



 
Hi George, When I look up a z value of 1 in my table of standard normal table I get the value of Pr(0 < z < 1) as in the diagram below, and the value in the table is 0.3413, or if you prefer 34.13%. Pr(0 < z < 1) It looks like you have added 50% to this value to obtain 84.13%. That would give you Pr( z < 1). Pr( z < 1) When the statistics texts state that "in a normal distribution, 1 standard deviation covers 68%" they mean that 68% of the probability (area) is between 1 and 1. Pr(1 < z < 1) To find the value of Pr(1 < z < 1) from the table you need to double the value of Pr(0 < z < 1). Hence the value is 2 × 0.3413 = 0.6826 or approximately 68%. I hope this helps,  


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