## 2 Materials and Methods
### Technical Measurement Uncertainty  
The technical measurement uncertainty for the ddPCR method was determined according to the global approach (top-down approach) described in ISO 19036:2019 [31]. In order to ensure an efficient and a reliable estimation of in-house reproducibility, a factorial design was implemented. Seven factors were chosen to represent the range of conditions during routine testing (Table 4). In this design, measurements were performed in eight different runs, with each run corresponding to a combination of factor levels. Depending on the spiking level of the chicken neck skin with _C. jejuni_ or _C. coli_ live/dead cell standards, a total of five artificially contaminated samples (TU1 to TU5, Table 5) were analyzed for every run.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample**} & **Level and** & **Bacterial** & _C. jejuni_ & _C. jejuni_ & \\  & **Organic Matter** & **Contamination** & _C. coli_ & _C. coli_ & **Biological** \\  & **Runse/mL** & **Level** & **Live** & **Dead** & **Replicates** \\  & **Runse/mL** & **CFU/mL** & **Cells/mL** & \\ \hline A1 & Low/12 mg & low & 200 & 2000 & 5 \\ A2 & Low/12 mg & low & 500 & 5000 & 5 \\ A3 & Low/12 mg & medium & 1000 & 10,000 & 5 \\ A4 & Low/12 mg & medium & 2000 & 20,000 & 5 \\ A5 & Low/12 mg & high & 5000 & 50,000 & 5 \\ A6 & Low/12 mg & high & 10,000 & 100,000 & 5 \\ \hline \end{tabular}
\end{table}
Table 2: Spiking plan for the accuracy study. For each sample (A1 to A6), 3 mL chicken neck skin rinse were spiked with live- and dead-cell standards of _C. jejuni_ and _C. coli_, respectively, in five biological replicates.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample**} & **Level and Organic** & **Bacterial** & _C. jejuni_ & _C. jejuni_ & \\  & **Matter Content of** & **Contamination** & _C. coli_ & _C. coli_ & **Dead** \\  & & **Meat Rinse/mL** & **Level** & **CFU/mL** & **Cells/mL** & \\ \hline T1-LM & & Low/8 mg & low & 500 & 5000 & 1 \\ T2-LM & & Low/8 mg & low & 1000 & 10,000 & 1 \\ T2-MM & Chicken neck & medium/37–48 mg & low & 1000 & 10,000 & 1 \\ T3-LM & skin/Chicken & Low/8 mg & medium & 2000 & 20,000 & 1 \\ T4-LM & breast/Turkey skin & Low/8 mg & medium & 5000 & 50,000 & 1 \\ T5-LM & & Low/8 mg & high & 20,000 & 200,000 & 1 \\ T5-MM & & medium/37–48 mg & high & 20,000 & 200,000 & 1 \\ \hline \end{tabular}
\end{table}
Table 3: Spiking plan for the trueness study. For each sample, 3 mL each of chicken neck skin, chicken breast, and turkey skin meat rinse were artificially contaminated with live- and dead-cell standards of _C. jejuni_ and _C. coli_, respectively.  
The low bacterial contamination samples (TU1 and TU2) were analyzed in three biological replicates for each of eight runs: 26 mL of chicken neck skin rinse were spiked (24 mL for ddPCR with PMA and 2 mL for the reference method). The medium (TU3) and high bacterial contamination (TU4 and TU5) were analyzed in two biological replicates for each of eight runs: 18 mL of chicken neck skin rinse were spiked (16 mL for ddPCR with PMA and 2 mL for the reference method). For the reference method, two biological replicates were analyzed immediately after spiking.