If you wish to find out something about a particular population , and it is not feasible to test the whole population, then you may test a sample of that population providing that the sample is an accurate refection of the population. In order to get a valid sample one needs to select randomly across the the entire population. Something to be avoided is self-selection.; this is in fact the major flaw in virtually all drug trials, in that they ask for volunteers, and this leads to bias. You might be thinking What's wrong with using volunteers?
Well, consider the following: Suppose you wished to ascertain the feeling of the general public towards the recent Olympic games. You might perhaps create a web-site asking people to complete a survey about the games. The flaw in the sample is here quite easy to spot. Only those who were interested in the games would volunteer, and therefore your sample would not reflect the the general population, some of whom must not be interested in the games.
This sampling problem is of course due to having to obtain consent from the subject when dealing with human-beings, you can't force people to take part.. Although the US government have done precisely that in the past. Chemical and biological weapons were used on civilian populations without their consent in the following locations:
Watertown, NY and US Virgin Islands (1950)
SF Bay Area (1950, 1957-67)
St. Louis (1953)
Washington, DC Area (1953, 1967)
Savannah GA/Avon Park, FL (1956-58)
New York City (1956, 1966)
One type of study that does not suffer from this flaw is a mortality study. So for instance you might wish to find out if LDL cholesterol increases mortality .from heart disease Here you can simple select a period , say a year, and then record the age of death and the LDL cholesterol level of the deceased,
In fact a major study along these lines was conducted along these lines:
In an eight-year study of about 26,000 men and women in Isehara, Kanagawa Prefecture, the death rate of men whose LDL cholesterol levels were between 100 mg/dl and 160 mg/dl was low, while the rate rose for those with LDL cholesterol levels of less than 100 mg/dl.
The LDL figures exhibited less influence on women, but the death rate still rose for women with LDL cholesterol levels less than 120 mg/dl.
A separate study of 16,850 patients nationwide who suffered cerebral stroke showed the death rate of people with hyperlipemia who died from a cerebral stroke was lower, and their symptoms more slight.
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