**Continuous Random Variable**

首先回顧以前提過的相對次數累積分布圖 **Relative frequency histogram**
它有幾個需要注意的重點, 以及當推廣到連續型的隨機變數時情形.

- 1.
- The total area under the histogram is 1.
- 2.
- For two points
*a*and*b*such that each is a boundary point of some class, the relative frequency of measurements in the interval*a*to*b*is the*area*under the histogram above this interval.

Because probability is interpreted as long-run relative frequency, the curve
obtained as the limiting form of the relative frequency histograms represents
the manner in which the total probability 1 is distributed over the interval of
possible values of he random variable *X*. This curve is called the
**probability density curve** of the continuous random variable *X*. The
mathematical function *f*(*x*) whose graph produces this curve is called the
**probability density function (pdf)** of the continuous random variable *X*.