How Much Must You Give for a White House Invite?

The NY Times today ("White House Welcomes Donors...", Sunday, April 15) had a page 1 story by Mike McIntire and Michael Luo about President Obama's invitations to the White House. Fascinating stuff.

Biggest surprise for me was the degree of White House transparency. The Federal Election Commission (FEC) keeps track of campaign donations (see www.data.gov) and the Executive Office of the President keeps a public log of visits to the White House that is posted on www.data.gov. Inevitably, someone was going to tie these together.

The donation totals are cumulative over three election cycles, i.e., from 2008 through 2012 (first quarter). Surprise (not) - there is a pretty close relationship between how much money you give and the probability of a White House invitation - for this purpose a visit equates to an invitation. The Times story doesn't provide a formula, but here is what I got inputting the numbers from the Times chart - see my chart immediately below.

PROBABILITY OF AN INVITATION y=26.3% + 40.2%x$GIFT (in thousands of dollars).
So if you give $100,000 over a 5.5-year period, your probability of an invitation is 26%+40%x100, which is (26+40)% = 66%.
What this means is that two out of three donors at the $100,000 level visit the White House. 
But a few caveats are needed, and a couple of questions must be asked.

First off, the FEC disclosure reports include gifts of only $200 and above. That excludes many small gifts via the Internet. So there will some invitations to dinner to the Internet givers that wouldn't show up on the FEC reports. The probability at the "zero" gift level is the probability that someone gets a White House invitation after giving only (at least) $200. The probability applies to givers of $200 or more.

Second, the White House visitors' list is hard to navigate for purposes of reconciling the numbers. What rules were followed in deciding who is included from the list of invitees? For example, many overseas visitors show up on the site and they are not permitted to give. Many of visits are labeled as events ("Holiday Party"). Some of the locations to which visitors are invited are strictly public rooms. How was the list narrowed down?

Finally, to draw conclusions that would characterize the Obama White House in any way, would we not need some data from other presidencies? How does the Obama era differ from Bush 43, for example?