BIS is consulting on a new statistical indicator for widening participation - the proportion of young entrants previously in receipt of free school meals (FSM). They can calculate this by matching the individualised HESA data from universities and colleges to the individualised schools data from previous years. If you have a Jiscmail account you can read Mike Milne-Picken's detailed analysis on the Admin-Planning list.
The existing measures are Socio-economic class (NS-SEC), state school, and low participation neighbourhoods, all of which are derived entirely from the HESA data provided by institutions. SEC in particular is troublesome and getting more troublesome, because applicants are increasingly unwilling to answer questions about their (or their parents') occupations, or they answer them untruthfully because they think that middle-class parentage will count against them in the admissions process. Whether or not you went to state school is fairly uninformative about social disadvantage. The postcode data are easy to collect but, especially in London, there are weaknesses where rich and poor people live too close together.
From my perspective, through, the main weaknesses of the existing data are their poor coverage of PT students and mature FT entrants. PT entrants will not usually pass through UCAS so the SEC data are not usually collected, whilst neighbourhood data is particularly unreliable if it describes the area where you currently live rather than the area where you were brought up. The FSM measure will be even worse for this group than our current approaches, though, because matching to decade-or-more-old schools data will be effectively impossible.
Young FT entrants are obviously an important group, but so are mature and PT entrants, and so often the young full timers seem to hog all the limelight. The mature and PT entrants are not like the young entrants, in particular they are from different ethnic and social groups - precisely the kinds of differences that should be relevant to WP policy. If the published data concentrate even more attention on young students, that cannot lead to good policy making.
A second problem relates to transparency. Rightly, the schools data is not going to be shared with institutions, so we will not know which of our students are FSM or not-FSM. This will be a big frustration to university managers who can currently tell pretty easily who their WP students are - at l;east among that young FT group.
Given the increasing weakness of the NS-SEC data, FSM may be the right solution. I suppose my hesitation is that I see it as a solution to the wrong problem.