Clear DefinitionsBlock Time Variance
1 min read
Pronunciation
[blok tym vair-ee-uhns]
Analogy
Like the unpredictable arrival times of city buses—you expect one every 10 minutes on average, but actual arrivals vary.
Definition
The statistical variability in the time intervals between successive blocks, influenced by consensus randomness and network factors.
Key Points Intro
Block time variance affects UX, difficulty adjustment, and orphan rates.
Key Points
PoW randomness: Mining is a Poisson process, producing exponential interarrival times.
Network delay: Propagation lag can shift timestamps and apparent intervals.
Difficulty adjustment: Designed to target average block time over a window.
User impact: Affects transaction confirmation time distribution.
Example
Technical Deep Dive
In PoW, each hash attempt is independent; block discovery follows exponential distribution with mean 1/λ, where λ is network hash rate over difficulty. Variance σ²=1/λ². Difficulty retarget algorithms (e.g., every 2016 blocks in BTC) adjust to maintain target mean but cannot reduce variance.
Security Warning
High variance periods can spike orphan rates, aiding selfish miners or chain reorgs.
Caveat
Variance is inherent to probabilistic consensus; smoothing via off‑chain channels (Lightning) may be required for UX guarantees.
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