In statistics a population proportion, generally denoted by
P
{\displaystyle P}
or the Greek letter
π
{\displaystyle \pi }
, is a parameter that describes a percentage value associated with a population. A census can be conducted to determine the actual value of a population parameter, but often a census is not practical due to its costs and time consumption. For example, the 2010 United States Census showed that 83.7% of the American population was identified as not being Hispanic or Latino; the value of .837 is a population proportion. In general, the population proportion and other population parameters are unknown.
A population proportion is usually estimated through an unbiased sample statistic obtained from an observational study or experiment, resulting in a sample proportion, generally denoted by
p
^
{\displaystyle {\hat {p}}}
and in some textbooks by
p
{\displaystyle p}
.
For example, the National Technological Literacy Conference conducted a national survey of 2,000 adults to determine the percentage of adults who are economically illiterate; the study showed that 1,440 out of the 2,000 adults sampled did not understand what a gross domestic product is. The value of 72% (or 1440/2000) is a sample proportion.
Placebo/control group has 800 people. They aren't given the vaccine. 60 of them develop the disease
Treatment group has 1000 people. They're given the vaccine. 15 develop the disease
It seems there's a formula viz ##\frac{p_1 - p_2}{\sqrt{p (1 - p)}\left(\frac{1}{n_1} + \frac {1}{n_2}\right)}##...