The mean blood pressure of a group of people is 105. The standard deviation was 10. What is the range of values for the 95% confidence interval?
Ref: High Yield Biostatistics, Pg: 14, 15, 22 & 23
95% confidence interval includes values within 2 standard deviations of the mean (µ±2SD). Since the SD is 10, twice of the SD would be 2X 10 = 20. Hence,
µ - 2SD = 105 – 20 = 85 &
µ + 2SD = 105 + 20 = 125
So 95% CI would include values ranging from 85 – 125.
This is the central 95% of the distribution and the remaining 5% of the distribution would be distributed equally on the two sides of this confidence limit. Hence, there will be 2.5% of the values in this distribution which will be smaller than µ - 2SD (< 85) and another 2.5% of the distribution which will be greater than µ + 2SD (> 125).