How to write a recommendation letter if you are not sure about its appropriateness As supervisors, many times we are asked to write recommendation letters.
In my case, I have been recently asked to write one for a student, who is looking to enroll in a Ph.D. program.
Due to the short time working under my supervision and the lack of results, it is not easy to highlight their strong points.
In fact, It is also difficult to emit my recommendation.
What would you do if you were in my shoes?
Decline on the grounds that, indeed, you've worked too little together. 

However (and actually adding to Peter K.'s answer), a discussion in order to understand why they chose you might be useful -- meaning that usually one asks for recommendation from people with whom they've worked longer. If their background and intentions don't seem suitable, maybe also tip them that another career path would be better?