comment from post 2

A T score and Z score are used in hypothesis testing. 

When to use a T Score:

• The sample size is below 30
• The population standard deviation is unknown (estimated from your sample data).
• the variation of scores in the two groups is not reliably different.

You must know the standard deviation of the population and your sample size has to be above 30 in order to use the Z score. If it doesn’t meet these criteria then use the T score. 

T-scores are used when the conversion is made without knowledge of the population standard deviation and mean

When to use, a Z score: 

• The standard deviation must be known and the sample size must be above 30. 

A Z-score tells you how many standard deviations from the mean your result is. The standard deviation must be known and the sample size must be above 30. 

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