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Probability factor
Leakage indicator (P_LI)
In today’s practise leakage is the single most important factor, when selecting which water mains are selected for rehabilitation. This should still be true, and therefore much labour was used to develop the leakage indicator. Just counting the number of leakage repairs on a water main section is too simple, and will lead to wrong decisions.
Information on leakage frequency is varying among the municipalities, except for the last 2 years where registration of leakages has been good due to the establishment of active leakage control in the region. However, for the pilot area registration of leakages has been done for the last 20 years.
The leakage indictor includes both the leakage frequency and the absolute number of leakages according to the formula:

L_N : Number of leakages on the water main section. [0 ... 4]

If L_N > 4 => P_LI = 6. (In this case the water main should always be rehabilitated)

L_F : Leakage frequency of the water main section, normalised t: no./100 m/10 year.
The leakage frequency is calculated for 3 periods:

1. 
   The lifetime of the pipe (L_F)
   
2. 
   The lifetime – last 5 years (L_F1)
   
3. 
   The last 5 years (L_F2)
   

If L_F2 > L_F1 Then F_Y = 1.5 else F_Y = 1