The Rotter Incomplete Sentence Blank can tell you what you're like in 40 sentences, most of which will be written by you. The responses to very short prompts are scored according to how positive or negative they are. (I wish they had some scoring guidelines for how sarcastic they are, though.)
Complete the following sentences:
"Most girls... "
"The only trouble... "
"I hate... "
As you can see, you don't get a lot to work with when you complete the 40 sentences of the Rotter Incomplete Sentence Blank. (It's not named Rotter because the person administering it thinks you are one. It's named for Julian Rotter, the man who developed it.) These sentences are from the "college edition" of the test. It's meant for college students and references school, girls (and presumably boys), and the future. The person taking the test gets 20 minutes to complete it, and the psychologist gets a look into a subject's overall mental state.
The scoring system is loose, but important. There are neutral responses, which tend to be things like well-known phrases or cliches. Someone who completes a phrase like, "Look... " with "both ways before crossing the street," is giving a neutral answer. Unless every other answer is about the need to be cautious, this one does not say anything about the subject's personality. There are positive responses, which are grouped into three categories, from the mildly positive ("Reading... is something I enjoy.") to the wildly positive ("People... love me and I love them."). Then there are conflict responses. These are also grouped into three categories, from mildly negative, ("At bedtime... I feel antsy.") to the really unpleasant ("I hate... getting up in the morning.").
The balance of positive, conflict, and neutral responses gives the psychologist an idea of the subject's state of mind. The freedom that the test allows the subject can also give the psychologist something more. People's responses tend to run along certain lines, and these lines can reveal either temporary problems or chronic mental illnesses. Take a look at different personality studies, and you will run across the test as a prelude to the study. It's a good way of selecting a group of people who all have the same issues from a large swath of the population. I once saw a study (sadly lost to me now) which included a group of people for whom an Incomplete Sentence test showed a high degree of "sex guilt." With all due respect and compassion for those suffering people, I would have loved to see those completed sentences.