How likely is ‘likely’?

Summary

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More or Less – "How likely is ‘likely’?"

Podcast: More or Less, BBC Radio 4
Host: Charlotte McDonald
Date: April 4, 2026
Guest: Adam Kucharski, Professor of Epidemiology and author of The Uncertain Science of Certainty

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Episode Overview

This episode delves into the ambiguity of probability language—words like "likely" or "possible" that often crop up in news, intelligence reports, and everyday conversation. Host Charlotte McDonald and epidemiologist Adam Kucharski explore how these words are interpreted by experts and the general public, and why miscommunication around them can have real-world consequences.

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Key Discussion Points & Insights

1. The Problem with Probabilistic Language

• Charlotte explains that terms like "likely," "probable," or "possible" are used to convey probability but are inherently ambiguous. People's interpretations of these words often vary widely.
  "Do you know how likely likely is and what probability probable is meant to convey? In some cases, it seems you probably don't." (01:58)

• Adam Kucharski defines probabilities in numeric terms:
  "If there's 10 events, all of which that have an 80% chance of happening […] we'd expect eight of those 10 to happen. If it's a 10% chance, we'd only expect on average one of those 10 events to actually occur." (03:10)

2. The History of Ambiguity: Sherman Kent and the CIA

• Adam recounts a famous incident from 1951, when CIA analyst Sherman Kent wrote of a "serious possibility" of Soviet action and realized colleagues had wildly different ideas of what that meant numerically.
  "He […] realised that even in the team who are co writing these things, they had very different numbers in mind for the kind of language they were using." (04:08)

• This ambiguity persisted for decades until recent efforts to standardize the language, especially in intelligence settings.

3. Modern Efforts to Standardize Probability Terms

• Intelligence agencies have adopted "probability yardsticks":

  • "Highly likely" means an 80-90% chance.
  • "Realistic possibility" indicates a 40-50% chance. (04:54)

• Despite these efforts, Charlotte points out that these phrases now frequently show up in public domains but lack consistency in interpretation.

4. Kucharski’s Large-Scale Probability Language Quiz

• Adam developed a quiz to test the public's understanding of these words, with over 5,000 respondents.

• The quiz had two parts:

  1. Comparing pairs of phrases to judge which implies higher probability.
  2. Assigning a numerical probability to a single phrase, e.g. "may happen." (05:52)

• Findings:

  • "The most consistent was the phrase 'about even', which you'd hope people would land on about 50%." (06:33)
  • High clarity for:
    • "Will happen" ≈ 100%
    • "Almost certain" ≈ 95%
    • "Almost no chance" = 1-5%
    • "Remote chance" and "highly unlikely" = 1-10%
  • Ambiguous phrases:
    • "Realistic possibility" had the widest spread, interpreted as anywhere from 10% to nearly 100%, average just over 50%. (07:42)
    • "Probable" and "likely": broad range (50%-100%), means just under 75%. (08:00)

• Charlotte demonstrates with a mini-quiz:

  • "Which is more likely: 'might happen' or 'could happen'?"—most people did not answer consistently. (08:54)

5. Sociodemographic Patterns

• Adam notes minor trends from the data:
  • More educated respondents were slightly more pessimistic.
  • US respondents were marginally more optimistic than UK respondents.
  • However, these were small effects; ambiguity was the dominant theme. (09:17)

6. Addressing Ambiguity: What Should Be Done?

• Adam’s recommendations:
  • "Just ensuring that what the person coming up with an estimate is saying is what the person is hearing at the other end." (10:00)
  • Avoid hiding behind vague words to dodge accountability:
    • "All of us, I think, can have some benefit in actually challenging ourselves to make a judgment that we can really go back and ask if we genuinely got that right, rather than having these kind of ambiguous words where we can always wriggle out of it later if we want." (10:15)

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Notable Quotes & Memorable Moments

• On the crux of the problem:

  “Do you know how likely likely is […] In some cases, it seems you probably don't.”
  — Charlotte MacDonald, 01:58

• On historical confusion in intelligence:

  “Even in the team who are co writing these things, they had very different numbers in mind for the kind of language they were using.”
  — Adam Kucharski, 04:08

• On the ambiguity of ‘realistic possibility’:

  “It really does seem like a word that means a lot of different things to a lot of different people.”
  — Adam Kucharski, 07:56

• On accountability in communication:

  “All of us, I think, can have some benefit in actually challenging ourselves to make a judgment that we can really go back and ask if we genuinely got that right, rather than having these kind of ambiguous words where we can always wriggle out of it.”
  — Adam Kucharski, 10:15

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Important Timestamps

• [01:48] — Episode begins; Charlotte introduces topic of ambiguous probability language in news and reports.
• [03:10] — Adam explains the basics of probabilistic thinking using numeric examples.
• [04:03] — Story of Sherman Kent and the “serious possibility” CIA report.
• [04:54] — Intelligence agencies introduce “probability yardsticks.”
• [05:52] — Adam describes his large-scale public quiz.
• [06:33] — Results: clarity and ambiguity in phrase interpretations.
• [07:31] — “Realistic possibility” as the most variable phrase.
• [08:54] — Charlotte’s on-air quiz: “might happen” vs. “could happen.”
• [09:17] — Demographic patterns in responses.
• [10:00] — Adam’s advice for communicators of risk.
• [10:15] — The importance of accountability in using probabilistic language.

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Summary

This episode underscores how words intended to convey risk or likelihood are open to wide interpretation and miscommunication, sometimes with critical implications. Adam Kucharski’s research reveals that while some probability phrases are well-understood, many—including commonly used ones like "likely" or "realistic possibility"—are not. The hosts urge greater precision and accountability in communicating risk, especially by experts, to ensure vital information is clear and actionable.

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Transcript

[A]

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[B]

Hello and thanks for downloading the more or less podcast. We're the programme that looks at the numbers in the news, in life and in highly ambiguous language. I'm Charlotte MacDonald. When you're reading the news, you'll often stumble across words that are meant to communicate the probability of something happening. A terrorist attack is a realistic possibility. The spread of a certain strain of viruses Highly likely. The relegation of your favourite football team is possible, even if you did get a last minute equaliser against Liverpool. But when you hear these terms, do you really know what kind of probabilities they're trying to get across? Do you know how likely likely is and what probability probable is meant to convey? In some cases, it seems you probably don't.

[C]

I'm Adam Kacharski, I'm a professor of epidemiology and author of the Uncertain Science of Certainty.

[B]

Adam has a very healthy obsession with the problem of communicating probabilities and it would be remiss of us if we didn't start out by making sure you understand what the numbers you're going to hear actually mean. So what does it mean if we say there's an 80% chance of something happening?

[C]

Intuitively, we can think of the chance of something happening as the proportion of those events that would come to fruition in reality. So, for example, if there's 10 events, all of which that have an 80% chance of happening in the next year, say on average, we'd expect eight of those 10 to happen. If it's a 10% chance, we'd only expect on average one of those 10 events to actually occur.

[B]

Okay, so we can express probabilities with numbers and we can also express them with words like probable, unlikely and so on. Ideally the words and numbers match up somehow, but that's not always the case.

[C]

A real standout example for this kind of problem came in 1951 when a CIA analyst called Sherman Kent wrote a report that said that there was a serious possibility that what was then USSR would try and invade Yugoslavia that year.

[B]

A serious possibility of war seems like quite an important thing to be clear about.

[C]

He then bumped into someone who was a chairman of an important committee in a corridor and the chairman said, what did you mean by serious possibility? And Kent said he thought about 65%. And the chairman said, I thought you meant something much lower than that. And Kent kind of had this panic and went back and talked to a lot of his team who'd been writing these dozens of reports and realised that even in the team who are co writing these things, they had very different numbers in mind for the kind of language they were using.

[B]

The invasion never happened. But this ambiguity between language and numbers rumbled on over the decades. However, Adam says that in the last 25 years there's been a real effort to pin them down, particularly from the intelligence agencies which are trying to communicate serious risks to politicians.

[C]

So there was a real effort to introduce in UK what was known as a probability yardstick, where it's saying if you're using these kind of words, this is the probability that you mean. So if you're using words like highly likely, you're saying there that there's an 80 to 90% chance that that's going to occur. If you're using the phrase we realistic possibility, you mean a 40 to 50% chance.

[B]

James Bond and his analysts in the back office should be communicating clearly now, but what about the rest of us? This probability language isn't just found in government reports. It also percolates out into the public in news stories about wars and pandemics and climate change. Does the public have a clear idea what these words actually mean? Adam says some small studies have looked at this, but he wanted much more data, so he built a quiz.

[C]

There were two main parts to the quiz. The first was comparing phrases. So you had a series of pairs, and it might say for each pair, which has the higher probability of something occurring. The second part of the quiz got people to put specific numbers to phrases. So if you're given a phrase like may happen, pick a number between 1 and 100% that you think matches the probability of that phrase occurring.

[B]

More than 5,000 people took part in Adam's probability quiz. You can have a go yourself if you click on the link on our website. And this gave Adam a pretty good idea of which words were highly ambiguous and which everyone agreed on.

[C]

One of the things that was possible was to see where the highest amount of ambiguity was. So, in terms of just the variation in what people say, the most consistent was the phrase about even, which you'd hope that perhaps people would land on about 50%.

[B]

There was also more clarity in some high and low probability phrases. For instance, most people thought the phrase will happen meant there was a 100% chance and almost certain something around 95%. At the other end, most people thought the phrase almost no chance meant between a 1 and 5% chance. The phrases remote chance and highly unlikely both came in between 1 and 10%. Those are the phrases where you can be pretty sure people know what you're saying. However, some were far less clear.

[C]

The most variable phrase was realistic possibility. So one that actually happens to be in intelligence guidelines and often used internationally in headlines to report risks of events.

[B]

Respondents to Adam's quiz thought the phrase realistic possibility could mean anything from a 10% chance of it happening to it being a near certainty, with the average answer just over 50%.

[C]

So it really does seem like a word that means a lot of different things to a lot of different people.

[B]

The terms probable and likely both had ranges from around 50 to 100%, with the averages just under 75%. Adam also asked people to compare probability words side by side to see if people were using probability in a consistent way. So here goes. Let's try a little quiz of our own. Which is more likely to happen, Something that is very unlikely or something that is highly probable? Ok, that one was easy. Obviously, highly probable is the answer. How about this, though? Which is more likely, something that might happen or something that could happen? Turns out people's judgments on these close comparisons are not consistent at all.

[C]

Some phases particularly that might happen, it could happen, which arguably are just very hard to pin down. But even if you give people that and then give them the same judgment a few questions later, they'll flip between them somewhat arbitrarily.

[B]

While the study was self selecting, he did end up getting a reasonable spread of responses from people in different groups and he could see a few patterns start to emerge.

[C]

There was a bit of a trend, for example, with values in education, so typically people who had more degrees tended to be a bit more pessimistic. We also found that people responding from the US were slightly more optimistic in their answers, all things being equal compared to the uk.

[B]

These, however, were only small effects in the scheme of things, moving the numbers by a few percent. Really the big picture here is that some probability language is just very ambiguous. That's a problem if you're trying to warn people about serious risks. But what should we do about that? For Adam, one thing is just making sure that the people communicating risk, whether they're scientists, politicians or journalists, know the risk.

[C]

Just ensuring that what the person coming up with an estimate is saying is what the person is hearing at the other end.

[B]

The second is to push the people calculating risks not to hide behind these words to avoid accountability.

[C]

All of us, I think, can have some benefit in actually challenging ourselves to make a judgment that we can really go back and ask if we genuinely got that right, rather than having these kind of ambiguous words where we can always wriggle out of it later if we want.

[B]

Thanks to Adam Kocharski. If you've seen a number in the news you think we should take a look at email more or lessbc.co.uk. we will be back next week. Until then, goodbye.

[D]

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[E]

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