# Should we always calculate the z in a confidence interval as 1 - (alpha/2)?

I’m currently beginning my studies in statistics and all of the examples of confidence interval questions I studied used (let’s assume a 95% confidence interval) as a z formula z = 1 - (0.05/2), which would result in 0.975 and then using a z-table in 1.96. The thing is, is the alpha always divided by 2 or are all the examples I studied using a two sided distribution?

Cross Validated Asked on November 14, 2021

Yes, it is because you're doing two-sided testing.

When you do two-sided testing, you have to allocate probability to both tails. You pick $$0.975$$ because $$2.5%$$ goes up high, and $$2.5%$$ goes down low.

Answered by Dave on November 14, 2021

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