Loss of relevance for personal notes

From summer 2008 to Autumn 2009, 55 people have been interviewed with an online questionnaire the following questions (among others):

• A) In one week, how many personal items do you write down?
• B) Of these personal notes, how many are still relevant (in percent)
  • ... after one day?
  • ... after one week?
  • ... after one month?
  • ... after one year?

The average values for (B) are 71,55%, 51,73%, 36,09%, 20,55% with rather high standard deviations (0,223, 0,265, 0,278, 0,243).
However, when computing the loss of relevance from one time point to another one we get:

• Loss of relevance on first day: 28,45% (0,223)
• In the next six days: 19,82% (0,161)
• In the next three weeks: 15,64% (0,139)
• In the next 11 months: 15,55% (0,146)

with somewhat nicer standard deviations (in parentheses). Interpolating this we get the fraction of relevant knowledge after n days as depicted in this chart:



Assuming one creates 10 new personal notes every day, then the personal notes collection contains after 100 days 1000 notes of which 389 (38,9%) are relevant. After 500 days, 1368 of 5000 notes (27%) are relevant.