Random thoughts of an economist

Generalization based on the sample size of one

Posted in Hong Kong, Statistics/Econometrics, teaching, Water by kafuwong on July 21, 2015

Recently, lead in water has occupied headlines of major newspapers in Hong Kong. Experts are consulted. Some experts appeared to mis-speak carelessly. The most laughable statement was made by a medical doctor, who is a consultant of the “Hong Kong Poison Control Network”. He remarked that lead poisoning can be caused by chewing on a pencil, among many other causes. It was so laughable that it is widely circulated on the internet.

Admit it, most of us do not know why this statement is laughable. OK. It is laughable because pencils (called “lead” pen in Chinese) do not contain any lead in the writing core nowadays. Although the writing core of some early pencils were made of lead, it has since replaced by the non-toxic grahpite. (http://pencils.com/pencil-history/) And, the statement came from a consultant of the “Hong Kong Poison Control Network”.

In fact, at least supposedly so, nowadays even the paint cover of pencils should not contain lead so that most pencils are safe for chewing (not encouraged).


Internet is a powerful tool. A friend appeared excited about the lead poisoning from pencil chewing story that he added a catchy title to his sharing of the news report about the doctor’s statement: “If you cannot trust doctors, who can you trust?”

It is this catchy and exaggerated statement that caught my attention. I have to admit, I like it.

Nevertheless, the statement is clearly too much a generalization based on the sample size of one. That “specific doctor” may not be trustworthy on this specific issue, but it does not mean that “specific doctor” is not trustworthy on other issues. Certainly, it does not mean that other doctors are not trustworthy.

Beware of similar generalization from a small sample of observations.


Reclaimed water

Posted in Economics, Environment, Hong Kong, Water by kafuwong on May 2, 2011

Stopped at Ngong Ping after six hours of hiking (the first three sections of Lantau Trail, http://en.wikipedia.org/wiki/Lantau_Trail).  Visited one of the cleanest and environmental friendly toilets in Hong Kong.  The toilet uses “reclaimed water”. 

According to the Hong Kong Government (http://www.gov.hk/en/residents/environment/water/usereclaimedwater.htm), “most sewage is treated to a certain standard before discharging into receiving water body – usually a river or the sea. In contrast, reclaimed water is more highly treated to make it clear in appearance, odourless and safe for reuse, and it forms part of the water supply.”  In fact, such reclaimed water is drinkable. 

I asked my friends whether they would be willing to drink reclaimed water.  They all said NO.  Yes, I agree that some of us may have trouble drinking reclaimed water as reclaimed water is linked to sewage.  However, like any other decision we make, the decision of “not to drink reclaimed water” is of course economic.  If there were no fresh water supply from mainland China and you were facing the choice of $1 per liter of reclaimed water versus $1000 per liter of mineral water, would you be willing to drink reclaimed water.  I would have taken the reclaimed water.  With such perspective, we can easily understand why in Singapore, we can buy bottled reclaimed water under the label of NEWater (http://www.adb.org/Water/Champions/2009/harry-seah.asp). 

Thanks God.  Cheap water supply is almost guaranteed by our motherland (mainland China).   However, if we are able to reclaim water from sewage at a reasonable price, we can use the water for purposes other than drinking.  Consequently, our reliance on the water from mainland will be reduced.  Our brothers and sisters in mainland will be able to enjoy more fresh water.