AMA tutorial part 3: puzzling outcomes

Ronny Kohavi, Microsoft
Slides available at
Wrap-up section based on KDD 2012 paper, co-authored by
Ronny Kohavi, Alex Deng, Brian Frasca, Roger
Longbotham, Toby Walker, and Ya Xu
How does one determine the OEC for a search engine?
What are some of the most surprising results we faced, and
how did we resolve them
An OEC is the Overall Evaluation Criterion
It is a metric (or set of metrics) that guides the org as to
whether A is better than B in an A/B test
In prior work, we emphasized long-term focus and thinking
about customer lifetime value, but operationalizing it is hard
Search engines (Bing, Google) are evaluated on query share
(distinct queries) and revenue as long-term goals
A ranking bug in an experiment resulted in very poor search
Distinct queries went up over 10%, and revenue went up over 30%
What metrics should be in the OEC for a search engine?
Degraded (algorithmic) search results cause users to search more
to complete their task, and ads appear more relevant
Analyzing queries per month, we have


where a session begins with a query and ends with 30-minutes of inactivity.
(Ideally, we would look at tasks, not sessions).
Key observation: we want users to find answers and complete tasks
quickly, so queries/session should be smaller
In a controlled experiment, the variants get (approximately) the
same number of users by design, so the last term is about equal
The OEC should therefore include the middle term: sessions/user
on a search result, additional JavaScript was executed
(a session-cookie was updated with the destination)
before navigating to the destination page
This slowed down the user experience slightly, so we
expected a slightly negative experiment.
Results showed that users were clicking more!
User clicks (and form submits) are instrumented and form
the basis for many metrics
Instrumentation is typically done by having the web browser
request a web beacon (1x1 pixel image)
Classical tradeoff here
Waiting for the beacon to return slows the action (typically
navigating away)
Making the call asynchronous is known to cause click-loss, as
the browsers can kill the request (classical browser optimization
because the result can’t possibly matter for the new page)
Small delays, on-mouse-down, or redirect are used
Click-loss varies dramatically by browser
Chrome, Firefox, Safari are aggressive at terminating such
reqeuests. Safari’s click loss > 50%.
IE respects image requests for backward compatibility
White paper available on this issue here
Other cases where this impacts experiments
Opening link in new tab/window will overestimate the click delta
Because the main window remains open, browsers can’t
optimize and kill the beacon request, so there is less click-loss
Using HTML5 to update components of the page instead of
refreshing the whole page has the overestimation problem
Primacy effect occurs when you change the navigation on a
web site
Experienced users may be less efficient until they get used to the
new navigation
Control has a short-term advantage
Novelty effect happens when a new design is introduced
Users investigate the new feature, click everywhere, and introduce
a “novelty” bias that dies quickly if the feature is not truly useful
Treatments have a short-term advantage
Given the high failure rate of ideas, new experiments are
followed closely to determine if new idea is a winner
Multiple graphs of effect look like this
Negative on day 1:
Less negative on day 2: -0.38%
Less negative on day 3: -0.21%
Less negative on day 4: -0.13%
Cumulative Effect
The experimenter extrapolates linearly
and says: primacy effect.
This will be positive in a couple of days, right?
Wrong! This is expected
For many metrics, the standard deviation of the mean is
proportional to 1 ⁄ √, where  is the number of users
As we run an experiment longer, more users are admitted
into the experiment, so  grows and the conf interval
The first days are highly variable
The first day has a 67% chance
of falling outside the 95% CI
at the end of the experiment
The second day has a 55% chance
of falling outside this bound.
Experiment Days
95% bound
21-day bound
The longer graph
Cumulative Effect
This was an A/A test, so the true effect is 0
X-axis: Treatment size
Y-axis: conf interval
Three lines: 1,2,3 weeks
Overlapping lines?
That’s the problem!
Confidence Inerval
Width (percent)
We expect the standard deviation of the mean (and thus the
confidence interval) to be proportional to 1 ⁄ √,
where  is the number of users
So as the experiment runs longer and more users are
admitted, the confidence interval should shrink
Here is the graph for sessions/user
Size of Treatment (relative factor)
1 week
2 weeks
3 weeks
The distribution changes
Users churn, so they contribute zero visits
New users join with fresh count of one
We have a mixture
Empirically, the coefficient of variation (ratio of the standard
deviation to the mean) grows at the same rate as √
Running an experiment longer does not increase statistical
power for some metrics; you must increase the variant size
Experiment is run, results are surprising.
(This by itself is fine, as our intuition is poor.)
Rerun the experiment, and the effects disappear
Reason: bucket system recycles users, and the prior
experiment had carryover effects
These can last for months!
Must run A/A tests, or re-randomize
OEC: evaluate long-term goals through short-term metrics
The difference between theory and practice is greater in
practice than in theory
Instrumentation issues (e.g., click-tracking) must be understood
Carryover effects impact “bucket systems” used by Bing, Google,
and Yahoo require rehashing and A/A tests
Experimentation insight:
Effect trends are expected
Longer experiments do not increase power for some metrics.
Fortunately, we have a lot of users

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