t-test using a sample less than 30 When I do the two tailed t-test using a sample less than 30, can I use this result? This is because I do also still see the t-test table which has degrees of freedom far less than 30. How further small can the sample size be?
 A: T-test are useful if the data is normally distributed and iid (@djima thank you). If the effect size is large you can use the t-test also if the sample size is small. So yes, you can use a t-test with a sample size which is smaller than 30.  The effect size can be calculated with Cohen's D.. Under certain circumstances other measures such as the Glass Delta or the Hedges G are more useful. 

"The present simulation study showed that there is
  no fundamental objection to using a regular t-test with
  extremely small sample sizes. Even a sample size as
  small as 2 did not pose problems. In most of the
  simulated cases, the Type I error rate did not exceed
  the nominal value of 5%. A paired t-test is also feasible
  with extremely small sample sizes, particularly when the
  within-pair correlation coefficient is high. (de Winter, 2013, p. 7)

J.C.F. de Winter (2013),
"Using the Student’s t-test with extremely small sample sizes,"
Practical Assessment, Research and Evaluation, 18:10, August, ISSN 1531-7714
http://pareonline.net/getvn.asp?v=18&n=10
