How cookie reach is estimated

Reach (generally speaking) is the number of people who have seen your ads — how many people your campaign reached.

Reach and cookies

As a basis for estimating reach, we start with cookies. Cookies enable Campaign Manager 360 to calculate reach because they can be used to distinguish individual browsers. When we count reach, we're really counting the number of browsers that have been served an ad or recorded a click on an ad. This measure is an estimate, because some people use more than one browser or computer, and some people share browsers, but it enables us to get a consistent picture of how widely an advertisement has been seen.

Adjusting for cookie deletion

We want to make sure that each cookie that's counted represents a unique browser. However, sometimes users delete their cookies. When they do, a new cookie is created the next time they're served an ad. In such cases, the same browser might be represented by two different cookies: the old cookie that got deleted, and the new cookie that was just created.

Avoid double-counting

We can make sure that each browser we count is unique by only counting mature cookies. Mature cookies are cookies that were created before the date range for which reach is being estimated. If we only count mature cookies, then we know that each cookie represents a unique browser, because all of them existed when the reporting period began, and two different cookies can't represent the same browser at the same time.

Mature cookies are defined in two different ways, depending on the structure of your report and the metrics you choose.

• Maturity based on the report's date range: For reports that don't break down the data by day, week, or month, mature cookies are cookies that were created before the start of the report's date range. For example, if you run a report from March 15 to March 31, cookies created through March 14 are counted as mature cookies. Cookies created on March 15 or after are discarded.
  
  This definition is also used for incremental reach metrics, even if you include Day, Week, or Month in the dimensions. That's because incremental reach measures how many additional new browsers were reached in each new time period, on top of those that were previously reached. To calculate this reach, Reporting needs to ensure that no browsers are double-counted from the beginning of the reporting period.

• Maturity based on day, week, or month: For reports that include Day, Week, or Month in the dimensions, reach (but not incremental reach) is estimated for each day, week, or month within the report. For each day, week, or month, mature cookies are cookies that were created before the start of that day, week, or month. For example, if you break down a report by month, then a cookie that was created in July is considered mature for the report row that shows reach for August, but not for the July row.

Correct the data

To avoid double-counting, Reporting discards all new cookies. But some new cookies are legitimate. To estimate reach more accurately, we have to estimate how many of the new cookies were actually legitimate.

To come up with an estimate, Reporting first looks at the number of impressions that are associated with mature cookies, and compares the number of impressions to the number of cookies. Then, based on the assumption that the ratio of mature cookies to mature impressions is the same as the ratio of all cookies to all impressions, Reporting calculates a value for the total number of cookies. That value is what we display as reach.

Show me the math

To estimate reach more accurately, Reporting applies a reach correction formula to the data:

(Mature Cookies / Mature Impressions) = (Total Cookies / Total Impressions)

In this formula:

• Total Cookies is the unknown value. It's the same thing as reach, and it's the value that is displayed in reports.

• Mature Impressions are impressions that are associated with mature cookies.

As you can see, the formula is comparing two ratios: the ratio of mature cookies to mature impressions, and the ratio of total cookies to total impressions. With the data Reporting collects, we can accurately calculate the first ratio. We then assume that the ratio of cookies to impressions is the same overall as it is for mature cookies and impressions.

Solving for Total Cookies, we can restate the formula as follows:

Total Cookies = Total Impressions × (Mature Cookies / Mature Impressions)