That is, when you create today a website with 100 working external links and checks your website after two year with a broken link checker, you will discover that rougly 50 links are broken.
How do you know?
I can almost hear you thinking "How do you know?". Well I will explain below.In the past I have copied as much data as possible of the directory Yahoo! This is because Yahoo! stopped, I have created a directory myself and I wanted to analyse the links and structure of this famous directory.
At January 4, 2016 I analysed the data I have and concluded that 77% (or more exactly 76.8387682%) of the links are fine.
Recently (October, 9 2017) I analysed the data again. Now 42% (42.0219319%) of the links are fine.
Based on this data I concluded that on an average day 0,093670021% of external links will get broken. That does not seem much. However the linkrot percentage per month is 2.81%.
Consequences
After a half year one sixth of the links are broken.After a year 30% of the links are broken.
After two years 50% of the links are broken. Hence the half-life of a link is two year.
See also this graph below
So when you think 3% broken links is acceptable, then you should check for broken links every month.
When 5% is acceptable, check every two months and when you think 10% is acceptable, check every 4 months for broken links.
Tip: Use the tool Maintenance Frequency at a Glance to find your optimal maintenance frequency.
Be wise, and check and repair your links at a regular base,
Hans
Update: After writing this blogpost I discovered that in the document "A longitudinal study of Web pages continued: a consideration of document persistence" it is stated that the half-time of a random web page is about 2.0 years. Great that's exactly what I concluded.
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