Private Click Attribution
Apple's Private Click Attribution is designed to provide less accurate, incomplete and time-delayed data to marketers about the events associated with their advertising. Apple acknowledges that for "ad click attribution to happen, some bits of data about what happened across two websites need to be sent."
Apple proposes that the browser send time-delayed information associated with the attribution to both buyers and sellers.
“Ad Click Attribution in a Nutshell
Here’s a simple example of ad click attribution:
An online store runs an ad on a... website. If a user clicks the ad and eventually buys something, both the online store and the... website where the ad was placed want to know; they want the purchase to be attributed to the ad click so that the store knows where to focus their advertising budget. Such attribution is used for measurement of which ads are effective…."
The steps in calculating this attribution include:
- The browser stores the click on every ad a visitor engages with
- The browser stores every conversion event
- The browser matches conversions to prior clicks
- The browser sends a time-delayed aggregate report to both buyers and sellers
Note the marketer must place publisher pixels to enable this data transfer.
By removing accurate, complete and real-time feedback to marketers, their ability to adjust the pricing and budget associated with advertising will be impaired. As the value marketers receive from advertising declines, so to will publisher revenues.
Apple's original proposal suggested marketers need only to measure a limited set of 64 campaigns, which is not practical to most marketers who run hundreds of campaigns or relying on sponsored search relying on thousands of keywords. They have since increased this limit, by reducing the granularity of conversion events that can be attributed to prior campaign interactions.
Apple's proposed Private Click Attribution relies on marketers sending the value of their conversion data to the publisher. However, Apple proposes that the data sent be a range rather than a specific value, which thus impairs the accuracy of the attribution algorithm. Moreover, W3C members have criticized this proposal on not improving privacy as well as forcing buyers to disclose proprietary data to sellers:
- “ad click destination may not want to share any details with ad click source besides the fact that conversion happened.”
- “It also doesn't seem more transparent than sending a request to a third party. The data being sent is already in aggregate and contains very little identifiable information so it's not clear what threat is being avoided.” 
- Marketers must place millions of publisher pixels if they want to advertise across millions of publishers.
- How will marketers value view-through attribution or multi-touch attribution?