Find your company's inflection metrics with Compass

Compass is a powerful feature that can help you identify behaviors that are predictive of retention or conversion. It identifies inflection metrics, or those that capture the moments when a user has reached a critical threshold in your product—which are instrumental in driving user growth.

For example, Facebook found early on that adding seven friends in the first ten days was the strongest signal of long-term retention. More recently, Netflix published an analysis on exactly how many episodes it takes per TV show to get hooked on the show.

Before you proceed, Amplitude suggests you read the Help Center documentation on Compass first. The rest of this article assumes that you have a general understanding of how the analysis works.

For ease of reading, Amplitude keeps the new user/retention use case but you can replace new user can with any base cohort, and replace retained user with any target cohort.

To find an inflection metric, you first need to decide your target cohort. Often, the inflection metric centers on the process of encouraging new users to become retained users. In the examples used in this article, the base cohort is new users and the target cohort contains retained users.

A base cohort is an initial set of users you are analyzing (for example, new users or logged in users). A target cohort is a set of users that have successfully completed a targeted action (for example, retention, conversion).

This is a common use case, and it's the default setup for Compass charts. However, you can easily edit your chart so that it reflects your specific analytic needs.

When looking for an inflection metric, keep in mind that it's not absolute. It doesn't mean that a user converts specifically at that point; instead, it suggests the type of behavior you want your organization (for example, the product and marketing team) to encourage in your users. 

Get started with Compass

The best way to begin using Compass is to ask yourself which events might be good predictors of retention. Once you have selected an event to analyze, you should start thinking about correlations that might have potential to reveal something interesting about user behavior.

Proportion above threshold

The proportion above threshold tells you how many new users actually triggered the event in their first N days. This matters because there must be a large enough sample of users meeting the threshold for Compass to understand how well it correlates with retention.

One way to change the proportion is by increasing the number of performance days in the window (Amplitude allows between one to seven days). More performance days gives users more time to reach the threshold, hence increasing the proportion. If you are investigating an event property, consider looking at the complete event, as that may have a high enough proportion above the threshold.

There's no perfect proportion above the threshold. Too low and it's unlikely you can get new users to perform that event that many times; too high and you don't have any room for improvement.

There are some extreme cases where a low proportion above the threshold can still result in a high correlation. For example, if a web application has high traffic but forces login for all new users.

This metric is important because it takes into account the balancing of finding your inflection metric. Referring back to the Facebook example, getting a new user to add one friend isn't a great choice for an inflection metric, because most users do that. However, getting a new user to add 100 friends, while highly correlated with retention, is hard because a very low percentage of users actually reaches that level.

True positive ratios: PPV and sensitivity

If you have a reasonable proportion above the threshold, view the correlation between reaching the event frequency and retention. Do this by looking at the positive predictive value (PPV) and sensitivity.

PPV looks at the ratio of users who reached the event frequency and retained (known as true positive) to all users who reached the event frequency (true positive + false positive). Sensitivity looks at the ratio of users that retained and reached the event frequency (true positive) to all users retained (true positive + false negative). You want both to be high.

For more information about true positives, false positives, and other values in a contingency matrix, see Confusion matrix on Wikipedia

Example 1: High PPV, low sensitivity

For example, imagine that PPV is high but sensitivity is low. This means that this event is a predictor of retention, but few new users reach the threshold. It's therefore a promising candidate for experimentation, to see if you can encourage more users to trigger it. It also means that there might be another inflection metric you haven’t looked at yet; something else is likely correlated with retention, since people who aren’t reaching this frequency are still retained.

Example 2: Low PPV, high sensitivity

In this example, the event frequency is capturing a lot of the retained users, but the total retention for the product is likely low. This is not a good candidate for an inflection metric, because either the product’s retention is low, or a high percentage of users meeting the event frequency aren't retained.

True negative ratios: NPV and specificity

While the inflection moment should be a positive predictor, you also want to ensure that when a user fails to reach the threshold, it's a negative predictor of retention—in other words, churn. Amplitude captures this through the negative predictive value (NPV) and specificity.

NPV looks at the ratio of users who both didn't reach the event frequency and didn't retain (true negative) to all users who didn't reach the event frequency (true negative + false negative).

Specificity looks at the ratio of users who both didn't reach the event frequency and didn't retain (true negative) to all users who didn't retain (true negative + false positive). As in the examples above, you are hoping to maximize both of these values.

Example 3: High NPV, Low Specificity

In this example, one of two things is likely happening. Either the PPV is also low (as in Example 2), or the proportion above the threshold is so high to prevent any improvement by encouraging this action. Neither are great inflection metrics.

Example 4: Low NPV, high specificity

Here, either the sensitivity is also low (as in Example 1), or the retention is so high that there aren’t many users to convert—a good problem to have.

As you might have guessed by now, the Compass analysis tries to uncover event frequencies that maximize the upper left (true positive) and bottom right (true negative) quadrants of the contingency matrix (or, if you are familiar with statistics, minimize the Type I and Type II errors).  

These inflection metrics tend to balance all five of the detailed statistics described to do this, and depending on the type of product, a good correlation is in the range of 0.2-0.4 depending on the number of performance days (1-7) for the event.

It's also important to check that the sample size is enough to draw conclusions. There's no magic number for this, as it depends on your total user volume, but you can see the effect of sample size by clicking on the blue +- number (the 95% confidence interval) next to the correlation. Change the date range to increase Sample size can: You can use up to 90 days of data.

Understand that Compass exposes correlations from your data—hypotheses you can now test by making changes to your product and/or lifecycle marketing. The only way to prove a causal relationship is to run an A/B or split test to isolate those changes. Read more about how you can analyze A/B test results on Amplitude.