Last time I pointed out that Ancestry.com's motive is financial: shareholder value, revenues and profits. Owned by The Church of Jesus Christ of Latter-day Saints, FamilySearch's motive is religious: eternal families and temple ordinances. Today let's look at the terms of the deal in terms of the motives.
If you need to refer back to the text of the announcement, it can be found on FamilySearch.org and the clarification can be found here at the Ancestry Insider. As usual, this is my opinion, not confidential information from either organization.
Ancestry.com's old images will not be as good as FamilySearch's new scans. Ancestry.com created its images back when modem's were much more prevalent than high-speed connections, so it scanned the images at lower resolutions to keep download times smaller. Plus, FamilySearch is scanning first-generation ("master") microfilms while Ancestry used second-generation (copies of master) films. (See "Census Image Quality.") Presumably, if FamilySearch is scanning first-generation microfilms, then NARA must be a partner also, providing access to the master microfilms. It appears that Ancestry.com will bear the cost of hosting the images and paying for the bandwidth to serve the images.
Ancestry gets to offer the better images via subscription. The Church gets more temple ordinances by offering the images for free at family history centers, which are frequented by Church members and genealogically-active members of the public. NARA gets the better images for free public use at their facilities. Ancestry.com and FamilySearch (and NARA) all get what they need for their objectives.
While in these deals we speak of indexes, which are document finding aids, it may be more descriptive to call them transcriptions and abstracts, the information extracted from a document. (See Indexes are not transcripts for the difference between transcriptions and abstracts.) And instead of keying we should be talking about indexing. But I digress...
The work of both organizations will be used to form enhanced indexes that will be given back to both organizations. For census years that FamilySearch has already indexed, the two organizations' indexes will be combined. It was not clear from the announcement which organization would bear the cost of this surprisingly expensive operation.
Ancestry.com's indexes have good and bad points. The law of diminishing returns would predict that as the accuracy of a census abstractions approach 100%, it becomes more and more expensive to achieve greater accuracy. So the bad point of Ancestry.com's census indexes is the limit that profitability places on accuracy. The good point is that Ancestry has allowed customer corrections for quite some time. I don't care whether your indexer is a Chinese professional or a random American volunteer, a user with intimate knowledge of a family's names is going to produce the most accurate names. I understand that Ancestry is supplying these user corrections as part of the deal.
For census years that FamilySearch has not yet indexed, FamilySearch can use the Ancestry indexes as the A keying and its own volunteers for the B keying. This cuts its A/B keying effort in half, allowing it to focus its volunteers elsewhere. FamilySearch could also use statistics from its A/B arbitration system to compare the quality of Ancestry's indexes to its own. If the quality is high enough and FamilySearch is not keying additional fields, they could skip the B keying altogether.
Again, Ancestry gets money from the indexes by charging for subscriptions (after offering them free for an unspecified length of time). The Church gets more temple ordinances because the indexes are available for free on FamilySearch.org, for free in NARA reading rooms and for free for a limited time on Ancestry.com.
If the Church could offer the images for free at home to Church members and the genealogically-active members of the public, it would. And it will. Next time I'll look at a new concept announced in the 28-July clarification: the notion of a FamilySearch Member. In the mean time, if you want to help produce a free index of the 1920 U.S. Census, visit www.familysearchindexing.org.