Image copyright Getty Images

One of your friends on social media takes a drenching from a bucket of icy water and suggests you do the same. The aim is to raise funds for the ALS Association, a US-based charity. Do you fill the bucket right away - or stop to ask if this is the charity you most want to donate to?

The cause is a good one.

ALS (amyotrophic lateral sclerosis) - or Motor Neurone Disease as it's known in the UK - is a progressive neurodegenerative disease that affects 140,000 people globally. The average sufferer lives just 39 months after diagnosis and there is no known cure.

The sacrifice you need to make to help is a small one - you prove your bravery to all your friends by being doused in icy water, and then help financially by texting a donation to the charity.

Since the "ice bucket challenge" started going viral at the end of July, the ALS Association has received more than $100m (£61m) in donations. To put this in context, the charity's latest tax return shows that it received just under $24m (£15m) in the whole of last year.

Just Giving - the online platform for charity giving - says that in the UK, 850,000 people have donated £5.4m ($8.8m) through ice bucket challenges.

But there are thousands of other charities out there that do a lot of great work for their respective causes. How do you decide which is the most deserving?

Image copyright Getty Images Image caption Hundreds of thousands of people die from malaria each year

Niel Bowerman, co-founder of the Centre for Effective Altruism, says one way to measure charities is to ask how much they improve people's quality and length of life.

How does the ALS Association spend its money? The association spends some money on staff salaries and expenses. Of the rest: A third is spent on public and professional education

Just over a quarter is spent on research

A fifth is spent on helping patients

"Some charities do this incredibly cheaply, for example by distributing malaria nets it costs only between $10 and $100 (£6 and £60) to give someone an additional year of healthy life whereas the cost for an ALS sufferer can be something like $200,000 (£123,000) to sustain someone in the final years of the disease," he says.

The World Health Organization (WHO) estimates there are between 135 million and 287 million people in the world suffering from malaria and it accounts for between 473,000 and 789,000 deaths annually - mostly among African children.

Many people may feel uncomfortable making such a cold and calculating assessment about where to give their money. People donate for different reasons - they may want to help friends who are fundraising, or they may have personal reasons for favouring one charity over another.

But Niel Bowerman argues that identifying which charity is going to do the most with your funds shouldn't be a case of looking at videos of smiling people having fun.

"For better or for worse this has developed into a science of its own and now there are thousands of people around the world doing analysis on which charities are going to have the most impact with your money," he says.

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"Many people think that if you follow their recommendations you get more bang for your buck."

One organisation that evaluates charities and produces recommendations for donors is GiveWell. Their approach is to take independent, academic evidence and then ask pertinent questions, such as - how large an effect does the charity have and how costly is it to obtain that effect. They also evaluate what's going wrong, what's going well and how they are likely to perform in the future. And they look for organisations that have a pressing need for additional funding.

Where would Elie Hassenfeld, co-founder of GiveWell, recommend someone give their donation?

Image copyright SPL Image caption Freshwater snails carry the parasite that causes schistosomiasis

"Two of the organisations that we recommend right now both work on de-worming programmes, which is treating parasitic infections in the developing world.

"The Schistosomiasis Control Initiative based in the UK and the Deworm the World Initiative based in the US… broadly speaking these are programmes that for a very small amount of money (about 50 cents to $1 per person) can treat young people, rid them of this disease and have the ability to affect their cognitive development long-term in a way that allows them to lead more productive lives as adults."

What is schistosomiasis? Also known as bilharzia, schistosomiasis causes stomach pains and diarrhoea - and may eventually damage the liver or kidneys, or cause bladder cancer

The parasites that cause the disease are released by infected freshwater snails into rivers, ponds or lakes

They enter the human body through the skin, migrate to the lungs and then to the liver - some varieties migrate further to the kidneys and bladder

Adult worms are 10mm long

The WHO estimates that between 1.4 and two billion people suffer with parasitic worms, of whom about 210 million suffer from Schistosomiasis. It kills tens of thousands people every year.

Another concern that has been raised about the huge donation of money to ALS is that other charities will see their donations drop as a result. This has been labelled "charity cannibalisation" and its existence is the cause of much debate.

Niel Bowerman thinks it's a real problem. He says that people have a fixed budget in mind for charitable giving and an unplanned donation to ALS will result in them giving less to others.

That claim is disputed by Prof Stephen Lee, of the Centre for Charity Effectiveness at the Cass Business School in London.

"There is no evidence to suggest that charity cannibalisation exists," he says. "Things like the ALS ice bucket challenge result in spontaneous giving. Regular giving to charities of someone's choice is much more measured and a one-off spontaneous donation is highly unlikely to affect that."

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