I came across a question stating:
We have a message consisting of 10,000 characters. After computing its message digest using SHA-1, we decide to change the last 19 characters in the original message. How many bits in the digest will change if it is recomputed, and why?
Until and unless this is an explicit attack on the hashing scheme, the new hash would be different than the original one (sort of exhibiting an avalanche effect).
But I am unable to understand how does small changes in the input text affect their corresponding hashes. Like does there exist a relationship on how many bits of the hash will be affected, if a certain change is done to the input text?
P.S.: I am not a 100% sure, but I believe I have read it somewhere that a hashing algorithm should change the bits of hash by $X$% if a new character/bit is added/substituted to the previous input text.