H2 Maths: When to use T test ? When to use Z test?

• quailmaster

  19 May 11, 23:18

   

  Case 1: small sample, from normal population of known variance --> Use Z test?

  Case 2: SMALL sample, normal population of unknown variance, variance estimated --> Use t test?

  Case 3: LARGE sample, normal population of unknown variance, variance estimated --> Use t test?

   

• qdtimes2

  21 May 11, 11:22

  Case 1: Z test

  Case2: T test

  Case 3 (either known variance or unknown variance and large sample) : Z-test

  so, T test is only used when sample is small and has unknown variance in the distribution

• ThunderFbolt

  22 May 11, 23:07

  Regarding case 2 & 3, I kinda also remember my teacher saying if N > 50, use Z. Otherwise, use T. Not sure about other schools. 

• frekiwang

  25 May 11, 14:04

  Case 1: small sample, from normal population of known variance --> z-test Case 2: SMALL sample, normal population of unknown variance, variance estimated --> t-test Case 3: LARGE sample, normal population of unknown variance, variance estimated --> z-test Here is a full summary for you: 1. Population distribution known to be normal. 1.1 population variance known(large/small sample size): z-test. 1.2 population variance unknown(large sample size): z-test.(t(n-1) is very close to N(0,1) when n is large). 1.3 population variance unknown(small sample size): t-test. 2. Population distribution unknown. Large samples, we can assume a normal distribution by CLT. z-test regardless of variance.