you can estimate, from a sample, the population expected values.

只有部分样本的话你可以来估计总体均值
The population mean is often written "X-bar."

总体均值一般写作X拔
If you have a sample with n observations,

如果你有n个观测值
it's the summation I = 1 to n of xi/n--that's the average.

对Xi从i=1到n求和再除以n
You know that formula, right?

能看懂这个公式吧
You count n leaves--you count the number of leaves.

你数了n片叶子...不对
You have n branches on the tree and

一共有n根树枝
you count the number of leaves and sum them up.

你数了叶子的数量然后把他们加起来
One would be--I'm having a little trouble putting this

这个1是指我好像把这个式子
into the Rituparna story,

和睿都巴若那的故事弄混了
but you see the idea. You know the average, I assume.

但你们应该懂我的意思应该明白均值的概念
That's the most elementary concept

均值是最基本的概念