What is the disadvantage in comparing line segments by mere observation?

Solution:

A line segment is a part of a line that has a fixed length or we can say that both the ends of a segment are fixed.

Two line segments whose legths have a difference which is evideltly seen can be compared by mere observation

Example: 

Two line segments wherein the lengths are almost same it becomes difficult to compare them by a mere observation.

Thus, It becomes difficult to compare the line segments whose difference in lengths is not significantly seen or known and hence, it may increase the chances of errors.

What is the disadvantage in comparing line segments by mere observation?

Summary:

It becomes difficult to compare the line segments whose difference in lengths is not significantly seen or known and hence, it may increase the chances of errors.