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Joshua Schachter's (del.icio.us) views on auto-incrementing: http://joshua.schachter.org/2007/01/autoincrement.html

Another problem with auto_increment occurs when the user id comes from an auto_increment field. By signing up as a new user once a week and observing your own user id you can measure the rate of growth of a competitor.

"By signing up as a new user once a week and observing your own user id you can measure the rate of growth of a competitor."

I've done similar with cheques. In a previous job, I was paid by cheque. Each month, I logged the cheque number. From this and taking into account other payment methods, I determined that the company had significantly fewer transactions than the boss claimed.

Using this technique I've found out some interesting things about Digg: http://news.ycombinator.com/item?id=106063

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