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Table 1 Results of full model, final model 1 (automated step-wise backward selection based on akaike information criterion AIC) and 2 (manual backward selection): coefficients (Coeff.), standard error (SE) and p-value of the wald test (Wald) for each separate category of a variable as well as likelihood ratio-test (LRT) indicating the p-value of the overall variable (final models only)

From: Cross-sectional study on risk factors for Porcine Reproductive and Respiratory Syndrome virus sow herd instability in German breeding herds

Variable/category Full model Final model 1 Final model 2
Coeff. SE Wald Coeff. SE Wald LRT Coeff. SE Wald LRT
(Intercept) − 1.00 2.59 0.70 − 1.10 1.00 0.27   − 1.75 0.91 0.05  
Suckling period (days)
 ≤ 21            
 ≥ 22 − 1.53 1.02 0.13 − 1.37 0.80 0.09 0.08 − 1.49 0.71 0.04 0.03
Sows—genetic
 Danbred            
 BHZP or Topigs − 1.27 1.02 0.22         
 PIC, Hülsenberger, Hermitage, other − 1.97 1.08 0.07         
Time interval between purchase of gilts (weeks)
 < 9            
 9 − 1.63 1.38 0.24 − 0.72 0.95 0.45 0.06     
 10–52 − 2.55 1.10 0.02 − 1.82 0.84 0.03      
Time interval in which sows are sent to slaughter (weeks) 0.08 0.29 0.79         
Time interval between pick-up of carcasses (days) 0.02 0.10 0.83         
Distance between sow barn and pick-up location for carcasses (m) − 0.02 0.01 0.09 − 0.02 0.01 0.03 0.01 − 0.01 0.01 0.06 0.03
Quarantine/acclimatization unit—number of gilts per compartment (n) 0.01 0.03 0.71         
Pig density in county
 Low            
 High 1.48 0.89 0.10         
Number of other pig farms within 1000 m distance
 ≤ 1            
 ≥ 2 3.94 1.33 < 0.01 2.88 0.92 0.00 < 0.001 2.29 0.81 0.01 < 0.001
Boar semen—purchase from PRRSV unsuspicious boar stud
 No            
 Yes − 0.83 0.81 0.31         
Hygiene lock
 No clear separation            
 Clear separation 0.88 1.21 0.47         
Employment of external staff
 No            
 Yes 1.29 0.86 0.13 1.70 0.74 0.02 0.01 1.51 0.65 0.02 0.01
Farrowing rhythm
 1- or 2-weekly            
 3-, 4- or 5-weekly − 0.90 0.85 0.29 − 1.06 0.66 0.11 0.10     
Suckling pigs—age at ear tagging (day)
 1st            
 ≥ 2nd − 0.96 0.86 0.26         
Suckling pigs—2nd standard antibiotic treatment
 No            
 Yes − 0.42 0.82 0.61         
Sows—vaccination against PRRSV
 No            
 Yes − 0.09 1.32 0.94         
AIC 94.9    81.8     84.0    
  1. BHZP “Bundes Hybrid Zucht Programm”, German commercial provider of hybrid sows, PIC “Pig Improvement Company”, North American commercial provider of hybrid sows