I cant understand the concept of internal path length (IPL). I cant understand what this value mean in the tree? For instance if a IPL of a tree is 30 then what does it mean for the tree. Tree does not have 30 nodes so what the path length means?

I am calculating the IPL for the tree which i have attached using the formula:

Summation (i-1) * Li

for the attached figure(one node at level 0, two at level 1, four at level 2, four at level 3and 2 at level 4:

1 * 0+2* 1 + 4 * 2+ 4 * 3 + 2 * 4= 30.

IPL = 30 & there are only 13 nodes in the tree. Book says that "IPL is the sum of all path lengths of all nodes". If there are only 13 nodes, then it means

IPL does not represent nodes of tree nor it represents the sum of nodes & edges of a tree, so what is the purpose of this value?

Zulfi.

allthat