Because t-score isn't as accurate as z-score, you should use 40 as a safety sample size, rather than 30 as you would for a z-score.
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T-scores and z-scores measure the deviation from normal. The normal for T-score is 50 with standard deviation of 10. if the score on t-score is more than 50, it means that the person scored above normal (average), and vise versa. The normal for Z-score is 0. If Z-score is above 0, then it means that person scored above normal (average), and vise versa.
Because as the sample size increases the Student's t-distribution approaches the standard normal.
A high z-score (or t-score, depending on what info you've been given for the data) means that a number is very far away from the mean (average) number. This number might be an outlier.
It approaches a normal distribution.
When you don't have the population standard deviation, but do have the sample standard deviation. The Z score will be better to do as long as it is possible to do it.