Effective altruism aims to quantify charity, and GiveWell's job is to grade charitable outfits; they know quantity cannot be the sole measurement of quality giving.
"We’ve come across a lot of charities making claims in the eight years since GiveWell was founded. You’ve probably seen the advertising: “Your $5 donation can save a life!” Often, those statements don’t hold up under closer scrutiny. Some gloss over the facts (yes, $5 can buy a mosquito net to prevent malaria, but not every mosquito net saves a life), while others involve mathematical errors (in one instance, a widely cited cost-effectiveness estimate for deworming, a treatment for parasitic worm infections, was roughly 100 times off the mark). As a result, we’re cautious when assessing charities’ claimed cost-effectiveness and when making similar estimates ourselves."--Catherine Hollander, The Washington Post