Distribution of incubation period for each clinical form of 37 invasive cases of listeriosis (box-plot). Line in the middle of boxes represents median of data. Boxes extend from the 25th percentile (X) to 75th percentile (X), representing interquartile range (IQR). Lines emerging from boxes extend to upper and lower adjacent values. The upper adjacent value is defined as the largest data point ≤X + 1.5 x IQR. The lower adjacent value is defined as the smallest data point ≥X – 1.5 x IQR. Dots are outliers (every point more than 1.5 x IQR from the end of a box).