Statistical analysis plays a central role in medical research. With exception for case reports, P-values and significance statements appear in almost all published papers. However, deeper insight into statistical principles seems not wide-spread. P-values and significance statements often appear to be interpreted as obscure properties of examined patients; differences between groups exist only if statistically significant; the interpretation of clinical and statistical significance is confused, etc. Understanding basic principles is necessary both for statistically analyzing data and for interpreting the results of the analyses. Without this understanding severe mistakes are easily made. A few minutes spent on statistical fundamentals may be an investment with an exceptionally high return of the invested time. This is the reason OAC plans a series of short methodological reviews, of which this is the first.