It can be difficult for nonprofits to measure their impact. The bottom line for nonprofits is essentially qualitative, not quantitative. I just re-read a great article by Jan Masaoka in Blue Avocado about the attempts to rate nonprofits and where these objective rating systems are falling short: http://blueavocado.org/content/charity-raters-part-i-charting-bad-and-bad.
Some of these ratings systems have improved, but the bottom line for donors and volunteers: get to know a nonprofit by building relationships with them, understanding their cause, and giving your time and money on an on-going basis, rather than looking at ratings. You can confirm that an organization does have IRS nonprofit status in good standing at www.guidestar.org and that it has California charity status at http://rct.doj.ca.gov/Verification/Web/Search.aspx?facility=Y.