The semi-quantitative method used provides interval data and has
a limit of detection (LoD) of 0.1 cfu/g. As food products may be contaminated
below this LoD, the observed prevalence will most likely
not be the “true” prevalence of contaminated products. To facilitate
the risk assessment, a lognormal distribution was fitted to the semiquantitative
data. The estimated prevalence (pest) indicates the proportion
of food products for which the 10 based log of the concentrations
(log cfu/g) can be described by a normal distribution with
mean (μ) and standard deviation (σ). Estimates for pest, μ and σ
were obtained for each meat category and year using maximum likelihood
estimation (MLE). This allows the incorporation of censored
data (Lorimer and Kiermeier, 2007).