P value is a fascinating thing. Everybody loves it, and hates it. When the p value is less than 0.05 (probably out of luck), everybody gets excited; but when it is not, everybody starts mumbling the F word. For many number crunchers, it’s the way of life.

Here is an interesting graph I made in Excel. The x-axis is the z value from a standard normal distribution, and the y-axis is the p value. Based on this figure, one can reach several conclusions:

(more…)

量子关于老魏文章中Error-bar 的疑问

=====

我在那篇文章里先是假定error bar的含义是指观测误差，即当你看到了平均体积为X，而其真实值是X周围某点。error bar表示X周围点的可能范围，即真实值的范围。对error bar的含义作此理解主要是因为大家都说过体积难以精确测量。在这个意义上讲，error bar当然不能涵盖负值。

我在文中并未排除用error bar表示离散度的可能性。但在这种情况下我的问题已经不是负体积问题了。
这离散度是试验本身的一个重要结果。但是在只有10只样品且不断处死的情况下，这离散度已不具备可重复的科学价值。

=====

在用图表描述数据分布和结果的时候，常见的有：
１）均数＋/-标准差: MEAN+/-SD
２）均数＋/-标准误: MEAN+/-SE，（ＳＥ＝ＳD／SQRT(N))
３）均数＋９５％可信区间: MEAN+/-1.96*SE

我不知道老魏本人在文章里面用的是那一种，但我认为不是前面有人提到的１。因为如果是均数加一个标准差的话，毕竟这种类型的图要表达的是现有样本的分布，是确实存在的值，所以不应有负值，图中的标棒也不会到０以下。

(more…)

In today’s NYT, an article described the labor shortage in Shenzhen, China. I think this article gave a false cry. I can’t imagine that China will lack cheap labors. I read Chinese news reports. All I see is there are hundreds of thousands of unemployed fresh high school and college graduates and there are still millions of layoffs in many cities.

Shenzhen is a different city. In early days, due to the special economic policy, it had recruited many young and cheap workers from rural areas to work in factories with very low payment. Overnight, Shenzhen became one of the most prosperous cities in China. One consequence is that the living cost there is skyrocketing. Most migrating workers now just cannot afford living there.

Since the middle of 1990s, economics is also booming everywhere. Opportunities are abundant. A physically capable labor worker can find equally tough and low paid job in any city. Educated young people are also more willing to stay in civilized cities such as Beijing and Shanghai (Including satellite cities) rather than in culture desert such as Shenzhen.

(more…)