Review lectures

[DiaPorntipa]

Hello @jstac, this PR contains all minor changes of lectures 3 and 4 edited by @HengchengZhang , @orectique and me.

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[HengchengZhang]

Thanks @DiaPorntipa for creating the PR, but there's still some changes I need to commit before merging.

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🚀 Deployed on https://64a24ed7bad6fb40384128a8--taupe-gaufre-c4e660.netlify.app

[jstac]

Thanks @DiaPorntipa ! Very helpful.

Please see the small requests above.

[HengchengZhang]

Hi @jstac, I've applied the review changes and make other modifications.

[jstac]

Thanks @HengchengZhang , nice work.

I think this code block should be hidden:

# transfer the survey weights from absolute into relative values
...

And in the code block below it, we should stick to PEP8 --- in particular, lines should be 80 characters max, so those long function calls need to be spread across two lines.

[HengchengZhang]

Hi @jstac, I see that's because there's error in codes and now they are fixed.

Similar errors are in LA lecture as well and I will fix it next reading group.

[DiaPorntipa]

Thanks @HengchengZhang for the update.

[jstac]

Nice work @DiaPorntipa and @HengchengZhang . Merging.