Hello readers!

Today I was thinking about why we keep books at home. This post is about the development of this idea.

**Motivation**
Looking at my bookshelf I realized that there are books and books. Books that we actually read regularly and others we accumulate just as trophies of accomplishments.

It seems that it is common to have an affection upon our books. But is it rational? Is it wise to keep a lot of books at home when someone outside of our little world could be making a better use of it? Why not get rid of some? Why not give one to a child, a student or a friend?

I decided I was willing to give

**some** books, keep my shelf cleaner and share knowledge.

**The problem**
But which books?

What is going to be the criteria to evaluate if a book is ready to be given or not?

It seems rational to choose the books I'm not going to need again. But this carries a bit of uncertainty and subjectiveness. It may deal with emotions. When we look at an old book a lot of memories may come into our minds. And this is an understandable argument to keep a big shelf.

**The invisible potential to recover memories.**

But it doesn't happen with all of our books...

**Developing a formula**
Then, I started to think in a formula to evaluate if it's time to get rid of a book or not.

I think it is reasonable to suppose that if I didn't pick up a book for 10 years, it is very unlikely that I'm going to pick it up again in the future. So, the chances of needing a book again is inversely proportional to how many days passed since the last day I used it until today. Technically speaking,

N ∝ 1/L

where N = chance of needing a book again and L = number of days since my last usage.

The frequency of times I took a book to read is strikingly relevant too. If I picked it up many times, it should be the case that it is still very relevant for me. Thus, the number of days I took a book to read must contribute in a directly proportional way to its usefulness. Hence,

N ∝ F

where F = number of days I read it.

Finally, a subjective metric must be taken into account. There are books we read a lot recently that we are very unlikely to read again. This is the case of text books used in a course that we didn't like and have no plan to read again. But the chances would be high with the 2 variables above only. Then, a rating to the book may be used to deal with these cases. Thus,

N ∝ R

where R = personal rating from 0 to 10.

**The formula**

Gathering all those considerations, we can devise a formula to determine the chances of needing a book again, which is:

N(L, R, F) = (**k**FR)/L

where k is a constant that is personal to the reader.

In order to find the value of

**k,** I took a book that I'm very confident in needing again. It is a book about

time, not so popular, but I like it.

I read it, approximately, during 60 days, so F = 60.

I think it is a nice book, so I gave it a rating of 9, then R = 9.

I didn't take it for about a month, so L = 30.

I want to obtain N = 1 when the chances are high of getting a book again. Given that, replacing F, R and L for N = 1, I obtained my

**k **as 0.055.

**Conclusion**
If you tried to use the formula, you could see that the value of N can be sometimes greater than 1. It is not perfect, but it worked well for some books I tried.

Maybe I should use other units to define the values of F and L. Sometimes it's hard to remember how many days we read a book. The same to remember how many days passed since the last reading.

But it is a beginning.

You may be asking yourself, why all this?

Well, because it's fun! =)

Thanks for reading!

Till the next post,

Ronald Kaiser

If you have any point or critics to my model, please feel free to leave a comment!