data collected over several time periods is what type of data?
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Since this is regarding statistics I assume you mean lower case sigma (σ) which, in statistics, is the symbol used for standard deviation, and σ2 is known as the variance.
Observations
The lower and upper limits of a class interval are known as Class Limits.
A scientist who studies the growth and density of populations and their vital statistics is known as a demographist!
They give estimates of unknown parameters which can then be used to make predictions based on distributions which are better known.