The Central Intelligence Agency has specialists who analyze the frequencies of l
ID: 3179618 • Letter: T
Question
The Central Intelligence Agency has specialists who analyze the frequencies of letters in the alphabet in an attempt to decipher intercepted messages that are sent as ciphered test. In standard English text, the letter T occurs at a rate of 7.33%.
a.) Find the mean and standard deviation for the number of times the letter T will be found on a typical page of 2700 characters and explain what they mean in a complete sentence.
b.) In an intercepted ciphered message sent to Spain, a page of 2700 characters is found to have the letter T occurring 267 times. Using the rule range rule of thumb, explained if this is an unusually low or an unusually high value in a complete sentence. I need to see all of your work!
Explanation / Answer
a) here mean =np=2700*0.0733=197.91
this is not integer hence meaning less for one sample of 2700 charecters; but average value of a large number of samples of 2700 charecters
b) std deviation =(np(1-p))1/2 =13.542
as 267 lies (267-197.91)/13.542 =5.1 std deviation away from mean ; hence it is highly unusual value cause as per range rule of thumb above a nd below 2 std deviation away values from mean are unusual