I was just reading an article about different ranking algorithms. One thing I'm a bit confused about is stumbleupon's algoritm:
(Initial stumbler audience / # domain) + ((% stumbler audience / # domain) + organic bonus – nonfriend) – (% stumbler audience + organic bonus) + N
N is a "safety variable" so that the assumed algorithm is flexible. It represents a random number.
Could someone explain the purpose of N to me? I don't understand how it's making the algorithm more flexible.
By introducing N you ensure that stuff that is not rated very often yet gets shuffled to the top occassionally (if N turns out high). I guess this ensures that everything gets seen at least a few times initially, and thus gets a few shots at obtaining more stumbles (or whatever they're called). Basically just slight randomization of new stuff to remove top-spot bias.
That is my guess anyway.
It helps the results degrade at an unpredictable rate. If you've ever stumbled for a couple of hours you'll likely have noticed a serious decline in the quality and relevance of each stumble as you start scraping the bottom of the barrel.
Without the random variable the results would degrade in a very predictable rate. The link you were most likely to love would be first and each subsequent link would likely appeal a little less to you. The variable attempts to negate this somewhat.
It's one of the many clever ways stumbleupon is designed to be addictive:
The results were unanimous in the fact that when one performed an action over and over again, and was given a reward randomly, dopamine levels rose. If the reward was given consistently, i.e. every four time the action was performed, the dopamine levels remained constant