Just a short blog entry about something that’s been bothering me for many years.
I think the best way to explain the problem is by starting with a simple riddle.
My site had visits 31,000 for one month, and then 30,000 visits the next month but the overall trend for visits was up.
How could that be?
The answer is simple.
Not every month has the same number of days.
If I had 31,000 visits in January (31 days) and 30,000 visits in February (28 days) I’d say the overall trend is up.
July / August and December / January are the only consecutive months that actually do have the same number of days in the month.
All this means is that when comparing metrics for two different months, be sure to use relative number and not absolute ones.
For example, comparing average visits per day for the month is better than comparing visits for the entire month.
To make matters even worse, most sites will see a large variance between weekdays and weekends.
Therefore the BEST way to compare month to month will factor this in as well.
I’ve seen two different ways to handle this:
1 – Compare the month in question to a time frame exactly 28 days (4 weeks) prior to the first and last days of the month you’re looking at (yes, you’ll probably have a few days which are overlapped, but that’s OK).
For example, we can compare May which is 31 days to the time frame of April 3rd (May 1st minus 28 days) to May 3rd (May 31st minus 28 days).
2 – Use metrics like “visits per weekday” or “visits per weekend day”. Not perfect since there is still some variance between specific days, but this is better than “visits per day” for the entire month.