Do you understand redundancy? Do you know how to calculate redundancy?

2022-09-30

Everybody knows the word redundancy, but not everybody knows what it exactly means or how it applies. Look at the picture. There are three men, but how many are needed to keep the ladder up? These men should not be doing this in the first place, but it is an interesting picture to talk about redundancy.

Dictionaries define redundancy as "multiple means to carry out the same function". We have three men to carry out the same function. But the question is, how many are really needed?

Is one man enough? Do we need two, or do we need all three men to hold up the ladder? If we need three men, we have no redundancy or 0 redundancy. We only have redundancy if we need only one or two men to hold the ladder.

But how much redundancy do we have? Can we express redundancy as a number? Yes, we can. But we must be very clear about how many failures we assume when we define it.

Let us say we only need one man to hold up the ladder, but we have three. Then we are three redundant if we assume no failure, two redundant if we assume one man failed, and one redundant if two men failed to hold up the ladder.

But, if we have three men and we need two to hold up the ladder, the story changes. Then we are three redundant if no men failed, one redundant if one man failed, and we have no redundancy if two men failed.

If you want to calculate redundancy, you must first express how many failures you assumed.