What is Cambridge Analytica? The company linked to Trump and Brexit explained

Here are answers to some of the key questions about the British firm.

What is Cambridge Analytica?

Cambridge Analytica is a British company which uses personal information from social media users to help clients try to influence voters or consumers, crafting messages targeted specifically to people's hopes, fears or desires.

The firm was founded as an offshoot of SCL Group, a strategic communication and military operations firm, in 2013.

It is largely owned by Robert Mercer, an American billionaire with a history of funding conservative political campaigns, who named Mr Trump's former campaign architect Steve Bannon as vice-president before he stepped into politics.

Mr Bannon even chose the company name, according to the New York Times, because it largely employed researchers from Cambridge University to construct its data-crunching software.

What do they do?

The firm describes itself as delivering "data-driven behavioural change" for its clients in both political and commercial fields, using large amounts of personal data from social media and other sources.

On a practical basis, the company's services are perhaps best described by chief data officer Alex Taylor.

"If you're collecting data on people and you're profiling them, that gives you more insight that you can use to know how to segment the population, to give them messaging about issues that they care about, and language and imagery that they're likely to engage with," said Mr Taylor, in a secretly filmed meeting with Channel 4 News, broadcast on Monday.

"We used that in America and we used that in Africa. That's what we do as a company."

Managing director Mark Turnbull went further.

"The two fundamental human drivers when it comes to taking information on board, effectively, are hopes and fears," Mr Turnbull said in another secretly filmed meeting.

He added: "It's no good fighting an election campaign on the facts, because actually it's all about emotion."

Where have they worked?

Cambridge Analytica famously switched from working with Ted Cruz in the 2016 US election to aiding Mr Trump's team, but it had previously helped 44 congressional and Senate campaigns in the 2014 mid-term elections.

Both chief executive Alexander Nix and the leaders of the Leave.EU campaign boasted about working together on the Brexit campaign but have since retracted their claims, saying no contract was signed and no work was completed.

Employees talked of working on political and commercial campaigns around the world in the Channel 4 expose, from Mexico and Malaysia to Brazil, Kenya, Australia and China.

Are their methods effective?

The jury is still out on whether Cambridge Analytica's tactics have the effect they claim, mainly because it is so difficult to measure.

The company boasts about helping their clients by targeting small groups of individuals - middle-aged men who live in Kent and are concerned about immigration, for example - with "psychographic" political advertising which plays to their fears and influences their vote.

However, by definition, the targeted individuals would be the only people able to see the adverts on social media, and they may not even be aware of it.

Mr Turnbull said: "It has to happen without anyone thinking 'that's propaganda' because the moment you think 'that's propaganda' the next question is 'who's put that out?' So we have to be very subtle."

Politicians in the UK and US are growing increasingly wary of such "dark" advertising but marketing experts are often more sceptical, seeing the claims and boasts from executives as more sales pitch than reality.

Without the relevant data and research, it is hard to be sure whether Cambridge Analytica influences elections in the ways they claim.

So what is the controversy?

Top of the list are topics raised by Mr Nix in a meeting with Channel 4 News journalists, in which he discussed "deep digging" on opposition candidates, and disinformation and entrapment as possible tactics for fighting elections, on top of its targeted messaging service.

Facebook banned Cambridge Analytica from using its platform on Friday, days before a whistleblower claimed the company had harvested and stored data about more than 50 million Facebook users without their permission.

The majority of those users were in the US but the UK's Information Commissioner has now issued a warrant to search the company's London offices after it failed to respond to a previous request about the possible illegal use of data.

Mr Nix gave evidence to the Commons Digital, Culture, Media and Sport Committee in February but chairman Damian Collins has now accused him of giving "false statements" and called Facebook's answers to repeated questions "misleading" in light of the recent revelations.

The Electoral Commission has previously faced calls from politicians to investigate links between Cambridge Analytica and Leave.EU, after the retracted boasts of working together in the EU referendum.

What happens next?

Facebook announced an independent audit of its relationship with Cambridge Analytica after suspending the company's accounts last week but has since been asked to stand down by the Information Commissioner's Office.

Mr Collins has now written to Facebook founder Mark Zuckerberg requesting he give evidence to the fake news inquiry following the Cambridge Analytica revelations.

He has promised to write to Mr Nix to ask him to explain his previous comments to the committee.

In the US, a number of attorney generals have launched investigations into how Facebook data has been used. Mr Nix and Cambridge Analytica have denied all allegations of wrongdoing.

Mr Zuckerberg said at the start of the year that he wanted to fix the issues which have plagued Facebook in recent years.

The past week's developments have only added to his already lengthy list.