# Question about confidence interval and its relationship to standard deviation

I calculated the sample mean, and it came out to be 100. The standard deviation given in the problem is 70. And the 95% confidence interval of $\mu$ I obtained is (50, 140). So if I know that 70 is the actual standard deviation, why is my CI not reliable? Is it because if I add or subtract one SD from the mean, I will end up outside of my CI?

• Check or show your calculation please. Your lower bound is 50 lower than the mean and upper bound is 40 higher than the mean. Also, explain what you mean by "not reliable." – Penguin_Knight Nov 11 '14 at 9:51
• Please include in the question the formula you used to calculate the confidence interval. Currently the result looks wrong – Alecos Papadopoulos Nov 11 '14 at 10:32