diff --git a/api.bs b/api.bs
index 7dab622..7a97973 100644
--- a/api.bs
+++ b/api.bs
@@ -27,11 +27,11 @@ The primary goal of this API is to enable attribution for advertising.
## Attribution ## {#s-attribution}
-Attribution is the process of identifying [=actions=]
+In advertising, attribution is the process of identifying [=actions=]
that precede an [=outcome=] of interest,
and allocating value to those [=actions=].
-For advertising, actions that are of interest
+Actions that are of interest to advertisers
are primarily the showing of advertisements
(also referred to as impressions).
Other actions include ad clicks (or other interactions)
@@ -117,32 +117,108 @@ with multiple purveyors of personal information that is traded for various purpo
## Goals ## {#goals}
-The goal of this document is to define a means of performing attribution
+The goal of this document is to define an means of performing [=attribution=]
+for advertising
that does not enable tracking.
-The primary challenge with attribution is in maintaining privacy.
-Attribution involves connecting activity on different sites.
-If that information were directly revealed,
-it would enable unwanted
-[[PRIVACY-PRINCIPLES#dfn-cross-context-recognition cross-context recognition]],
-thereby enabling tracking.
-This document avoids cross context recognition by ensuring that
-attribution information is aggregated using an [=aggregation service=].
-The aggregation service is trusted to compute an aggregate
-without revealing the values that each person contributes to that aggregate.
+## End-User Benefit ## {#user-benefit}
-Strict limits are placed on the amount of information that each browser instance
-contributes to the aggregates for a given site.
-Differential privacy is used to provide additional privacy protection for each contribution.
+The measurement of advertising performance creates new cross-site flows of information.
+That information flow creates a privacy risk or cost--
+of [[PRIVACY-PRINCIPLES#dfn-cross-context-recognition|cross-context recognition]]--
+that needs to be justified in terms of benefits to end users.
-Details of aggregation service operation is included in [[#aggregation]].
-The differential privacy design used is outlined in [[#dp]].
+Any benefits realized by end users through the use of [=attribution=] is indirect.
+End users that visit a website
+pay for "free" content or services
+primarily through their attention
+to any advertisements the site shows them.
+This "value" accrues to the advertiser,
+who in turn pays the site.
+The site is expected to use this money to
+support the provision of their content or services.
-## End-User Benefit ## {#user-benefit}
+
+
+path:images/value.svg
+
+Value exchange for advertising-supported content and services
+
-New additions to the
+Participation in an [=attribution=] measurement system
+would comprise a secondary cost to Web users.
+
+Support for attribution enables more effective advertising,
+largely by informing advertisers about what ads perform best,
+and in what circumstances.
+Those circumstances might include
+the time and place that the ad is shown,
+the person to whom the ad is presented, and
+the details of the ad itself.
+
+Connecting that information to outcomes
+allows an advertiser to learn what circumstances most often lead
+to the outcomes they most value.
+That allows advertisers to spend more on effective advertising
+and less on ineffective advertising.
+This lowers the overall cost of advertising
+relative to the value obtained. [[ONLINE-ADVERTISING]]
+
+Sites that provide advertising inventory,
+such as content publishers and service providers,
+indirectly benefit from more efficient advertising.
+Venues for advertising that are better able to
+show ads that result in
+the outcomes that advertisers seek
+can charge more for ad placements.
+
+Sites that obtain support through the placement of advertisements
+are better able to provide quality content or services.
+Importantly, that support is derived unevenly from their audience.
+This can be more equitable than other forms of financial support.
+Those with a lower tendency or ability to spend on advertised goods
+obtain the same ad-supported content and services
+as those who can afford to pay. [[EU-AD]][[COPPACALYPSE]]
+
+The ability to supply "free" services
+supported by advertising
+has measurable economic benefit
+that derives from the value of those services. [[FREE-GDP]]
+
+
+## Collective Privacy Effect ## {#collective}
+
+The use of aggregation--
+if properly implemented--
+ensures that information provided to sites is about groups and not individuals.
+
+The introduction of this mechanism therefore represents collective decision-making,
+as described in [[PRIVACY-PRINCIPLES#collective-privacy]].
+
+Participation in attribution measurement carries a lower privacy cost
+when the group that participates is larger.
+This is due to the effect of aggregation on
+the ability of sites to
+extract information about individuals from aggregates.
+This is especially true for central [[#dp|differential privacy]],
+which is the mathematical basis for the privacy design used
+in this specification.
+
+Larger cohorts of participants also produce more representative--
+and therefore more useful--
+statistics about the advertising that is being measured.
+
+If attribution is justified,
+both these factors motivate the enablement of attribution for all users.
+
+Acting to enable attribution measurement by user agents
+will not be positively received by some people.
+Different people perceive the costs and benefits
+that come from engaging with advertising differently.
+The proposed design allows people the option of appearing to participate in attribution
+without revealing that choice to sites; see [[#opt-out]].
## Attribution Using Histograms ## {#histograms}
@@ -268,7 +344,7 @@ The aggregation service:
from the provided inputs
and that there are enough conversion reports,
-2. adds the histograms including sufficient [[#dp noise]]
+2. adds the histograms including sufficient [[#dp|noise]]
to produce a differentially-private aggregate histogram, and
3. returns the aggregate to the site.
@@ -783,6 +859,11 @@ depend on the type of [=aggregation service=].
+## Optional Participation ## {#opt-out}
+
+TODO
+
+
# Security # {#security}
TODO
@@ -812,6 +893,17 @@ spec:infra; type:dfn; text:user agent