Difference between Z Distribution and T Distribution
Topic Six

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Difference between Z Distribution and T Distribution

The Z distribution is a special case of the normal distribution with a mean of 0 and standard deviation of 1. The t distribution is similar to the Z distribution, but is sensitive to sample size and is used for small or moderate samples when the population standard deviation is unknown. At large samples, the z and t samples are very similar.

The t statistic is used to test hypotheses about an unknown population mean u when the value of ό is unknown.  The formula for the t statistic has the same structure as the z-score formula, except that the t statistic uses the estimated standard error in the denominator.  The only difference between the t formula and the z-score formula is that the z-score uses the actual population variance, ό 2 (or the standard deviation) and the t formula uses the corresponding sample variance (or standard deviation) when the population value is not known.  Simply put, the basic difference between these two is that the t statistic uses sample variance (s 2) and the z-score uses the population variance (ό 2).  To determine how well a t-statistic approximates a z-score, we must determine how well the sample variance approximates the population variance.  Basically, for small samples, the t-statistic is used.

Figure 1

The mean must be known prior to computing the sample variance.  This places a restriction on sample variability such that only n-1 scores in a sample are free to vary.  The value n-1 is called the degrees of freedom (df) for the sample variance.  Degrees of freedom describe the number of scores in a sample that are free to vary.  Because the sample mean places a restriction on the value of one score in the sample, there are n-1 degrees of freedom for the sample.

The t Distribution and Shape of the t Distribution

The t distribution will approximate a normal distribution in the same way that a t statistic approximates a z-score.  How well a t distribution approximates a normal distribution is determined by degrees of freedom (df).  The greater the sample size (n) is, the larger the degrees of freedom (n-1) are, and the better the t distribution approximates the normal distribution.  The exact shape of a t distribution changes with df.   As df gets very large, the t distribution gets closer in shape to a normal z-score distribution.  Distributions of t are bell-shaped and symmetrical and have a mean of zero.  The t distribution tends to be flatter and more spread out, whereas the normal z distribution has more of a central peak.

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