In Sentiment Analysis, I've seen the sentiment of an entire string of words (tweets, paragraphs, sentences, etc.) computed by summing the sentiment of all of the terms in the string.  I know there is a monotonic relationship between summing and averaging.  So, if sentences are only ever compared to other sentences, then of course it wouldn't make a difference.  So, why is it that the average is not used instead of the sum?  Especially if there are cases where strings could contain only one word, or many words, it seems to me that an average sentiment would be the way to go.  Is that not true?  
Why use sum and not average for sentiment analysis?
machine learning;sentiment analysis