Two-tailed question

If one has a specific problem, which can be converted into a statistical question, it is important to know if the question is one-tailed or two-tailed. Typically one-tailed are questions for limits, such as Is the limit exceeded or not (e.g. alcohol in blood). If we want to know whether a measured value lies within a certain range this is a typically two-tailed question. The value could be too low or too high. 

We get the critical value from the table of t-values. We have a two-tailed question, we want to have 95% confidence level, and with 10 measurements we have 9 degrees of freedom. Therefore we get from the table a critical value of 2.262. 

If we have a one-tailed question we have to use other t-values. In the table shown in this shde we have to select the level of confidence from the first row for one-tailed and from the second row for two-tailed questions. 

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