Data exploration
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1. Explain what are the numerical and categorical variables.

2. Explain what is the difference between experimental studies
   and observations.
   What it is a confounding variable. When we can draw conclusion
   about correlations and when about causation?

3. What is the difference between census and the sampling?
   What are the sampling methods?
   What are the possible sources of the sampling bias.

4. How we define point estimate of sample statistics:
   measures of center and spread.
   What does it mean "robust statistics".
   Which definitions have these features.

5. What is the box plot.
   Could you draw example and explain meaning of its content.
   How we deduce that distributions has a tail, outlayers, etc.