# AJAX PROBLEM QUESTION....

If the samples for Ajax Extreme and Ajax Normal are 25, why the solution is with z score????? ..... According to past lessons, it should be done with Students T table.

Thanks for the answer

I had the same question. Then, googled a bit and found the following answer in researchgate:

"

Good question. Most of the Statistical book shows when sigma is known and less than 30 sample size then z-test is appropriate.

"

So, I think z-test is ok here. Need to learn more though.

Md. Ariful Amin: Can you share the research gate link? I am also confused why z test is appropriate in this case.

Dear ALVARO JAVIER SOTO CORONEL, were you able to finally get an answer why z-test is preferred in this case over t-test? I am still not convinced.

When variance is known and Sample size is more than 30 then we use Z-test.

here sample size is 50, 25 Extreme Ajax+ and 25 Ajax bottles, total 50 bottles