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Incidence vs. Prevalence.
Incidence measures NEW disease incidents compared to all people at risk over time.
Prevalence measures ALL diseased people compared to all people being studied over time.
Let's say for 12 months, you observe 100 people who do not change their diet or exercise.
If the "12-month prevalence" of obesity is 10%, that means 10 people out of the 100 people
started obese and one year later, they are still obese.
In this example, the incidence of obesity is 0%, because there are no new obese people.
On the other hand, if you started out with 100 people with a normal BMI,
and then you gave them unlimited cake and took away their treadmills,
What happens? People would get fatter, there would be NEW
cases of obesity developing, and now the INCIDENCE of obesity would increase.
Let's say for the first year, there is a 10% incidence of obesity. That means that 10 people
out of 100 became obese. For the second month, there is still a 10%
incidence of obesity. That means that 9 people became obese.
The denominator is NOT the 100 people who are being studied, the denominator is the
90 people who are AT risk for obesity. The 10 people who became obese in the first
month are no longer "at risk"
Who cares?
Low incidence and high prevalence indicate a chronic condition.
If the incidence of diabetes is 1% per year, and the prevalence is 40%
that suggests that, after getting diabetes, people keep living for 40 years.
Equal incidence and prevalence indicate an acute condition.
If the incidence of New Plague for the first year is 25%, and the prevalence is also 25%
that means there were ZERO cases of New Plague before the study began.
If, during the second year, New Plague incidence and Prevalence are still 25%
that means everyone who got “New Plague” the first year died.
Otherwise, the second year's 25% diseased people would add to the first year's 25% diseased
people, getting about a 50% prevalence.
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