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Q: Correlation does not equate to causation. ( Answered ,   4 Comments )
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 Subject: Correlation does not equate to causation. Category: Science > Math Asked by: sl7-ga List Price: \$200.00 Posted: 30 Jun 2004 15:20 PDT Expires: 30 Jul 2004 15:20 PDT Question ID: 368317
 ```I am looking for at least five really good examples of statistically valid but nonsensical correlations. For example I believe I have heard it said that if the American League wins the World Series, then "X" is likely to happen whereas if the National League wins the World Series then "Y" is likely to happen. Although historically the "X/Y" phenomenon has been observed with a very high degree of accuracy clearly neither "X" nor "Y" has anything to do with baseball. I believe that there is another correlation that says if women's skirt length in the new fall fashions is lower then "X" is likely to happen, and if higher then "Y" is likely to happen. I am looking for examples of correlations that actually exist and that can be documented with hard numbers as existing, but where it is clear that the correlation, though undeniable, is not the causation for the relationship that statistically appears to exist. WAYNE C. CURTIS, STATISTICAL CONCEPTS FOR ATTORNEYS 157 (Quorum Books 1983), states, "a high degree of positive correlation . . . does not necessarily mean that there is a cause-and-effect relationship between two variables. A high correlation can occur by chance alone or because both variables are related to some other variable which has not been considered in the analysis.? I would like to find at least 5 real life good examples of the fact that a high degree of positive correlation does not equate to cause and effect. I am offering \$200 for 5 documented examples of nonsensical correlations (with specific numbers quantifying the degree of correlation), but if you have fewer than 5, I would offer \$40 per example. It would be helpful if some of the examples were of the variety caused by pure chance alone (such as the American League/National league example) and other examples were of the variety caused by the fact that the two correlated events were actually both caused or influenced by a third factor not considered in the correlation. Thank you for your help.```
 Subject: Re: Correlation does not equate to causation. Answered By: tox-ga on 01 Jul 2004 06:52 PDT Rated:
 ```Hi sl7-ga! Your question, I must admit, was a rather fun one to investigate. The fact that correlation never implies causation, which may be lost on many people, is extremely important. Causation is just one of three possible relationships between two correlated variables: a) Causation - a change in X causes a change in Y b) Common Response - both X and Y change in common to some third, unseen variable c) Confounding - the effect of X and Y is mixed up with the effects of other explanatory variables on Y. To establish causation, a carefully controlled designed experiment must be run. I have found many silly examples of correlation without causation. For example, ice cream sales and shark attack frequency are strongly correlated. This is not because sharks start attacking in response to ice cream, but because the two variables exhibit a common response to the warm season. Another example is strong correlation exists between the number of cavities in elementary school children and their vocabulary size. No one advocates eating more candy to increase knowledge though; these variables are both tied with age. One class of relations you may be familiar with are economic indicators. Examples are skirt hemlines which rise with stock market prices, or the 'Big Mac' index which uses the price of Big Macs around the world to gauge the currency exchange market. The following link describes the Super Bowl index, which operates on the principle that when a team from the National Football Conference or from the pre-1970 NFL wins the Super Bowl, stocks rise: http://www.fool.com/Fribble/1996/Fribble960112.htm This has held true for 26 of 29 Super Bowls! Also, this link describes the correlation between the Napa Valley Wine Auction and the Dow Jones: ://www.google.ca/search?q=cache:ngKUTO0F194J:biz.tizwine.com/stories/storyReader%244298&hl=en&lr=&ie=UTF-8&c2coff=1&safe=off&strip=1 There is not a direct causal relationship between the two, but a rather indirect, confounded one. Your question requested correlations which were backed by numbers and generally these are only available with scientific studies; either medical or psychological. I have provided five such examples which I feel seem like nonsensical links at first glance. If you are dissatisfied with any aspect of these for any reason, you only need to let me know and I will be glad to work with you until you are completely satisfied. Cheers, tox-ga ----------------------------------------------------------------------------- ----- Shaving habits and risk of stroke ----- It would be silly to think one could change their chances of having a stroke by shaving more. Yet, a study by British researchers has found a correlation between shaving habits and stroke risk. Shaving less than once a day means increasing your risk for a stroke by 70%! In fact, your risk of dying from _any_ cause rises 30%. The lead researcher feels that the cause behind this correlation has to do with hormones. Testostorone has already been used to link baldness to higher risk for heart disease. Source: Shaving habits linked to stroke risk http://news.bbc.co.uk/1/hi/health/2729457.stm ----- Developing breast cancer and working the nightshift ----- Of course having breast cancer does not cause one to take a job with a nightshift; neither could working at night cause breast cancer. Yet, there appears to be a statistical connection. In fact, women who, for at least six months, worked predominantly at night are 50% more likely to develop breast cancer. This is the result of a Danish study of 7000 women, which were adjusted to consider other possible factors, including alcohol consumption and age at birth of first and last kids. This is not the first study to link the two, but it is the most statistically sound. It is not known how the two are linked. One theory is that the exposure to light during nighttime hours triggers hormonal changes which lead to breast cancer. Source: Nightshift link to breast cancer http://news.bbc.co.uk/1/hi/health/1108590.stm ----- Prayer and successful impregnation ----- Can you increase your chances of conceiving a child through prayer? If correlation implied causation, then this study would suggest that is true. A team of scientists from Columbia University found that a group of women undergoing in-vetro fertilization had double the chance of concieving when they were being prayed for. The scientists were confident that their study was methodically sound because the women did not know they were being prayed for. In fact, they were so confident, they published their findings in the Journal of Reproductive Medicine. I have no idea why this worked and neither do the scientists. I believe this may fall under the category of correlations caused by chance alone. Source: Study finds prayer 'double chance of IVF success' http://www.ananova.com/news/story/sm_414237.html?menu=news.latestheadlines.wor ----- Breast implants and suicide rates ----- It does not make any sense to think that anything about breast implants--simple bags of silicone--could impact mental health, especially to the point of causing suicide. Neither is there any reason to believe that being suicidal leads a woman to get breast impants. Nevertheless, a Swedish study involving over 7000 women found triple the suicide rate among those with breast implants as compared to an otherwise similar group of women. The likely explanation for this strange correlation is that a desire for breasts strong enough to get surgery could be, in some women, a symptom of insecurity and self esteem issues. In a few cases, these greater issues could lead to a suicide attempt. Source: Breast implant suicide link http://news.bbc.co.uk/1/hi/health/2826933.stm ----- Tooth loss and heart disease ----- It is intuitively nonsense that losing teeth can lead to heart disease, or that heart disease causes tooth loss. However, a strong correlation was found between the two by American researchers. Studying their subjects, they found that "in those who had lost up to nine teeth, 45% had carotid artery plaque. Just under two-thirds of those with 10 or more teeth missing had plaque build-up in the vessels." The source of the relationship is not entirely clear. Some studies have linked gum disease to heart disease, while others link infectious diseases to increasing the risk of blocked arteries. Some doctors simply see tooth loss as an indicator of lifestyle. Source: Tooth loss link to heart disease http://news.bbc.co.uk/1/hi/health/3114013.stm```
 sl7-ga rated this answer: and gave an additional tip of: \$100.00 ```Thank you for a really terrific and prompt response to my question. You were 100% on point with reference citations to back up your examples. I'm impressed.```

 ```Thank you very much for your generous rating, feedback, and tip! It is certainly appreciated. I'm very glad that you were satisfied. I look forward to having an opportunity to work with you in the future. Cheers, tox-ga```
 ```Great work, Tox! Your answer was both educational and entertaining. ~Pink```
 ```One example of this that I remember from college is that people who own washing machines are more likely to die in a car accident. I don't know what the actual numbers were. No, clean clothes don't make you a bad driver, people who own washing machines are more likely to own a car and therefore more likely to be driving.```
 ```This might be a little out of the scope, but a friend of mine determined that (x) the shorter her skirt and/or the lower her top was, (y) the more free drinks she received.```