50 is your sample size have to be, at minimum in standard deviation.
What is standard deviation?
The variability in your dataset is measured by its standard deviation.
It reveals the average deviation between each result and the mean.
A low standard deviation denotes that values are grouped closely to the mean, while a large standard deviation denotes that values are often spread widely from the mean.
We want n * p and n * ( 1 - p ) both atleast 10.
0.2 * n ≥ 10
n ≥ 10/0.2
n ≥ 50
Learn more about standard deviation here:
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