What Does a 30% Chance of Rain Mean? (Understanding Risk, with Gerd Gigerenzer) | Big Think

What Does a 30% Chance of Rain Mean? (Understanding Risk, with Gerd Gigerenzer)
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Gerd Gigerenzer on seeing and understanding the risks around you.
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Gerd Gigerenzer:

Gerd Gigerenzer is Director at the Max Planck Institute for Human Development and Director of the Harding Center for Risk Literacy in Berlin. He is former Professor of Psychology at the University of Chicago and John M. Olin Distinguished Visiting Professor, School of Law at the University of Virginia. He is also Batten Fellow at the Darden Business School, University of Virginia, and Fellow of the Berlin-Brandenburg Academy of Sciences and the German Academy of Sciences. Awards for his work include the AAAS Prize for the best article in the behavioral sciences and the Association of American Publishers Prize for the best book in the social and behavioral sciences. His award-winning popular books Calculated Risks: How To Know When Numbers Deceive You, and Gut Feelings: The Intelligence of the Unconscious were translated into 18 languages. His academic books include Rationality for Mortals, Simple Heuristics That Make Us Smart and Bounded Rationality: The Adaptive Toolbox (with Reinhard Selten, a Nobel Laureate in economics). In Better Doctors, Better Doctors, Better Decisions (with Sir Muir Gray), he shows how better informed doctors and patients can improve healthcare while reducing the costs. Gigerenzer has trained U.S. federal judges, German physicians, and top managers in decision making and understanding risks and uncertainties.
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TRANSCRIPT:

Gerd Gigerenzer: In the good old times people learned how to read and to write. That’s no longer sufficient in the high tech twenty-first century. We also need to know how to deal with risk and uncertainty. And that is what I mean with risk savvy.

Here is a simple example. You hear on the weather report that there is a 30 percent chance of rain tomorrow. Thirty percent chance of what? Now I live in Berlin and most Berliners believe that it means that it will rain 30 percent of the time, that is seven to eight hours. Others think it will rain in 30 percent of the region. Most New Yorkers believe that’s all nonsense. It means it will rain on 30 percent of the days for which this prediction has been made, that is, most likely not at all. Many psychologists think that people can’t learn how to deal with risk but in this case it’s the experts, the meteorologists, who have not learned how to communicate risk in an instinctive way, that is to say to what class 30 percent refers. Time or region or days? And if you have some imagination you can think about other classes. For instance, one woman in New York said I know what 30 percent means. Three meteorologists think it rains and seven not. Now getting soaked is a minor risk. But are we risk savvy when it comes to more important things?

For instance, 20 year-olds drive with their cell phones glued to their ears not realizing that they decrease their reaction time to that of a 70 year old. Or many Americans, about 20 percent, believe that they are in the top one percent income group. And as many believe they will soon be there. Or take health. So about an estimated one million children get every year unnecessary computer tomography CT scans. And that’s really because they’re not really clinically indicated. Which is not just a waste of time but also danger to the kids because a CT scan can have the radiation of a hundred chest x-rays and may lead in a small number of these kids later to cancer.

We deal everyday with risks but we haven’t learned how to understand them. And the problem is not simply in the human mind but also in experts who really don’t know what the risks are or don’t know how to communicate. Or in other areas like if it’s about finance or health have interests other than yours. So the key message is this. Everyone can learn to deal with risk. In that case everyone can learn to ask the question probability of what. And second, if you believe that you’re safe by your delegating the responsibility of your wellness and health to experts then you may be disappointed because many experts do not know how to communicate probabilities or try to protect themselves against you as in health care as a potential plaintiff. So you have to think yourself. And that’s the key message.

[CaptnDeadpool]

It means that out of 100 hypothetical scenarios of today's weather, in 30 of those scenarios it's gonna rain.

[cubedude]

i still want to know what 30% chance of rain means

[ParaditeRs]

30% chance of rain means that there is a 30% chance that it might rain at a given time. I have never met anyone who has seen it as anything other than that and I have lived in 9 states.

[ftpmarco]

What does 30% rain mean? Well, you have to figure that out yourself.
it means 100% -1 vote for bigthink.

[HexerPsy]

What does 30% rain mean? Well, you have to figure that out yourself.
Yeah...

[maillolhelloworld]

There are a varieties of solutions to deal with risk, but the utmost important thing is that you should know how to deal with your own problem yourself. Knowing the cause of risk and the assumption of experts can help to figure out what you concern and take action.
Thanks for sharing.

[IXOY]

I always thought, along with everyone I know, that 30% chance of rain meant that there is a 30% chance that it will rain at some point in the day. And there is a 70% it won't rain at all that day. I have never heard of it being anything else.

[Pectusx]

Nahh i live in germany too and ive never heard of somebody who misinterpreted the chance of rain like this...

[Iionios]

It seems there's alot of misunderstanding here, what Dr. Gigerenzer is trying to say is the need for reasoning when dealing with risk prevention. If the experts can't, or won't, communicate the meaning of a percentage directly, we need to ask the experts for more details. "Percent chance of what, exactly?"
With research you can learn that 30% chance of rain is a forecast saying "3 of 10 meteorologists say it will rain". But unless you actaully search for this information, as a consumer, you will never be told.

[MacDKB]

Obviously, it means there is a 3 in 10 chance that it will rain. Is it true that most people don't understand this? _Really_? Well, it explains a lot. In fact, I have noticed that anything under 50% almost always means it won't rain. And only when it reaches 60% can you be sure there will be rain...

[hdpasdlol]

Just to get this straight.
30% chance of rain, means that for example if we had 10 days with the same conditions as the day in question, it would rain in 3 out of the 10 days right? That is what i understand.

[itsbobfoster]

With respect to your example about a percentage probability of rain, I thought about the three main variations in meaning some time ago and called the national weather service in my country, which is Canada. When I posed the question and the three examples of possible meanings, the person answering my question was honest enough to say that he really didn't know what it meant.

I would suggest that there is another dimension here and that is deciding what constitutes rain. If I go outside and single drop falls on my head, but I see no other rain for the entire day, did the probability when the drop hit me change to 100%?

[jordonsoper]

I may be wrong but doesn't 30% chance of rain mean that if that exact day were to occur 100 times (with identical weather conditions based on those meteorological predictions) on thirty of those days it would rain regardless of how much,   how long or where it would rain on those days?

[clintgirard]

if u cant get past the fact that u still dont know what 30% means, then u missed the entire point.

[seonteeaika]

I think it's 30% chance that it rains *at least once* during given time frame, say "afternoon" which would mean several hours. It doesn't account at all on how much it could rain, or how many small rain clouds could randomly pass by.

Additionally you could say that to define "rain", certain amount of water during the time frame would need to drop. So in that sense it does affect a little on how much it could rain.

[adnanilyas]

30% chance of rain tomorrow? 
Where I live, that means we will have snow tomorrow. 

[TheUnchainedMind]

I find people's interpretation of a "30% chance of rain" to be puzzling.
To me it's just that, that there is a one in three chances of rain.

[AtomicRooster]

Another part of what 30% chance of rain means is that computer models indicate there's a 30% chance of rain occurring in that FORECAST AREA.  Those areas are LARGE.  So if there's a shower in a small part of the forecast area, but in 95% of that area there was no rain at all, then the meteorologists weight the event with 100% certainty that rain did occur. 

This is why forecasts seem to cry wolf.  The forecasts are not designed to be digested by an individual who occupies a point in a forecast area.  Forecast percentages only make sense to people with Doppler radar.

[misterpurplesky]

'30% chance of rain' means: if you care 30% about being dry and comfy, take an umbrella

[Dyslexic-Artist-Theory-on-Time]

This is like understanding the statistics risk of nuclear accident!!! 
Think for your self!!!