“Some questions are more easily asked than answered” Edward Sapir(?)

Possibly the most frequently asked and least frequently answered question is why does the definition of the standard deviation involve division by n-1, when n might seem the obvious choice. This is a question which perplexes introductory statistics students and calculator manufacturers alike. The explanations given in calculator manuals tend to range from obscure to fanciful, and both options are given on the keypad, usually labelled s_n-1 and s_n to add to the confusion. (The symbol s is reserved for the standard deviation of a random variable or a population/distribution, an entity which lecturers valiantly try but usually fail to keep distinct from its sample counterpart.) Australian students first meet the standard deviation in secondary school, where the definition given does indeed involve division by n. This definition is preferred to avoid the question about the n-1 being raised it would seem. Secondary school teachers have a hard enough life as it is. And it must be remembered that the difference between the two definitions is largely academic for all but the smallest of sample sizes (say, less than 10 observations). So, don’t get too agitated by the revamped definition. The truth can be told but the telling usually quells the desire to know. If the fire still burns in your belly, read on.
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转自泰克Cisco网络技术论坛

传统的网络服务程序，如:ftp、pop和telnet在本质上都是不安全的，因为它们在 网络上用明文传送口令和数据，别有用心的人非常容易就可以截获这些口令和数据。而且，这些服务程序的安全验证方式也是有其弱点的，就是很容易受到"中间人 "(man-in-the-middle)这种方式的攻击。所谓"中间人"的攻击方式，就是"中间人"冒充真正的服务器接收你的传给服务器的数据，然后再 冒充你把数据传给真正的服务器。服务器和你之间的数据传送被"中间人"一转手做了手脚之后，就会出现很严重的问题。

SSH的英文全称是Secure SHell。通过使用SSH，你可以把所有传输的数据进行加密，这样"中间人"这种攻击方式就不可能实现了，而且也能够防止DNS和IP欺骗。还有一个额 外的好处就是传输的数据是经过压缩的，所以可以加快传输的速度。SSH有很多功能，它既可以代替telnet，又可以为ftp、pop、甚至ppp提供一 个安全的"通道"。

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美国空军的雷鸟(Thunderbird) F-16战机正编队飞过肯尼迪航天中心的车辆装配大楼。在今年末，雷鸟将会出现在肯尼迪航天中心举办的世界航天博览会开幕周的飞行表演中。此次博览盛会将展出那些曾在同一地点聚集过的航空文物、装备和人物，同时它将是世界上最大的航空展出之一。 图像来自：U.S. Air Force/TSgt. Justin D. Pyle

Zend Debugger 5.2.5 and Zend Studio 5.5 are both newest products from Zend, the PHP company. Zend Debugger is a full-featured server side php debugger engine, an interactive tool that helps you debug PHP scripts. Zend Studio is a fantastic IDE for PHP programming(I am really happy with its code assistant). If they work together, we can use them to debug any PHP codes on a web server,  and have a good studying and development enviroment of PHP.

The installation of Zend Studio 5.5 is very easy. Just follow its setup sequence. However, the installation and configuration for Zend Debugger 5.2.5 is demanding. Fortunately, we have a really good tutorial here. Following its steps start from "Find the extension directory" and you will have this image from call phpinfo() method which indicate the successful of installation.

If you can’t get Zend Debugger to work although you have followed instruction of that tutorial. Please try following solution:

1. open your php.ini (you can find it by calling phpinfo() method and looking up the "Configuration File (php.ini) Path" entry (more…)

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