We sometimes hear on various forums that spam is always on the increase and that email servers are getting blasted with it. I decided to investigate the relationship between spam volume one week to the next. Specifically, I decided to determine what the week-over-week relationship is in total spam traffic. If it increases one week, is it more likely or less likely to increase the next week? If we get a sudden blast in spam, will that spam blast continue?
To determine this, I collected all of our historical data going back to the beginning of 2005. I then calculated the week-over-week % change in overall message traffic. Specifically, I calculated this week's traffic to the previous week's traffic, then I calculated this week's traffic to next week's traffic. I then calculated the % differences between the two.
When comparing the % change in this week's traffic to last week's traffic, the average change is 1.6%. When comparing this week's traffic to next week's traffic, the average change is 1.5%. These two values are pretty much the same, this tells us that, on average, spam is increasing by about 1.5% per week. However, the relationship between the total volume of spam from one week to the other is not at all similar. I calculated the correlation between this week's % change in volume to next week's % change in volume, and the correlation coefficient -0.22. In other words, if we get an increase in spam this week, we are more likely to see a drop in spam next week. The difference will be made up when the next increase occurs and it will be larger than the average 1.5% increase.
Next, I decided to check the relationship for two week periods. For example, I calculated the total amount of mail between Jan 1 - Jan 14, then Jan 15 - Jan 28, Jan 29 - Feb 11, and so forth, and then ran the same tests. The results for the average change is that spam volume increases by about 2.5% every two weeks. However, the correlation coefficient for bi-week over bi-week change in volume is 0.032. In other words, if we get an increase in spam over a two-week period, we cannot make an estimate as to the total amount of spam we can expect to see over the following two weeks.