Confusion regarding when to use $z$-statistics vs. $t$-statistics

I was referring to this video lecture for calculating the confidence interval. However, I have some confusion. This guy is using $z$-statistics for the calculation. However, I think it should have been a $t$-statistics. We aren’t given the true standard deviation of the population. We are using the sample standard deviation to estimate the true one.

So why did he take normal distribution for the confidence interval rather than $t$?

Answer

You are correct, it should be a t-distribution. But since the sample size is 36 (i.e > 20), a z distribution would also be appropriate.
Remember, as the sample size grows, the t-distribution becomes more similar to the z-distribution in shape.

Attribution
Source : Link , Question Author : user34790 , Answer Author : John

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