I've done some work with federal agencies and how they process FOIA requests:
A request for information under the FOIA can be granted, partially denied, or denied. If the request is granted, the exact records requested are returned unedited. If the request is denied, one or more reasons (exceptions) must be stated from a list of allowed exemptions. If a request is partially denied, one or more exemptions must be stated and what the requester receives back will either be a subset of what was asked for or will be redacted to remove sensitive information. For example, PIA (personally identifiable information - like SSNs, birth dates, medical records, etc.) is an exemption and is grounds for a partial denial, but it usually only means that this information will be redacted from the requested records.
So if you are looking at statistics (annual FOIA reports are required by law from every government entity and the reports themselves are either published or available via FOIA request themselves), you need to know the total number of new requests, the total number of requests held over from the previous fiscal year, the number of requests granted, the number partially denied, and the number totally denied. There are also individual statistics for denials and partial denials broken down by exemptions. There isn't anything on the annual report about how many exemptions were applied to individual requests - that would just have to be averaged out.
The Obama administration did encourage more release of records under the FOIA and a relaxing of exemptions. The idea was to assume that any record could be released unless an exemption prevented it. The previous directive was to presume that any record could not released and then try to justify it. If they couldn't justify denying it, they would grudgingly release it. The other thing that has been encouraged is pre-emptive release. For any request that is granted (no exemptions) there is no reason to not put that record on the agency's public web site to avoid processing any future requests for it. Or if there are certain types of records that can be released and that get requested often, go ahead and publish them. Theoretically this will reduce the number of FOIA requests processed, but I think it's probably too early to see a difference based on this policy.