Why the second source matters more than the first

Most investigations begin with a single source: a document, a conversation, a record that does not look right. That first source is valuable. It points the work in a direction. But it rarely establishes anything on its own, however credible it appears.

The step that turns a lead into a finding is corroboration. A second source that confirms the first, and that does not depend on it, is what allows a researcher to write a sentence with confidence. In our work, it is also what allows a client to act on that sentence.

Independence is the point

Two sources are only better than one if they are genuinely independent. A news report that repeats a press release is not a second source for the press release. Three people who heard the same story from the same person are, for the purposes of verification, one source.

Checking independence means tracing each source back to where its information came from. It is slow work, and it often reveals that an apparently well-supported claim rests on a single origin. When that happens, we say so.

Two sources repeating one origin are one source. The work is in finding out which is which.

Access and motive

Before counting sources, we ask what each one could actually know. A document produced by the body it describes has direct access to the facts, and also an interest in how it presents them. A former employee may have had real access for a period and none since. A published account may rest on sources it does not name. Each of these can be valuable, and each has limits that shape how much weight it can carry.

Motive matters as much as access. People and organisations record and recount events for reasons of their own, and those reasons shape what they include and what they leave out. Recognising a motive does not mean discounting a source. It means looking for corroboration from someone whose interests are different.

Documents and testimony

The strongest corroboration usually comes from different kinds of evidence. A witness account supported by a contemporaneous record is more reliable than two accounts from people who know each other. A company filing supported by a court record is more reliable than two filings by the same company.

• Primary records are created at the time, for a purpose unconnected with the question.
• Secondary reports describe or interpret what others have recorded or said.
• Testimony is what people tell us, and is assessed for access, consistency and motive.

When a second source cannot be found

Sometimes the corroborating source does not exist, or cannot be reached lawfully. The finding may still matter. In those cases we report it, but we grade it accordingly and explain what it rests on. A reader should never have to guess how much weight a sentence can bear. In practice, that often means two sentences where one might seem enough: one setting out what the source says, and another setting out what supports it.

This is why every finding in our reports carries a stated level of confidence. The levels are described on our Approach page.

Recording the chain

Corroboration is only useful if it can be shown. We log each source as it is found, record how it was obtained and note what it depends on. The result is a report whose conclusions can be traced back, line by line, to the evidence behind them. The record also protects the client. If a finding is later challenged, by a counterparty, a regulator or a court, the file shows where it came from and how it was tested.

The discipline is not new. Careful investigative journalists and intelligence analysts have long worked this way. It remains the most reliable way we know to find out what is true.