Z Score vs. T Score

Difference Between Z Score and T Score

You would come across the terms Z score and T score in statistics and are called standard score. These are used to show the SD data above and below the mean. Z score is used in Z-test while T score is for population. These are quite similar and this is why we tend to get confused. Here are some differences listed below to make these terms clearer to you.

  • T –test can be used if you do not have any information about the population and just have the sample data while Z test is better to use if you know the standard deviation and population mean for a population.
  • T- Test can be performed when there are samples different from each other, samples matching with each other and for a single sample as well. Z test is performed on large sample size usually more than 30 while T test is good for sample size below 30 because with a small sample you do not get correct estimation of standard deviation.

Z scores are commonly used in hospitals in measuring bone mass density of a person. There are different kinds of units to measure bone density and Z score is used to bring uniformity in interpretation of score.

A person scoring zero and 50percentalie on z -score is average and to be above average you need to score above zero. Z-score is also used by pediatricians for measuring height of growing children. If a child is short according to his or her age then he or she would score –i.65 at 5th percentile.

The formula to calculate z and t scores are

Z score = (patient’s BMD- expected BMD)/SD

Z score= T score – reference T score


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