# 4-1. Random Variables {% file src="" %} Chapter 4-1 (Korean) {% endfile %} {% file src="" %} Chapter 4-2 (Korean) {% endfile %} {% file src="" %} Chapter 4-3 (Korean) {% endfile %} {% file src="" %} Chapter 4-1 (Chinese) {% endfile %} {% file src="" %} Chapter 4-2 (Chinese) {% endfile %} {% file src="" %} Chapter 4-3 (Chinese) {% endfile %} {% file src="" %} Examples of Chapter 4-1 {% endfile %} {% file src="" %} Examples of Chapter 4-2 {% endfile %} {% file src="" %} Examples of Chapter 4-3 {% endfile %} {% file src="" %} Examples of Discrete Distribution Functions of Rstat {% endfile %} > *A* **random variable** *is a numerical quantity that is generated by a random experiment.* We will denote random variables by capital letters, such as $$X$$ or $$Z$$ , and the actual values that they can take by lowercase letters, such as $$x$$ and $$z$$ . | Experiment | Number $$X$$ | Possible Values of $$X$$ | | ------------------------------------------- | ------------------------------------------- | ---------------------------------- | | Roll two fair dice | Sum of the number of dots on the top faces | 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 | | Flip a fair coin repeatedly | Number of tosses until the coin lands heads | 1, 2, 3,4, … | | Measure the voltage at an electrical outlet | Voltage measured | $$118 ≤ x ≤ 122$$ | | Operate a light bulb until it burns out | Time until the bulb burns out | $$0 ≤ x < ∞$$ | > *A random variable is called* **discrete** *if it has either a finite or a countable number of possible values.* > > *A random variable is called* **continuous** *if its possible values contain a whole interval of numbers.* {% file src="" %} Discrete Random Variables {% endfile %} \[看到这个网站] [**概率分布**](https://zh.wikipedia.org/zh-cn/%E6%A6%82%E7%8E%87%E5%88%86%E5%B8%83)