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        riddles >> general problem-solving / chatting / whatever >> The not-so obvious to many
        
(Message started by: BenVitale on Jun 12^th, 2009, 4:34pm)

Title: The not-so obvious to many
Post by BenVitale on Jun 12^th, 2009, 4:34pm

Why do many misinterpret ....

... a 40% chance of precipitation
... a 20% chance of rain?

According to an article, many misunderstand a probabilistic weather forecast (http://www.suncoaststormwatch.com/3/post/2009/04/new-study-many-people-misunderstand-rainy-weather-forecast.html)

And in this article, only half the population understands (http://www.usnews.com/articles/science/environment/2009/04/20/rainy-weather-forecasts-misunderstood-by-many.html) what a precipitation forecast means well enough to make a fully informed answer

Title: Re: The not-so obvious to many
Post by rmsgrey on Jun 12^th, 2009, 6:45pm

One answer I've heard is that, while a better sense of range and direction will help a neolithic hunter spear dinner (and not get lost on his way back to the cave), so geometric intuition is generally pretty good, anything more than a crude "likely", "unlikely", "almost certain", "only in grandparents' tales" in the way of probabilistic intuition doesn't help much - random events are too... random for the difference between 40% and 50% to make a big difference in long-term planning - in both cases, your best bet is to take low-cost precautions and wait and see whether it actually happens...

Title: Re: The not-so obvious to many
Post by towr on Jun 13^th, 2009, 4:26am

I think one very important reason is that no one has ever been told what it means.
Does it mean that if I stay outside the whole day there is a 20% probability I will be hit with at least one drop of rain? Or does it mean that if I go out the door at some given time, there is a 20% chance I will find it rains? (Which would be the case if it rained 20% of the time).

Apparently it means

Quote:

"The probability of precipitation is the probability of any particular point area within a forecast area receiving measurable precipitation in a given time period."

All I can say is that nobody ever told me that before. And I'm not at all sure how useful this type of forecast is if that's what it means. Essentially, if it's known that for one brief second it will rain ever so slightly in the entire forecast area, then there is a 100% probability of rain; and if it pours and pours on only your house for 24 hours, there is a 0% chance of rain (because any particular point in the forecast area is very unlikely to be your house).
I would expect people to assume a forecast is somewhat useful and informative, and apparently, it isn't. Well, ok, we really all suspected that somewhere, didn't we.
Well, granted, when I look up the weather forecast it includes the expected amount of rain that will fall, which is a lot more useful to know.

Title: Re: The not-so obvious to many
Post by BenVitale on Jun 13^th, 2009, 10:47am

If a meteorologist on TV predicts a 20% chance of rain, some people will think of 20% of an area, others 20% of the time.

Given that probability is taught in high schools, it is quite remarkable that a simple statement "a x% chance of rain" confuses a lot of people.

I did some reasearch to find out more about it ... and becoz friends and relatives have asked me about the significance of these probabilities.

I jokingly say, "If the weather talks about a 20% chance of anything, he's right 100% of the time."

But seriously, I found that according to the U.S. Weather Service Forecast Office (http://www.srh.noaa.gov/ffc/html/pop.shtml)

And, according to the Canadian Weather Service (http://www.weatheroffice.gc.ca/mainmenu/faq_e.html#weather8)

Quote:

Q.What is the Probability of Precipitation, or POP?
A.The probability of precipitation (POP) is the chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any point of the forecast region during the forecast period. For example, a 30% probability of precipitation means that the chance of you getting rained over (or snowed over in winter) is 3 in 10. In other words, there is a 30% chance that rain or snow will fall on you, and, therefore, a 70% chance that it won't. It must also be noted that a low POP does not mean a sunny day: it only means a day where the chance of rain or snow is low.

This fact sheet (http://www.msc.ec.gc.ca/cd/brochures/probability_e.cfm) explains the Probability of Precipitation:

And, The probability of precipitation (POP) is not always indicated in the forecast.

Title: Re: The not-so obvious to many
Post by towr on Jun 13^th, 2009, 2:12pm

on 06/13/09 at 10:47:44, BenVitale wrote:

Given that probability is taught in high schools, it is quite remarkable that a simple statement "a x% chance of rain" confuses a lot of people.

No, it's not. Because it doesn't mean what it should mean if it were to be a useful term.
(And also people are very good at forgetting what they're taught in highschool)