When faced with a problem of data analysis, the first step is to decide

whether to use a parametric or a nonparametric procedure. A parametric

procedure should be used when both of the following are true:

1. The data are collected and analysed using an interval or ratio scale of

measurement.

2. All of the assumptions required for the validity of that parametric procedure can be verified.

Otherwise, a nonparametric procedure should be used. This means that

a nonparametric procedure is appropriate when any of the following is true:

1. The data are counts or frequencies of different types of outcomes.

2. The data are measured on a nominal scale.

3. The data are measured on an ordinal scale.

4. The assumptions required for the validity of the corresponding parametric procedure are not met or cannot be verified.

5. The shape of the distribution from which the sample is drawn is unknown.

6. The sample size is small.

7. The measurements are imprecise.