Take for example, a study that found people who owned two cars
live longer than people who owned only one car.
Those who owned three cars were even more likely to live a longer life, and so on.
Does this mean buying more cars will allow you to avoid death?
No, of course not.
In this case, the lurking variable is the car buyers affluence.
Richer individuals own more cars and tend to live longer.
Probably because they have access to better medical care and healthier food.
The cars have nothing to do with it.
Of course, sometimes causation seems to be the only reasonable explanation of
the relationship between an explanatory variable and a response variable.
We can see this in the case of smoking and lung cancer.
>> Why do you buy Winston?
>> Believe it or not, there was a time when cartoons sold cigarettes on TV.
>> A Winston tastes good, like a cigarette should.
[SOUND] >> People smoked in hospitals and
smokers didn't give a second thought to the health risks they might be taking.
A far cry from today, when anyone can tell you smoking causes lung cancer.
[SOUND] It's even printed on cigarette packs.
But how can we say for sure that smoking causes lung cancer?
Yes, there's a strong association between the two.
But how do we know there are no lurking variables affecting our conclusions?
The best way to make a case for causation is to conduct an experiment.
An experiment imposes a treatment in order to observe it's effects.
In the case of smoking we could a local hospital and
randomly assign newborn babies to one of two groups.
Those we forced to smoke and those we prevent from smoking.
We could then keep our subjects in isolation and
as they get older compare the cancer rates between the two groups.
The only difference between the groups would be their smoking habits.
Of course, this particular experiment is highly unethical and
logistically impossible.
So if we can't conduct an experiment,
how do we prove a causal relationship between smoking and lung cancer?
The answer to that question was long incoming.
But offers a fascinating look at biostatistical research.
[MUSIC]
Cigarette smoking became increasingly popular in America after World War I.
When cigarettes were handed out to soldiers to boost morale.
Per capita consumption rose from 49 cigarettes in 1900 to
611 cigarettes in 1920.
Along with the rise in smoking came a disturbing rise in lung cancer rates.
[MUSIC]
A handful of doctors began to raise early warnings about the dangers of smoking.
One was Richard Overholt.
A young lung surgeon who noticed that his non-smoking patients
had quicker surgical recoveries and higher survival rates than the smokers.
He tried to get other doctors to encourage their patients to quit.
>> A good share of the doctors were smokers.
They didn't believe this and
it was very difficult to convince people that this was doing damage to them,
because they felt that they were getting pleasure from smoking.
[MUSIC]
>> During the 1930s and 40s,
the popularity of cigarette smoking soared as a new image of the sophisticated,
independent, and fun-loving smoker took hold.