[clinic · EX-11] BM25 by hand

[akash-coded]

Exercise clinic. One long-running thread per exercise. Ask here rather than opening a
new thread — the answers accumulate where the next person will look for them.

Exercise: EX-11 — BM25 by hand · difficulty 🟢

What this exercise asks

Read the exercise first; this thread is for when you are stuck, not instead of it.

Three ways people get stuck on this one

1. Using natural log for IDF and then comparing against the implementation's base. The ranking is unchanged; the numbers will not match and you will hunt a bug that is not there.

2. Forgetting the length normalisation term, then being surprised the long document wins.

3. Treating k₁ as a weight rather than a saturation rate — see the derivation.

How to ask here

Post the numbers you got, the config that produced them, and what you already tried.
A question with a measurement in it gets answered in one round trip; a question without one
takes three.

If you have finished it, post the write-up in Show and Tell with the
[solution · EX-11] prefix rather than here — this thread stays for people who are stuck.