When counting a given letter in sentences, why do people tend to omit occurrences in certain common words? I have noticed that when I, and presumably others, count the number of times the letter F appears in the following passage:

FINISHED FILES ARE THE RESULT
OF YEARS OF SCIENTIFIC
STUDY COMBINED WITH THE
EXPERIENCE OF YEARS


They typically get a count of three, yet the real answer is six. The letter F in the the word "OF" tends to be omitted from the count.

I imagine that this effect has something to do with top-down processing.

Questions


Have there been any scientific studies of this or a similar task?
What proportion of people complete the task successfully?
What explains the tendency of people to not count the letter F's in the word "OF"?
What explains the tendency of people to not count the letter F's in the word "OF"?

One possibility to explain this effect is related to the phenomenon of word skipping. When we read, we do not fixate every word, but skip a certain proportion of words while making educated guesses instead. Whether or not a certain word is skipped seems to depend on its length and its predictability, that is, how likely a word is given its predecessor (e.g, Rayner et al., 2011). For your example, the preposition "of" might be very likely to be skipped because it is very short and some grammatical clauses always ask for this preposition. 

Please see PubMed for a more comprehensive literature search on word skipping: http://www.ncbi.nlm.nih.gov/pubmed?term=%22word%20skipping%22

References
Rayner K, Slattery TJ, Drieghe D, Liversedge SP. (2011). Eye movements and word skipping during reading: effects of word length and predictability. J Exp Psychol Hum Percept Perform. 37(2):514-28