram_macros.h provides macros for some common count types such as memory or elapsed time, in addition to general count macros. These have reasonable default values; you seldom need to choose the number of buckets or histogram min. However, you still need to choose the histogram max (use the advice below).

If none of the default macros work well for you, please thoughtfully choose a min, max, and bucket count for your histogram using the advice below.

Count Histograms: Choosing Min and Max

For histogram max, choose a value such that very few emissions to the histogram exceed the max. If many emissions hit the max, it can be difficult to compute statistics, such as the average. One rule of thumb is at most 1% of samples should be in the overflow bucket. This allows analysis of the 99th percentile. Err on the side of too large a range versus too short a range. (Remember that if you choose poorly, you'll have to wait for another release cycle to fix it.)

For histogram min, if you care about all possible values (zero and above), choose a min of 1. (All histograms have an underflow bucket for emitted zeros, so a min of 1 is appropriate.) Otherwise, choose the min appropriate for your particular situation.

Count Histograms: Choosing Number of Buckets

Choose the smallest number of buckets that give you the granularity you need. By default, count histogram bucket sizes scale exponentially so you can get fine granularity when the numbers are small yet still reasonable resolution for larger numbers. The macros default to 50 buckets (or 100 buckets for histograms with wide ranges), which is appropriate for most purposes. Because histograms pre-allocate all the buckets, the number of buckets selected directly dictates how much memory is used. Do not exceed 100 buckets without good reason (and consider whether sparse histograms might work better for you in that case—they do not pre-allocate their buckets).

Timing Histograms

You can easily emit a time duration (time delta) using UMA_HISTOGRAM_TIMES, UMA_HISTOGRAM_MEDIUM_TIMES, UMA_HISTOGRAM_LONG_TIMES macros, and their friends, as well as helpers like SCOPED_UMA_HISTOGRAM_TIMER. Many timing histograms are used for performance monitoring; if this is the case for you, please read this document about how to structure timing histograms to make them more useful and actionable.

Percentage or Ratio Histograms

You can easily emit a percentage histogram using the UMA_HISTOGRAM_PERCENTAGE macro provided in histogram_macros.h. You can also easily emit any ratio as a linear histogram (for equally sized buckets).

For such histograms, you want each value recorded to cover approximately the same span of time. This typically means emitting values periodically at a set time interval, such as every 5 minutes. We do not recommend recording a ratio at the end of a video playback, as video lengths vary greatly.

It is okay to emit at the end of an animation sequence when what's being animated is fixed / known. In this case, each value represents roughly the same span of time.

Why? You typically cannot make decisions based on histograms whose values are recorded in response to an event that varies in length because such metrics can conflate heavy usage with light usage. It's easier to reason about metrics that avoid this source of bias.

Many developers have been bitten by this. For example, it was previously common to emit an actions-per-minute ratio whenever Chrome was backgrounded. Precisely, these metrics computed the number of uses of a particular action during a Chrome session, divided by length of time Chrome had been open. Sometimes, the recorded rate was based on a short interaction with Chrome–a few seconds or a minute. Other times, the recorded rate was based on a long interaction, tens of minutes or hours. These two situations are indistinguishable in the UMA logs–the recorded values can be identical.

The inability to distinguish these two qualitatively different settings make such histograms effectively uninterpretable and not actionable. Emitting at a regular interval avoids the issue. Each value represents the same amount of time (e.g., one minute of video playback).

Local Histograms

Histograms can be added via Local macros. These still record locally, but are not uploaded to UMA and are therefore not available for analysis. This can be useful for metrics only needed for local debugging. We don't recommend using local histograms outside of that scenario.

Multidimensional Histograms

It is common to be interested in logging multidimensional data–where multiple pieces of information need to be logged together. For example, a developer may be interested in the counts of features X and Y based on whether a user is in state A or B. In this case, they want to know the count of X under state A, as well as the other three permutations.

There is no general purpose solution for this type of analysis. We suggest using the workaround of using an enum of length MxN, where you log each unique pair {state, feature} as a separate entry in the same enum. If this causes a large explosion in data (i.e. >100 enum entries), a sparse histogram may be appropriate. If you are unsure of the best way to proceed, please contact someone from the OWNERS file.

Histogram Expiry

Histogram expiry is specified by the expires_after attribute in histogram descriptions in histograms.xml. The attribute can be specified as date in YYYY-MM-DD format or as Chrome milestone in M*(e.g. M68) format. In the latter case, the actual expiry date is about 12 weeks after that branch is cut, or basically when it is replaced on the "stable" channel by the following release.

After a histogram expires, it ceases to be displayed on the dashboard. However, the client may continue to send data for that histogram for some time after the official expiry date so simply bumping the 'expires_after' date at HEAD may be sufficient to resurrect it without any discontinuity. If too much time has passed and the client is no longer sending data, it can be re-enabled via Finch: see Expired histogram allowlist.

Once a histogram has expired, the code that records it becomes dead code and should be removed from the codebase along with marking the histogram definition as obsolete.

In rare cases, the expiry can be set to "never". This is used to denote metrics of critical importance that are, typically, used for other reports. For example, all metrics of the "heartbeat" are set to never expire. All metrics that never expire must have an XML comment describing why so that it can be audited in the future.

<!-- expires-never: "heartbeat" metric (internal: go/uma-heartbeats) -->

For all new histograms, the use of expiry attribute is strongly encouraged and enforced by the Chrome Metrics team through reviews.

How to choose expiry for histograms

If you are adding a histogram to evaluate a feature launch, set an expiry date consistent with the expected feature launch date. Otherwise, we recommend choosing 3-6 months.

Here are some guidelines for common scenarios:

If the listed owner moved to different project, find a new owner.
If neither the owner nor the team uses the histogram, remove it.
If the histogram is not in use now, but might be useful in the far future, remove it.
If the histogram is not in use now, but might be useful in the near future, pick ~3 months or ~2 milestones ahead.
If the histogram is actively in use now and is useful in the short term, pick 3-6 months or 2-4 milestones ahead.
If the histogram is actively in use and seems useful for an indefinite time, pick 1 year.

We also have a tool that automatically extends expiry dates. The 80% more frequently accessed histograms are pushed out every Tuesday, to 6 months from the date of the run. Googlers can view the design doc.

Expired histogram notifier

The expired histogram notifier notifies histogram owners before their histograms expire by creating crbugs, which are assigned to owners. This allows owners to extend the lifetime of their histograms, if needed, or deprecate them. The notifier regularly checks all histograms across the histograms.xml files and identifies expired or soon-to-be expired histograms. It then creates or updates crbugs accordingly.

Expired histogram allowlist

If a histogram expires but turns out to be useful, you can add the histogram's name to the allowlist until the updated expiration date reaches the stable channel. When doing so, update the histogram's summary to document the period during which the histogram's data is incomplete. To add a histogram to the allowlist, see the internal documentation: Histogram Expiry.

Testing

Test your histograms using chrome://histograms. Make sure they're being emitted to when you expect and not emitted to at other times. Also check that the values emitted to are correct. Finally, for count histograms, make sure that buckets capture enough precision for your needs over the range.

Pro tip: You can filter the set of histograms shown on chrome://histograms by specifying a prefix. For example, chrome://histograms/Extensions.Load shows only histograms whose names match the pattern "Extensions.Load*".

In addition to testing interactively, you can have unit tests examine the values emitted to histograms. See histogram_tester.h for details.

Interpreting the Resulting Data

The top of go/uma-guide has good advice on how to go about analyzing and interpreting the results of UMA data uploaded by users. If you're reading this page, you've probably just finished adding a histogram to the Chromium source code and you're waiting for users to update their version of Chrome to a version that includes your code. In this case, the best advice is to remind you that users who update frequently / quickly are biased. Best take the initial statistics with a grain of salt; they're probably mostly right but not entirely so.

Revising Histograms

When changing the semantics of a histogram (when it's emitted, what buckets mean, etc.), make it into a new histogram with a new name. Otherwise the "Everything" view on the dashboard will mix two different interpretations of the data and make no sense.

Deleting Histograms

Please delete code that emits to histograms that are no longer needed. Histograms take up memory. Cleaning up histograms that you no longer care about is good! But see the note below on Cleaning Up Histogram Entries.

Documenting Histograms

Document histograms in histograms.xml. There is also a google-internal version of the file for the rare case in which the histogram is confidential (added only to Chrome code, not Chromium code; or, an accurate description about how to interpret the histogram would reveal information about Google's plans).

Add Histogram and Documentation in the Same Changelist

If possible, please add the histograms.xml description in the same changelist in which you add the histogram-emitting code. This has several benefits. One, it sometimes happens that the histograms.xml reviewer has questions or concerns about the histogram description that reveal problems with interpretation of the data and call for a different recording strategy. Two, it allows the histogram reviewer to easily review the emission code to see if it comports with these best practices and to look for other errors.

Understandable to Everyone

Histogram descriptions should be roughly understandable to someone not familiar with your feature. Please add a sentence or two of background if necessary.

It is good practice to note caveats associated with your histogram in this section, such as which platforms are supported (if the set of supported platforms is surprising). E.g., a desktop feature that happens not to be logged on Mac.

State When It Is Recorded

Histogram descriptions should clearly state when the histogram is emitted (profile open? network request received? etc.).

Provide Clear Units or Enum Labels

For enumerated histograms, including boolean and sparse histograms, provide an enum= attribute mapping enum values to semantically contentful labels. Define the <enum> in enums.xml if none of the existing enums are a good fit. Use labels whenever they would be clearer than raw numeric values.

For non-enumerated histograms, include a units= attribute. Be specific: e.g. distinguish "MB" vs. "MiB", refine generic labels like "counts" to more precise labels like "pages", etc.

Owners

Histograms need owners, who are the experts on the metric and the points of contact for any questions or maintenance tasks, such as extending a histogram's expiry or deprecating the metric.

Histograms must have a primary owner and may have secondary owners. A primary owner is a Googler with an @google.com or @chromium.org email address, e.g. lucy@chromium.org, who is ultimately responsible for maintaining the metric. Secondary owners may be other individuals, team mailing lists, e.g. my-team@google.com, or paths to OWNERS files, e.g. src/directory/OWNERS.

It's a best practice to list multiple owners, so that there's no single point of failure for histogram-related questions and maintenance tasks. If you are using a metric heavily and understand it intimately, feel free to add yourself as an owner.

Notably, owners are asked to determine whether histograms have outlived their usefulness. When a histogram is nearing expiry, a robot files a reminder bug in Monorail. It's important that somebody familiar with the histogram notices and triages such bugs!

Tip: When removing someone from the owner list for a histogram, it's a nice courtesy to ask them for approval.

Components

Histograms may be associated with components, which can help make sure that histogram expiry bugs don't fall through the cracks.

There are two ways in which components may be associated with a histogram. The first and recommended way is to add a tag to a histogram or histogram suffix, e.g. UI>Shell. The second way is to specify an OWNERS file as a secondary owner for a histogram. If the OWNERS file contains a component, then the component is associated with the histogram. If the specified OWNERS file doesn't have a component, but an OWNERS file in a parent directory does, then the parent directory's component is used.

Cleaning Up Histogram Entries

Do not delete histograms from histograms.xml. Instead, mark unused histograms as obsolete and annotate them with the date or milestone in the <obsolete> tag entry.

If deprecating only some variants of a patterned histogram, mark each deprecated <variant> as obsolete as well. Similarly, if the histogram used histogram suffixes, mark the suffix entry for the histogram as obsolete.

If the histogram is being replaced by a new version:

Note in the <obsolete> message the name of the replacement histogram.
Make sure the descriptions of the original and replacement histogram are different. It's never appropriate for them to be identical. Either the old description was wrong, and it should be revised to explain what it actually measured, or the old histogram was measuring something not as useful as the replacement, in which case the new histogram is measuring something different and needs to have a new description.

A changelist that marks a histogram as obsolete should be reviewed by all current owners.

Deleting histogram entries would be bad if someone to accidentally reused your old histogram name and thereby corrupts new data with whatever old data is still coming in. It's also useful to keep obsolete histogram descriptions in histograms.xml—that way, if someone is searching for a histogram to answer a particular question, they can learn if there was a histogram at some point that did so even if it isn't active now.

Exception: It is ok to delete the metadata for any histogram that has never been recorded to. For example, it's fine to correct a typo where the histogram name in the metadata does not match the name in the Chromium source code.

Patterned Histograms

It is sometimes useful to record several closely related metrics, which measure the same type of data, with some minor variations. You can declare the metadata for these concisely using patterned histograms. For example:

<histogram name="Pokemon.{Character}.EfficacyAgainst{OpponentType}" ...>
  <owner>individual@chromium.org</owner>
  <owner>team@chromium.org</owner>
  <summary>
    The efficacy multiplier for {Character} against an opponent of
    {OpponentType} type.
  </summary>
  <token key="Character">
    <variant name="Bulbasaur"/>
    <variant name="Charizard"/>
    <variant name="Mewtwo"/>
  </token>
  <token key="OpponentType">
    <variant name="Dragon" summary="dragon"/>
    <variant name="Flying" summary="flappity-flap"/>
    <variant name="Psychic" summary="psychic"/>
    <variant name="Water" summary="water"/>
  </token>
</histogram>

This example defines metadata for 12 (= 3 x 4) concrete histograms, such as

<histogram name="Pokemon.Charizard.EfficacyAgainstWater" ...>
  <owner>individual@chromium.org</owner>
  <owner>team@chromium.org</owner>
  <summary>
    The efficacy multiplier for Charizard against an opponent of water type.
  </summary>
</histogram>

Note that each token <variant> defines what text should be substituted for it, both in the histogram name and in the summary text. As shorthand, a <variant> that omits the summary attribute substitutes the value of the name attribute in the histogram's <summary> text as well.

*** promo Tip: You can declare an optional token by listing an empty name: <variant name="" summary="aggregated across all breakdowns"/>. This can be useful when recording a "parent" histogram that aggregates across a set of breakdowns.

You can use the <variants> tag to define a set of <variant>s out-of-line. This is useful for token substitutions that are shared among multiple families of histograms. See histograms.xml for examples.

By default, a <variant> inherits the owners declared for the patterned histogram. Each variant can optionally override the inherited list with custom owners:

<variant name="SubteamBreakdown" ...>
  <owner>subteam-lead@chromium.org</owner>
  <owner>subteam@chromium.org</owner>
</variant>

As with histogram entries, never delete variants. If the variant expansion is no longer used, mark it as <obsolete>.

*** promo Tip: You can run print_expanded_histogram.py --pattern= to show all generated histograms by patterned histograms or histogram suffixes including their summaries and owners. For example, this can be run (from the repo root) as:

./tools/metrics/histograms/print_expanded_histogram.py --pattern=^UMA.A.B

*** promo Tip: You can run print_histogram_names.py --diff to enumerate all the histogram names that are generated by a particular CL. For example, this can be run (from the repo root) as:

./tools/metrics/histograms/print_histogram_names.py --diff origin/master

For documentation about the <histogram_suffixes> syntax, which is deprecated, see https://chromium.googlesource.com/chromium/src/+/refs/tags/87.0.4270.1/tools/metrics/histograms/one-pager.md#histogram-suffixes-deprecated-in-favor-of-pattern-histograms

When To Use Sparse Histograms

Sparse histograms are well-suited for recording counts of exact sample values that are sparsely distributed over a large range. They can be used with enums as well as regular integer values. It is often valuable to provide labels in enums.xml.

The implementation uses a lock and a map, whereas other histogram types use a vector and no lock. It is thus more costly to add values to, and each value stored has more overhead, compared to the other histogram types. However it may be more efficient in memory if the total number of sample values is small compared to the range of their values.

Please talk with the metrics team if there are more than a thousand possible different values that you could emit.

For more information, see sparse_histograms.h.

Team Documentation

Reviewing Metrics CLs

When reviewing metrics CLs, look at the following, listed in approximate order of importance:

Privacy

Does anything tickle your privacy senses? (Googlers, see go/uma-privacy for guidelines.)

Please escalate if there's any doubt!

Clarity

Is the metadata clear enough for all Chromies to understand what the metric is recording? Consider the histogram name, description, units, enum labels, etc.

It's really common for developers to forget to list when the metric is recorded. This is particularly important context, so please remind developers to clearly document it.

Note: Clarity is a bit less important for very niche metrics used only by a couple of engineers. However, it's hard to assess the metric design and correctness if the metadata is especially unclear.

Metric design

Does the metric definition make sense?
Will the resulting data be interpretable at analysis time?

Correctness

Is the histogram being recorded correctly?

Does the bucket layout look reasonable?
- The metrics APIs like base::UmaHistogram* have some sharp edges, especially for the APIs that require specifying the number of buckets. Check for off-by-one errors and unused buckets.
- Is the bucket layout efficient? Typically, push back if there are >50 buckets -- this can be ok in some cases, but make sure that the CL author has consciously considered the tradeoffs here and is making a reasonable choice.
- For timing metrics, do the min and max bounds make sense for the duration that is being measured?
The base::UmaHistogram* functions are generally preferred over the UMA_HISTOGRAM_* macros. If using the macros, remember that names must be runtime constants!

Also, related to clarity: Does the client logic correctly implement the metric described in the XML metadata? Some common errors to watch out for:

The metric is only emitted within an if-stmt (e.g., only if some data is available) and this restriction isn't mentioned in the metadata description.
The metric description states that it's recorded when X happens, but it's actually recorded when X is scheduled to occur, or only emitted when X succeeds (but omitted on failure), etc.

When the metadata and the client logic do not match, the appropriate solution might be to update the metadata, or it might be to update the client logic. Guide this decision by considering what data will be more easily interpretable and what data will have hidden surprises/gotchas.

Sustainability

Is the CL adding a reasonable number of metrics?
- When reviewing a CL that is trying to add many metrics at once, guide the CL author toward an appropriate solution for their needs. For example, multidimensional metrics can be recorded via UKM, and we are currently building support for structured metrics in UMA.
Are expiry dates being set appropriately?

Everything Else!

This document describes many other nuances that are important for defining and recording useful metrics. Check CLs for these other types of issues as well.

And, as you would with a language style guide, periodically re-review the doc to stay up to date on the details.

Becoming a Metrics Owner

If you would like to be listed as one of the OWNERS for metrics metadata, reach out to one of the existing //base/metrics/OWNERS. Similar to language readability review teams, we have a reverse shadow onboarding process:

First, read through this document to get up to speed on best practices.
Partner up with an experienced reviewer from //base/metrics/OWNERS.
Join the cs/chrome-metrics.gwsq.

Note: This step is optional if you are not on the metrics team. Still, consider temporarily joining the metrics gwsq as a quick way to get a breadth of experience. You can remove yourself once your training is completed.
Start reviewing CLs! Once you're ready to approve a CL, add a comment like "I am currently ramping up as a metrics reviewer, +username for OWNERS approval" and add your partner as a reviewer on the CL. Once at a point where there's pretty good alignment in the code review feedback, your partner will add you to the OWNERS file.

Processing histograms.xml

When working with histograms.xml, verify whether you require fully expanded OWNERS files. Many scripts in this directory process histograms.xml, and sometimes OWNERS file paths are expanded and other times they are not. OWNERS paths are expanded when scripts make use of merge_xml's function MergeFiles; otherwise, they are not.

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