Why does the median-past time average the last 11 block timestamps and not the last 12?

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As described in BIP-113. Wouldn't the last 12 blocks bring us closer to a one-hour offset (ie 120min, midpoint 60)?

Paul Razvan Berg

Posted 2018-06-20T22:14:18.057

Reputation: 227

Answers

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It is a median, not an average, thus it selects the 6th block's timestamp after sorting.

If you're a non-native English speaker, international maths heuristics can be quite tricky. Double-check on the Internet before translating into your head.

Paul Razvan Berg

Posted 2018-06-20T22:14:18.057

Reputation: 227

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BIP 113's goal is not to aim for a specific offset.

Its goal is guaranteeing monotonicity (treating every block's timestamp as strictly larger than the timestamp of each of its ancestors). It does this by leveraging the existing consensus rule which states that the median of the timestamp of a block has to be strictly larger than the median of its 11 predecessors.

As to why the number 11 eleven was chosen in that existing consensus rule, we don't know. It was chosen by Bitcoin's creator.

Pieter Wuille

Posted 2018-06-20T22:14:18.057

Reputation: 54 032

As I also answered my question, the problem was specifically around the one-hour offset. I confused median with average, as I'm still getting used with maths in English.

But thank you for your answer, it provided clear and helpful info! – Paul Razvan Berg – 2018-06-20T22:46:46.713

Median or average is not relevant. If the existing consensus rule would have been average, BIP113 would have chosen the same. – Pieter Wuille – 2018-06-20T23:12:28.013

I agree, that stands for BIP113. However, we cannot know for sure that they would have selected the same number, ie 11, had the Bitcoin creator chosen the average not the median. – Paul Razvan Berg – 2018-06-21T08:51:18.313

Asked: 2018-06-20T22:14:18.057

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Active: 2019-09-24T13:40:50.093