Also, I'd like to maximize the impact of my money. How do you assess the relative value of different agencies?
And, what should I do over the next 12 months to be better prepared next year?
A couple times every year I look at their top charity and drop a few hundred dollars on them. IMO if you want to maximize the impact of your donation, you can't go wrong with this approach.
They list "Deworm the world" as:
> The Deworm the World Initiative, led by Evidence Action, is one of our top-rated charities and an organization that we feel offers donors an outstanding opportunity to accomplish good with their donations.
Deworming children who have worms is obviously really important, but there's some debate around the effectiveness of population level deworming. Partly this debate is caused by different statistical methods: if you do heath stats you see less benefit; if you do economist stats you see more benefit. (The argument started with an attempt to replicate the results by looking at the data. Using economist stats this works; using health stats it doesn't. The mass deworming people say that some statistical shenanigans make that an invalid approach. The epidemiologist's point to missing data from the original study.)
http://community.cochrane.org/features/deworming-debate
http://www.bbc.co.uk/news/magazine-33972806
This is for one of their best, most recommendable, charities. It doesn't inspire confidence in the GiveWell methods or recommendations.