Z-score (Standard Score / Normal Score)

Z stands for Standard Normal Distribution.
It's fairly important in real life: Japan use Z-score on exam to estimate each student's study skills.

Z-score is the essential concept of Z-Statistics.

Percentiles

Before start you probably need to know: explanations of percentiles are quite confusing and different from each teacher teaches and different at each website you searched. Because there is NO standard definition of percentile.

Empirical rule (68-95-99.7 Rule)

This rule ONLY applies to Normal Distribution.

It’s also called the 68-95-99.7% rule, because for a normal distribution:

• ≈68% of the data falls within 1 standard deviation of the mean

Density Curves

Some times histograms aren't good enough to visualize large amount of dataset. And Density Curve plot will solve the problem, as it can take on any value in a continuum, they're not just thrown into some buckets.

Permutations & Combinations

Aside from probability, Permutations and Combinations are essential tools for statistics.

They’re to solve the problem: how many groups are there of if we choose some from some.

Population Parameters

Population

In statistics, the Population is the collection of all people, items, or objects that are required for a specific study.

Parameter

It’s also called the Population parameter.

Mean Absolute Deviation (MAD)

The Mean absolute deviation is the absolute average of all deviations.

The deviation is the distance from the value to the mean value. 
It's used to describe how the values looks like or how they're laid on the axis, are they close to each other or…

Intro to Probability

It’s easy but always confusing if you haven’t yet totally understood it in the first place.

The very first thing to do for solving a probability problem, is to CATEGORISE the problem and apply different formula.

• Single Event