What Two Assumptions Must Be Met When You Are Usin

What two assumptions must be met when you are using the z test to test differences between two means? Can the sample standard deviations s1 and s2 be used in place of the population standard deviations σ1 and σ2?Two assumptions that must be fulfilled when using z test are;Samples from the two populations are independent, i.e. observations from first sample do not influence/ affect observations from the second sample and vice versa.The considered populations (population from which samples are taken) are normally distributed Sample size is greater or equal to 30Can the sample standard deviations s1 and s2 be used in place of the population standard deviations σ1 and σ2?Yes we can use them assuming that s1 and s2 are the point estimates of the population standard deviation σ1 and σ2. However we will lose one degree of freedom