Applied Statistics

[huy.duong@ctsu.ox.ac.uk]

Dear friends,

>Hanoi, July 20 (AP)
>...
>An survey conducted by the National Assembly found that 86% of 5,000
>randomly picked voters across the country said they would vote for a
>Communist Party member, an Assembly representative said.

Although applied statistics uses priciples that are simple 
theoretically, there are more to the difficulties in applying 
than to meet the eye.

Take for example a survey of extra-marrital affairs. It is 
claimed that 10,000 married persons are randomly picked, and 
ask if they are having an affair and only 1% say yes.

First of all, we have to ask how random the selection of the
sample really is. For example, the surveyor might walk down 
the street and pick houses out at random and if there is a 
married person in the house then you interview them. This might
seem random enough, but it isn't. Younger people are less likely
to be at home and they might be the ones who are more likely to have 
affairs. Couples in which both the husband and wife work away from 
home are less likely to be at home and they might be the ones who
are more likely to have affairs. Also, you might pick a house and
say to the occupant that you're doing a survey on affairs and he 
says that he's busy, so you pick another house in your quest to 
interview 10,000 people. If a person is likely to refuse because
he is infact having an affair, then you sample will be biassed. 

Secondly, we have to be careful of interview bias. Eg, if you are
ask if you are having an affair and you spouse or children is present
then if you are you would be too worried about the consequences to
say `yes'. Or you are standing for National Assembly election and
you are worried that the answer `yes' might be used to disqualify or
blackmail you.

Thirdly, if some of the 10,000 are so diplomatic that you can't
work out whether their answer is yes or no, then you will have to
worry whether this introduces bias into your result.

Huy