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I have noticed that when I, and presumably others, count the number of times the letter F appears in the following passage:


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.


  • 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
up vote 10 down vote accepted

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:

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

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