# 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"?
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If my hypothesis is correct, then one of the edits changed the meaning of the original question for the worse: If the reader uses the "F" phoneme to scan the sentence for the letter "F", then "OF" will be missed because it has no "F" phoneme. I suggest removing "certain common words" from the edited question and don't add assumptions to questions. –  John Pick Jun 21 '12 at 2:54