value from e.g. 0.045 to 0.055 as a change from significance to non significance. Therefore the real P 

values are preferably reported, P=0.045 or P=0.055, instead of P<0.05 or P>0.05, so the reader can make 

his own interpretation. 

With regards to the interpretation of P values as significant versus not significant, is has been 

recommended to select a smaller significance level of for example 0.01 when it is necessary to be quite 

certain that a difference exists before accepting it.  When a study is designed to uncover a difference, or 

when  a life saving drug is being studied, we should be willing to accept that there is a difference even 

when the P value is as large as 0.10 or even 0.20 (Lentner, 1982).  The latter authors state:  The tendency 

in medical and biological investigations is to use too small a significance probability . 

Confidence intervals 

Whereas the P value may give information on the statistical significance of the result, the 95% confidence 

interval gives information to assess the clinical importance of the result. 

When the number of cases included in the study is large, a biologically unimportant difference can be 

statistically highly significant. A statistically significant result does not necessarily indicate a real biological 

difference. 

On the other hand, a high P value can lead to the conclusion of statistically non significant difference 

although the difference is clinically meaningful and relevant, especially when the number of cases is small.  

A non significant result does not mean that there is no real biological difference. 

Confidence intervals are therefore helpful in interpretation of a difference, whether or not it is statistically 

significant (Altman et al., 1983). 

Presentation of results 

It is recommended to report the results of the t test (and other tests) not by a simple statement such as 

P<0.05, but by giving full statistical information, as in the following example by Gardner & Altman (1986): 

The difference between the sample mean systolic blood pressure in diabetics and non diabetics 

was 6.0 mm Hg, with a 95% confidence interval from 1.1 to 10.9 mm Hg; the t test statistic was 2.4, 

with 198 degrees of freedom and an associated P value of P=0.02. 

In short: 

Mean 6.0 mm Hg, 95% CI 1.1 to 10.9; t=2.4, df=198, P=0.02 

Paired samples t test 

The paired samples t test is used to test the null hypothesis that the average of the differences between a 

series of paired observations is zero.  Observations are paired when, for example, they are performed on 

the same samples or subjects. 

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