How Accurate is your Spam Filter?

When researching spam filters, you'll frequently see reference to accuracy. Is a filter 90% accurate? 95% accurate? 99% accurate?

Here's how you can test the accuracy of spam filtering with whatever email system you're using:

1. In the morning, when you get into the office or fire up your computer at home, clear your spam folder. (i.e. Microsoft’s Junk e-Mail folder, Ella Review Spam etc.)


2. Jot down the number of messages you have in your inbox. [e.g. 523]


3. During the course of the day, don't delete any messages (good or spam). Leave them in the inbox or the spam folder


4. The next morning, determine the “denominator” – the total number of messages you received since yesterday morning
   
   a. Take the total number of messages in your inbox (including messages you consider to be spam), [e.g. 584 total messages in your inbox]
   
   b. Plus the number of messages in your spam folder, [e.g. 87 messages in your spam folder]
   
   c. then subtract the total number of messages that you wrote down in step 2 above [(584+87)-523 = 148]


5. Then determine the “numerator”
   
   a. the number of inbox messages that you consider to be spam. [e.g. 9 inbox messages considered to be spam]
   
   b. plus the number of any “false positive” messages (if any) that the filter might have misclassified (good messages in your spam folder) [e.g. 1 message]


6. Now divide the number of messages misclassified by your filter (the numerator) by the total number of messages you received. [10/148 = 6.76%]


7. Subtract the result from 100%. [100% – 6.76% = 93.24%]
   

This simple method should give you a true reflection of the accuracy of your spam filter.