The question was raised at a conference concerning the gospel of Mark, and the percentage of the gospel that is unique. At that time, I felt it safe to say that at least 20% and possibly 25% would be unique. Since then I have completed a count of words in this gospel, using the Newberry Englishman’s text. All that is printed as a separate part of speech, whether the definite article before proper nouns, or particles, etc., was included in the count. The total is 11,648. I used the Spanish text to mark out words and expressions that were not used in the other gospels. On comparing these portions with the Greek text, some simply disappeared. The total of unique words is 3,437 which represents 29.5% of this Gospel.
Others may wish to make a similar comparison using one of the modern Greek texts, but it’s pretty certain that the overall percentages would be little affected. Were anyone to claim, irrespective of the text used, that 25% of this gospel is unique, it’s a claim that could not easily be overthrown.
Personally I hold to the Traditional Text, which may have small variations from the Textus Receptus.