At the company’s inception, Tripadvisor executives were worried that the reader-generated reviews of hotels and restaurants would be overwhelmingly negative but they found much to their surprise that reviews were biased in the opposite direction: 

“We were worried it was going to be a gripe site,” said the chief executive, Stephen Kaufer. “Who the heck would bother to write a review except to complain?” Instead, the average of the 50 million reviews is 3.7 stars out of five, bordering on exceptional but typical of review sites.

In fact, we can reverse the logic: “Who the heck would bother to write except to praise?”

Imagine you are asked to write a letter of recommendation for someone up for tenure.  First, the university asks you if you are willing to write the letter.  You mentally measure the amount of time it is going to take to read the papers.  Add to that the time to write a clear and comprehensive letter.  Are you going o do all that just to say something bad?  Probably not.  But if you are going to write something nice that gives the candidate a job for life, that might give you the satisfied buzz to counterbalance the cost of writing a letter.  So, letters of recommendation will be biased towards the positive.

There is still some information: Bad candidates will get fewer letters than good candidates. Buts is this carefully noted?  Is the number of letter writers who refused to write letters even recorded?

Perhaps the main countervailing force is envy. Why does X deserve tenure at highly ranked University A while I the letter writer am at humble University B?  It is impossible not to write a letter for University A. If the candidate is bad, you are forthcoming.  If the candidate is good, you are begrudging.  But the quality of the letter is monotone in the quality of the candidate and information is aggregated. Only the very best universities are the object of envy.  The rest have to decode the positive bias in their tenure procedures just like Tripadvisor users.